Anesthesia Ventilators - Clinical Tree

Overview

Since the 1960s, the use of intermittent positive-pressure ventilation (IPPV) has become widespread during surgical and diagnostic procedures that require anesthesia. Today’s observer might wrongly conclude that the research, development, and methods for mechanical ventilation in the operating room (OR) had occurred only recently. However, much of the necessary experimentation and design took place much earlier.

Anesthesia ventilators are commonly compared with mechanical ventilators used in the intensive care unit (ICU); however, anesthesia ventilators are unique; they not only deliver oxygen and remove carbon dioxide, they also facilitate the delivery of inhalational agents used to render patients unconscious and maintain surgical anesthesia. Modern delivery systems typically are semiclosed systems, which imply the removal of carbon dioxide and conservation of potent inhalational agents. Low fresh gas flows (FGF) are commonly used to aid in agent sparing and reduce patient exposure to unheated gases. In contrast, the ICU ventilator breathing circuit is typically an open system because no gases are recirculated through the system; as such, a carbon dioxide absorber is not used. High gas flows can be used because elaborate gas-warming and humidification techniques are available and have proven cost effective in the ICU.

In the past, it was not uncommon to bring an ICU ventilator into the OR for oxygenation and ventilation of patients with extremes of pulmonary pathophysiology because these machines had more capability. Recent advances in ventilator technology have made the differences between ICU ventilators and anesthesia ventilators negligible. Outcome data continue to be lacking in the scientific literature regarding differences in modes used in the OR: synchronized intermittent mechanical ventilation (SIMV) or pressure-support ventilation (PSV). However, this has not stopped manufacturers from providing these (and other) modes of ventilation, nor has it stopped clinicians from using them while delivering an anesthetic during surgery.

In this chapter, the sections on pulmonary mechanics, physiology, and basic principles of mechanical ventilation delineate the technology and contemporary strategies for lung ventilation. Major features of commonly used anesthesia ventilators in the United States are described.

Of note, no chapter can substitute for the detailed information provided by each ventilator manufacturer, which is found in the educational materials and the operator’s and service manuals for each piece of equipment. The reader should refer to these documents for the most detailed pneumatic and electrical schematics.

History

Early recorded attempts to artificially ventilate the lungs of a person date from the 1400s. Baker found records of mouth-to-mouth resuscitation of a newborn in 1472 and of an asphyxiated miner in 1744. Paracelsus is credited with the first use of a bellows in 1530 to artificially inflate the lungs. Open-chest ventilation of a dog via an endotracheal reed was described by Andreas Vesalius in 1555, using mouth-to-tube pressurization, but it was later replaced with bellows ventilation by Robert Hooke in 1667. By the late 1700s, Denmark had initiated a formal campaign and monetary reward for those using a bellows to resuscitate victims of near-drowning. The metal endoral tube, with a conical adapter for the glottic opening, was introduced in 1887 by O’Dwyer for the treatment of patients with diphtheria. This tube was combined with George Fell’s manual ventilating bellows and valve device and was used to treat opium overdose in 1891. The resulting Fell-O’Dwyer apparatus ( Fig. 6.1 ) was simplified by removing the valve and placing a hole in the circuit; the hole could be occluded by the thumb during inspiration, thus providing positive-pressure ventilation and passive exhalation. In France in 1896, Tuffier and Hallion were able to partially resect the lung of a patient whose trachea they had intubated blindly with a cuffed tracheal tube and whose lungs they ventilated during the surgical procedure. Finally, in 1898, the first rudimentary anesthesia machine was developed by Rudolph Matas of New Orleans, who added an anesthetic vapor delivery system to the Fell-O’Dwyer apparatus, thus allowing the resection of a chest wall lesion under positive-pressure ventilation with anesthesia.

Attempting to circumvent the difficulties of tracheal intubation, in 1904, Sauerbruch developed a negative-pressure operating chamber that required the patient’s head to be sealed outside the chamber. Further development resulted in the electrically powered “iron lung” by Drinker and Shaw in 1928, which was widely used to treat patients with respiratory failure during the polio epidemics. In an alternative approach, in 1905, Brauer provided positive-pressure ventilation via the head, sealed within a chamber and thus eliminating the need for intubation or operating within a chamber ( Fig. 6.2 ).

Fig. 6.2, Brauer’s positive-pressure apparatus.

Modern techniques of endotracheal ventilation during general anesthesia were initiated by Magill in 1928 for head and neck surgery. The beginnings of modern mechanical ventilation are attributed to Engström and his ventilator during the polio epidemics in Denmark circa 1952. This ventilator was later modified for use during general anesthesia, which stimulated the development of a huge number of anesthesia ventilators with a wide diversity of characteristic behaviors, mechanisms, and power sources. Gradually, designs were modified and eventually replaced by pneumatically-controlled and fluidically time-cycled systems that were optional accessories for an anesthesia machine. These stand-alone ventilators substituted for the reservoir bag at the connection to the breathing circuit and took on the appearance of modern “bag-in-a-bottle” double-circuit systems.

Contemporary anesthesia workstations have replaced these freestanding ventilators by integrating the fresh gas delivery system, waste gas scavenging system, and ventilator into one unit. Modern ventilators have electronically controlled circuits and, in some cases, closed feedback loops with microprocessor-regulated flow control valves. Modern ventilators provide both digital and graphic displays to aid in ventilator management.

Physiology and Mechanical Concepts

Gas Exchange

The two major functions of the lung are taken into consideration during mechanical ventilation: ventilation, the elimination of carbon dioxide (CO ₂ ), and oxygenation, the intake of oxygen (O ₂ ). A clear distinction should be made between the elimination of carbon dioxide and the intake of oxygen, even though these two processes are mechanically coupled during natural, spontaneous breathing and are interrelated at the metabolic level. Each is capable of physiologically stimulating ventilation.

Carbon dioxide elimination depends on ventilation, which means the lungs are inflated with non–CO ₂ -containing gases. The carbon dioxide gas of metabolism enters the alveoli of the lungs, and the CO ₂ -containing gas is expelled from the lungs on exhalation. As such, alveolar ventilation determines the partial pressure of CO ₂ in the arterialized blood (PaCO ₂ ).

Oxygenation is best represented by the partial pressure of oxygen in the arterial blood (PaO ₂ ). Predictable improvements in oxygenation can be facilitated by enriching the inspired gas as dictated by the alveolar gas equation:

P_{A} O_{2} = Fi O_{2} (P_{B} - P_{H 2 O}) - (P_{A} C O_{2} / R)

where P _A O ₂ is the partial pressure of alveolar oxygen, FiO ₂ is the fraction of inspired oxygen, P _B is barometric pressure, P _H2O is partial pressure of water vapor at 37°C, P _A CO ₂ is the partial pressure of alveolar carbon dioxide, and R is the respiratory quotient. Clinically, the P _A CO ₂ is best estimated from the PaCO ₂ unless a significant diffusion barrier is present. Increased oxygenation also can be accomplished by increases in airway pressure, which can recruit collapsed alveoli and redistribute alveolar fluid. These changes may be largely independent of ventilation.

Carbon Dioxide Equilibrium

The quantity of carbon dioxide produced normally dictates the minute ventilation. With the exception of using cardiopulmonary bypass or an extracorporeal membrane oxygenator (ECMO), no alternative method has proved satisfactory for eliminating carbon dioxide. Breathing is essential. Normally, in the absence of disease, high altitude, and pharmacologic intervention, spontaneous ventilation results in a PaCO ₂ of almost exactly 40 mm Hg. However, the quantitative relationship between CO ₂ production and minute ventilation often is poorly understood.

Carbon Dioxide Production

A resting adult weighing 70 kg produces approximately 0.008 gram molecules (moles) of carbon dioxide per minute. At standard temperature (0°C) and pressure (760 mm Hg), one mole of any gas occupies 22.4 L. Therefore, 0.008 moles of carbon dioxide occupy approximately 180 mL. At body temperature (37°C), this is approximately 200 mL.

Carbon Dioxide Elimination

Expressing carbon dioxide production either in moles or in milliliters at atmospheric pressure provides no insight into the volume of ventilation required to maintain homeostasis. More helpful information is provided when the same quantity of carbon dioxide is expressed at different partial pressures, using Boyle’s law ( Table 6.1 ).

Table 6.1

Metabolic Production of CO ₂ Expressed at Different Partial Pressures

	Partial Pressure (mm Hg)	Volume Occupied (mL)
Normal atmospheric pressure	760	200
One-tenth atmospheric or double alveolar partial pressure	76	2000
Normal alveolar partial pressure	38	4000
Half alveolar partial pressure	19	8000

Each volume represents the minimum alveolar ventilation capable of achieving that partial pressure of carbon dioxide.

The volume of carbon dioxide shown at each partial pressure in Table 6.1 is the volume occupied by the metabolic production for 1 minute (0.008 moles). At each pressure, the volume shown is the volume of carbon dioxide produced and therefore is the least alveolar ventilation per minute that is capable of eliminating the carbon dioxide produced. Any lesser alveolar ventilation is insufficient to allow the carbon dioxide to escape at that partial pressure.

The patient’s minute volume is made up of both the alveolar plus the total dead space ventilation. Normally, the dead space ventilation (i.e., physiologic dead space) is one-third of the minute volume. For an alveolar minute ventilation of 4000 mL, the required total minute ventilation therefore is approximately 6000 mL. The mixed expired carbon dioxide has a partial pressure of approximately 27 mm Hg.

During IPPV under anesthesia, the total (i.e., physiologic) dead space typically increases to approximately 45% of the tidal volume. The same alveolar ventilation of 4000 mL/min thus requires a total minute ventilation of approximately 7275 mL, which will result in a mixed expired partial pressure for carbon dioxide of approximately 22 mm Hg.

Oxygen Uptake

At rest, the 70-kg adult human has an oxygen consumption of approximately 250 mL/min. Strictly speaking, ventilation is not essential for oxygenation. When the patient is breathing oxygen, the pulmonary reservoir represents approximately 12 minutes worth of metabolic consumption. Furthermore, if a denitrogenated apneic patient is connected to an oxygen supply (apneic oxygenation), oxygenation theoretically is unlimited. In that case, survival is limited by carbon dioxide accumulation, not by hypoxia.

Effect Of Respiratory Quotient

Respiratory quotient (RQ) is the ratio between carbon dioxide production and oxygen consumption. The production of carbon dioxide, such as 200 mL/min, normally is slightly less than the oxygen consumption, at 250 mL/min (RQ: 200/250 = 0.8). This discrepancy has interesting implications. With an RQ of 0.8, the sum of the arterial partial pressures for carbon dioxide and oxygen [PaO ₂ (100) + PaCO ₂ (40) = 140 mm Hg] will always be slightly less than the humidified inspired oxygen tension [PiO ₂ = 0.21 × (760 – 47) = 149 mm Hg] instead of being exactly equal to it. Because the minute volume inspired is slightly greater than that exhaled, a continuing, small, net inward movement of gas into the lungs is observed. At equilibrium, the partial pressure of nitrogen, or nitrous oxide (N ₂ O), is slightly greater in the lung than in the inspired gas and, of necessity, this causes an equal reduction in the space that would have been available for the respiratory gases, carbon dioxide, and oxygen.

Physics of Gas Flow

As spontaneous breathing occurs, work is done to move gas into and out of the lung. This work has been designated the work of breathing (WOB). Another way to view this is the energy expended to move the gas into and out of the lungs. It is well known that the total WOB (WOB _T ) is the sum of the work related to overcoming the elastic properties of the lung and chest wall (WOB _E ) and the work related to overcoming the resistance aspects of the circuit, endotracheal tube, and large and small airways (WOB _R ). Thus,

WO B_{T} = WO B_{E} + WO B_{R}

Under normal circumstances, the work related to overcome the elastance of the lung and chest wall is nearly 70% of the total WOB; the work related to overcome the resistance of the airways is nearly 25%, and approximately 5% is related to inertial properties of the tissues and gases. Elastance (E) of the chest wall is defined as the change of airway pressure (ΔP) divided by the change in volume (ΔV):

E = Δ P / Δ V

Elastance, however, is more commonly described by the inverse, compliance (C):

C = Δ V / Δ P

Experimentally derived, formulated, and published in the 1840s by Jean Louis Marie Poiseuille (1797–1869), Poiseuille’s law identifies the relationship of gas flow (Q) directly to the pressure gradient (ΔP) and identifies an inverse relationship to the resistance (R) of the system:

Q = Δ P / R

This equation can be manipulated to show that:

R = Δ P / Q and Δ P = Q \times R

Because the purpose of the ventilator is to perform the WOB, it becomes advantageous to examine these physical relationships. In so doing, a method for classifying and understanding ventilator function using the “equation of motion” has emerged.

The force exerted by a ventilator is measured as pressure. This pressure must overcome two distinct impedances to motion during inspiration: compliance and resistance . Exhalation is passive when mechanical ventilation is used and usually remains unaccounted. The pressure required to overcome the compliance properties of the lung and chest wall can be expressed as:

P_{C} = V_{T} / C

where P _c is pressure compliance. A second element of pressure required to overcome the resistance is found within the breathing circuit, the endotracheal tube, and the conducting airways. The pressure required to overcome this resistance may be expressed mathematically:

P_{R} = Q_{I} \times R

where P _R is pressure resistance and Q _I is inspiratory flow. Because the ventilator exerts pressure to overcome both the compliance and resistance, these two equations may be combined during inspiration:

P_{T} = P_{I} = P_{C} + P_{R}

where P _T is total pressure and P _I is inspiratory pressure, or:

P_{I} = ([V_{T} / C] + [Q_{I} \times R])

Compliance and resistance may be regarded as the “load” facing the inspiratory pressure that results in the two fundamental variables: tidal volume and inspiratory flow . Changes in inspiratory pressure result in changes in both tidal volume and inspiratory flow. Changes in a desired tidal volume can be achieved by changes in inspiratory pressure and/or flow. Changes in a desired inspiratory flow can be achieved as the result of changes in inspiratory pressure, tidal volume, or both. While using specific ventilator modes, a ventilator attempts to control the inspiratory flow rate to provide a “set” tidal volume or inspiratory pressure ( Fig. 6.3 ). When the inspiratory flow is matched to a desired tidal volume, the inspiratory pressure varies to the given load ( Fig. 6.4 ). When the inspiratory flow is matched to a desired inspiratory pressure, the tidal volume varies to the given load ( Fig. 6.5 ).

Fig. 6.3, Triad of ventilator parameters.

Fig. 6.4, Relationship of controlled inspiratory flow and “fixed” or “set” tidal volume. An example of fixed flow on a 3-L anesthesia circuit reservoir bag with volume control ventilation settings. Notice inspiratory flow is constant (a) and inspiratory pressure rises linearly (b) . P AW , airway pressure.

Fig. 6.5, Relationship of controlled inspiratory flow and “fixed” or “set” inspiratory pressure. An example of a set pressure limit on a 3-L anesthesia circuit reservoir bag with pressure control ventilation settings. Notice inspiratory pressure rises sharply to the set pressure limit (a) and inspiratory flow rises sharply, then decays before exhalation (b) . P AW , airway pressure.

Interdependence Of Ventilator Settings

Each ventilator discussed in this chapter has a selection of primary variables that the user may set. These may include any of the following:

Inspired pressure:	P _I
Peak inspiratory pressure:	P _Imax
Tidal volume:	V _T
Minute volume:	V _M
Inspiratory flow:	Q _I
Frequency:	f
Respiratory cycle time:	T _c
Inspiratory pause time:	T _plat
Inspiratory time:	T _I
Expiratory time:	T _E
Inspiratory/expiratory ratio:	I:E

Many of these respiratory variables are interdependent, such as minute volume, tidal volume, and respiratory rate
$V_{M} = V_{T} \times f$ . In some ventilators, the tidal volume is determined by dividing the minute volume by the respiratory rate:

V_{T} = V_{M} / f

Tidal volume is also related to the inspiratory flow rate and the inspiratory time:

V_{T} = Q_{I} \times T_{I}

An inspiratory pause (plateau time [T _plat ]) is a part of the inspiratory time. Because the inspiratory pause makes minimal contribution to the tidal volume, the equation for tidal volume may be modified:

V_{T} = Q_{I} \times (T_{I} - T_{plat})

Clinicians are also concerned about the relationship between inspiratory and expiratory time because the I:E ratio and the absolute times T _I and T _E affect ventilation and oxygenation. This ratio and the absolute time of inspiration are related to frequency and may be expressed mathematically.

Frequency determines duration of the respiratory cycle (T _c ), usually expressed in seconds, by the following relationship:

T_{C} = 60 / f

It follows that 60/f is equal to the sum of inspiratory and expiratory times:

60 / f = T_{I} + T_{E}

The relationship between T _I and T _E is conventionally expressed as the I:E ratio with 1 as the numerator. This ratio may be derived mathematically as follows, where R _I:E equals I:E ratio:

(1)
Seconds per minute devoted to inspiration:

f \times T_{I}

(2)
Seconds per minute devoted to exhalation:

60 - (f \times T_{I})

(3)
Dividing (1) by (2), the I:E ratio is:

R_{I:E} = (f \times T_{I}) / (60 - [f \times T_{I}])

(4)
The frequency and the inspiratory time can be derived by rearranging (3):

(60 \times R_{I:E}) - (f \times T_{I} \times R_{I:E}) = f \times T_{I}

(5)

$60 \times R_{I:E} = (f \times T_{I}) + (f \times T_{I} \times R_{I:E})$
(6)

$60 \times R_{I:E} = (f \times T_{I}) \times (1 + R_{I:E})$
(7)

$f \times T_{I} = (60 \times R_{I : E}) / (1 + R_{I : E})$
(8)

$T_{I} = (60 \times R_{I : E}) / ([1 + R_{I : E}] \times f)$

Depending on the manufacturer and the particular model of ventilator, the clinician must choose the R _I:E (I:E ratio), mean Q _I (average inspiratory flow rate), or both. The reader is encouraged to study the manufacturer variations shown in Figs. 6.6 to 6.13 .

Fig. 6.6, Interdependence of ventilator settings. Notice that the frequency and the I:E ratio are independent of one another. If tidal volume is fixed, changes in the respiratory rate (a) or I:E ratio (b) alter the inspiratory time. The inspiratory flow must then be adjusted (c) to maintain the fixed tidal volume at the new inspiratory time. f, Frequency or respiratory rate; Flow insp, inspiratory flow; I:E ratio, inspiratory time to exhalation time ratio; Paco2, partial pressure of arterial carbon dioxide; TE, exhalation time; TI, inspiratory time; VM, minute ventilation; VT, tidal volume.

Fig. 6.7, The GE-Datex-Ohmeda 7000 ventilator (GE Healthcare, Waukesha, WI) requires the user to set minute volume and rate. Tidal volume must be calculated. Numbered components indicate the following: set minute volume (1) , set frequency (2) , set I:E ratio (3) , warning lamps (4) , switch to test the warning lamps (5) , ventilator preoperative checklist (6) , switch to activate one manual cycle (7) , sigh function on/off (8) , and power on/off switch (9) .

Fig. 6.8, The Ohmeda 7800 ventilator (GE Healthcare, Waukesha, WI) allows the user to select inspiratory flow (5) to determine the rate at which the selected tidal volume is delivered. This selection alters the I:E ratio, which is announced in the liquid crystal display (7) , which also displays ventilatory parameters and alarms. Compared with the Ohmeda 7000, the Ohmeda 7800 adds a pressure limiter (3) and a fixed inspiratory pause option (1) . Additional numbered components include alarm limit sets (2) , oxygen calibration dial (4) , set frequency (6) , set tidal volume and apnea alarm disable (8) , power on/off switch (9) , and alarm silence button (10) .

Fig. 6.9, The Ohmeda 7900 ventilator (GE Healthcare, Waukesha, WI) allows the user to set desired tidal volume (as shown) or inspired pressure (3) in the volume-controlled or pressure generator modes, respectively. Compared with the Ohmeda 7800 ventilator, the user selects the I:E ratio (5) instead of the flow. Positive end-expiratory pressure (PEEP; 7) is an integral feature on the control panel. Dedicated displays of measured parameters are demonstrated. Other numbered components include the audible alarm silence button (1) , mechanical ventilation on/off switch (2) , select frequency (4) , select inspiratory pressure limit (6) , adjustment knob for the corresponding selection (8) , select menu (9) , and select apnea–volume alarm combinations (10) .

Fig. 6.10, A unique feature of the Dräger AV ventilators (Dräger Medical, Telford, PA) is that both the inspiratory flow and the I:E ratio may be set by the user, creating a variable inspiratory pause within the preset inspiratory time.

Fig. 6.11, The Dräger AV-2 ventilator (Dräger Medical, Telford, PA) control panel adds a pressure limiter and digital displays of frequency and I:E ratio. The inspiratory flow and I:E ratio are set by the user, creating a variable inspiratory pause.

Fig. 6.12, The Fabius ventilator controls

Fig. 6.13, The Air-Shields Ventimeter Controller II. Shown are the on/off switch (A) , the inspiratory flow control (B) , the inspiratory time control (C) , and expiratory pause control (D) .

The following example demonstrates how frequency, tidal volume, flow, and I:E ratio are interdependent. Assume that in a 60-kg patient V _T is 600 mL and f equals 10 breaths/min. As such, the cycle time is fixed at 6 seconds.

By choosing an I:E ratio of 1:2, the inspiratory time becomes fixed at 2 seconds, which mandates a mean inspiratory flow rate of 300 mL/sec or 18 L/min (300 mL/sec × 60 sec/min = 18 L/min). Choosing an I:E ratio of 1:1 increases the inspiratory time to 3 seconds, resulting in an inspiratory flow rate of 200 mL/sec or 12 L/min. An I:E ratio of 1:3 reduces the inspiratory time to 1.5 seconds, resulting in an inspiratory flow rate of 400 mL/sec or 24 L/min.

This situation is complicated somewhat by the selection of an inspiratory pause because the respiratory gases are not in transit into or out of the lungs. Exhalation begins when gases start leaving the lungs, so the inspiratory pause is considered part of the inspiratory phase of the respiratory cycle. When incorporated into the original example, with an I:E ratio of 1:2, the inspiratory time of 2 seconds would result in a mean inspiratory flow rate of 300 mL/sec or 18 L/min. In a situation in which T _plat equals 25%, T _I is selected, and 25% of the original inspiratory time is added to the inspiratory time. This results in a changed I:E ratio. In this example, the inspiratory time is 2 seconds and 25% of the inspiratory time is 0.5 seconds, which results in a total inspiratory time of 2.5 seconds. Superficial calculations of inspiratory flow rate would suggest 240 mL/sec or 14.4 L/min flows. However, the ventilator would continue to deliver the inspiratory flow at 300 mL/sec or 18 L/min. Inspiratory flow is stopped at 2 seconds, achieving the 600 mL tidal volume, but exhalation occurs 0.5 seconds later. As such, the I:E ratio is changed without affecting the tidal volume, respiratory rate, or inspiratory flow rate. This pause at end inhalation allows for a mild increase in mean airway pressure, increased alveolar recruitment, and improved oxygenation ( Fig. 6.14 ).

Fig. 6.14, The effect of the inspiratory pause on the I:E ratio.

Primary selection of an inspiratory flow mandates the inspiratory time for the given 600 mL V _T and thus determines the I:E ratio. For example, selecting a flow of 18 L/min (300 mL/sec) mandates an I:E ratio of 1:2
$V_{T} / Q_{I} = T_{I}$ :

600 mL \div 300 mL / \sec = 2 \sec

At f = 10, each cycle is 6 seconds. Therefore, expiratory time equals 4 seconds. The I:E ratio is 2 seconds relative to 4 seconds, or 1:2.

Some combinations of settings may exceed the capability of the ventilator. For example, a very low flow setting may not be able to deliver the 600 mL within the allotted 6-second cycle time. The alarm “VENT SET ERROR” will appear in the GE-Datex-Ohmeda 7800 ventilator (GE Healthcare, Waukesha, WI) display ( Fig. 6.15 ). In the GE-Datex-Ohmeda 7800, with Q _I equal to 18 L/min, selection of the inspiratory pause of 25% will prolong the T _I to 2.5 seconds, thus changing the I:E ratio to 1:1.4.

Fig. 6.15, The limitations of the GE-Datex-Ohmeda 7800 ventilator, illustrating the relationships among flow, frequency, and tidal volume. Only combinations of settings behind the shaded area are possible. In situation A, the message “VENT SET ERROR” appears and may be corrected by decreasing the rate C, increasing the flow B, or decreasing the tidal volume to D.

Other ventilators allow the selection of the I:E ratio and flow simultaneously. Three situations may result: (1) flow will be inadequate to deliver the selected tidal volume, (2) flow can be increased to create a variable end-inspiratory pause, and (3) flow can be just enough to depress the bellows to the bottom, thus delivering the desired tidal volume without a pause.

The final primary variable is the inspiratory pressure (P _I ). This variable can be set on some ventilators as a primary variable if the ventilator is in a pressure control mode. In so doing, airway pressure increases very rapidly to the set level and is maintained at that level for the duration of the inspiratory period. This behavior must be distinguished from that of an airway pressure limiter, a passive device that does nothing more than prevent airway pressure (P _AW ) from exceeding a certain value. A Venturi ventilator can be set to function like a pressure generator if the flow is set to a high level and the pressure limiter is carefully adjusted to the desired peak airway pressure.

Lung Function During Anesthesia And Mechanical Ventilation

During anesthesia with a tracheal tube, lung function is adversely affected by many factors. Most of these factors are related to the physical aspect of a tracheal tube: retention of secretions as a result of cough suppression, interference with and damage to the mucociliary elevator, increased insensible water loss by lack of humidification, inspissation of secretions by dry gases, and heat loss in exhaled gases. In addition, an increase in ventilation/perfusion (V/Q) mismatching occurs from changes in physiologic and mechanical (apparatus) dead space. The WOB may be increased from resistance changes to load related to the inner diameter and length of the tracheal tube.

Tracheal suctioning, one solution to the problem of airway secretions, has mixed risks and benefits. The removal of secretions from airways can improve oxygenation and ventilation, but the act of suctioning the secretions can cause negative airway pressure, resulting in atelectasis and entrainment of nitrogen that reduces the fraction of inspired oxygen. In addition, direct airway irritation by instrumentation can cause coughing and straining that can transiently affect the cardiac output and blood pressure.

IPPV causes an increase in intrathoracic pressure during inspiration. Elevated intrathoracic pressures can decrease the blood flow returning to the heart from extrathoracic blood vessels, which in turn decreases cardiac output. Venous return also can be decreased by positive end-expiratory pressure (PEEP), which increases intrathoracic pressure during exhalation.

Lung function can be improved by the use of a tracheal tube. Tracheal intubation can protect the airway from oral and gastric secretions, especially if a cuffed tube is used, and it can establish a patent conduit for ventilation to occur. The latter occurs when upper airway obstruction is present. Mechanical ventilation can reduce the WOB and allow fatigued respiratory muscles a chance to recover. Mechanical ventilation allows consistent and predictable ventilation patterns, which removes the need for an anesthesiologist to ventilate manually for long periods. This also makes it possible to change ventilation strategies in response to changes in the surgical process, patient condition, and indicators of oxygenation and ventilation, such as capnography, oximetry, blood gases, and mechanical parameters that include airway pressure and resistance, and lung compliance. In specific circumstances, appropriate modes of ventilation can improve the function of an abnormal lung during anesthesia. Finally, the delivery of medications such as potent inhalational agents, and aerosolized substances through the tracheal tube with the aid of mechanical ventilation can relieve bronchospasm.

Lung Protection Strategies

Institution of IPPV is associated with the ever-present risk of traumatic lung injury. Barotrauma, or injury related to pressure, can be grossly manifested as a pneumothorax or more subtly as physiologic and pathologic changes related to alveolar overstretching. Volutrauma, injury related to volume, also can be caused by alveolar overstretching. Damage related to shear stress from the opening and closing of the alveoli, called atelectrauma or shear trauma, can be caused by both pressure- and volume-related changes. Airway irritation that causes patient/ventilator dyssynchrony—coughing, bucking, and straining—can result in sharp changes in airway pressure, which can then cause lung injury. Usage of PEEP can lessen the injury from shear trauma but also can result in increases in physiologic dead space and reduced cardiac output.

Disease states that affect the uniformity of the lung can increase the risk of lung injury. This happens when small segments of the lung have reductions in compliance compared with their normal counterparts. Previous assumptions about the relatively predictable distribution of the volume, pressure, and perfusion to the lung segments may no longer hold true; nondiseased segments receive a greater share of the tidal volume and effects of the inspiratory pressure and PEEP. As a result, these “good” lung segments can be injured, and physiologic changes related to V/Q mismatch may be exaggerated. This can be seen in patients with congestive heart failure, pneumonia, and acute respiratory distress syndrome (ARDS).

Large tidal volumes (15 to 20 mL/kg) also have been used during anesthesia to maintain alveolar distension. Although this strategy effectively prevents atelectasis and reduces shunt fraction, it is now recognized as a potential cause of barotrauma. Overdistension of healthy alveoli can cause disruptions of the alveolar-capillary membrane and lead to pulmonary interstitial emphysema and pneumothorax. As already stated, the presence of diffuse lung disease may compound injury to remaining healthy alveoli. Tidal volumes of 6 to 8 mL/kg are now recommended, with the addition of PEEP. ^,

Complementing the recommendation of normal tidal volumes, safe peak inflation pressures now dominate ventilation strategies. Studies show that maximal alveolar pressures only slightly greater than 30 to 40 cm H ₂ O may be associated with lung injury. Recent designs of anesthesia ventilators have all incorporated peak airway pressure limiters.

Optimal PEEP recruits collapsed alveoli and maximizes functional residual capacity (FRC). However, increases in PEEP beyond this point may overdistend patent alveoli without further recruitment of others. New strategies analyze static pressure–volume (compliance) plots to determine optimal PEEP and safe peak inspiratory pressures. Optimal PEEP is usually 5 to 15 cm H ₂ O.

For the past 15 years, clinicians have been reducing tidal volumes even further for patients with acute lung injury (ALI) or ARDS. A landmark paper from the ARDS Network showed a reduction in mortality rate in a group ventilated with lower tidal volumes (6 mL/kg) compared with those in a control group (12 mL/kg). Ventilator settings that result in injury cause diffuse alveolar damage leading to pulmonary edema, activation of inflammatory cells, local production of inflammatory mediators, and leaks of these mediators into the systemic circulation. Prospective studies examining the use of lung protective strategies in the OR for non-ALI patients are lacking. The few randomized studies that have been done do not confidently demonstrate benefits in the OR, and some authors recommend the avoidance of high plateau pressures (>20 cm H ₂ O) and large tidal volumes (>10 mL/kg) in this patient population. The objective of this strategy is to minimize regional end-inspiratory stretch and thereby reduce alveolar injury and inflammation.

Big data analysis has focused attention on the effect of peak (driving) and plateau pressures on lung injury. Retrospective data from nearly 70,000 medical records indicated that tidal volumes >10 ml/kg ideal body weight and plateau pressures >16 cm H ₂ O were linked to postoperative respiratory complications. Plateau pressures less than 16 cm H ₂ O and PEEP of 5 cm H ₂ O were identified as lung protective.

Ventilation with tidal volumes of 6 ml/kg and low peak airway pressures are associated with an increased incidence of hypercapnia. Permissive hypercapnia promises to reduce ventilatory complications without significant adverse effects. Humans seem to tolerate respiratory acidosis well, even with an arterial pH of 7.15 and a PaCO ₂ of 80 mm Hg. A classic study by Frumin et al. demonstrated that patients who were apneic but well oxygenated tolerated P _a CO ₂ levels of over 200 mm Hg with few complications. This strategy may be contraindicated in patients with increased intracranial pressure, recent myocardial infarction, pulmonary hypertension, or gastrointestinal bleeding. This is because acute increases in PaCO ₂ increase sympathetic activity, cardiac output, pulmonary vascular resistance, and cerebral blood flow and also may impair central nervous system (CNS) function.

Inverse ratio ventilation (IRV) originated in the early 1970s as a method to improve oxygenation in neonates with hyaline membrane disease and was later extended to adults with ARDS. Although ratios as high as 4:1 were used, the benefits of this mode depend more on an absolute prolongation of inspiratory time in combination with a decreased peak inspiratory pressure. Typical I:E ratios of 1:2 to 1:4 have been lengthened to 1:1 or greater. The objective is to increase mean airway pressure and minimize peak pressure; the desired outcome is recruitment of collapsed alveoli without overdistension. Mean airway pressure directly corresponds with alveolar recruitment, reduction in shunt fraction, and increased oxygenation. Clinical evidence supports the contention that shunt fraction is reduced and oxygenation is improved, although modestly. Because pressure control is the desired outcome, this mode of ventilation is more commonly applied with pressure generators. Caution is advised when using IRV because this strategy may not allow adequate alveolar emptying, thus causing “breath stacking” and auto-PEEP. In addition, IRV is contraindicated in obstructive lung disease, such as asthma. It also may cause hypotension from a reduction in venous return since there is a higher inspiratory pressure for a longer portion of the respiratory cycle.

High-frequency ventilation (HFV), defined later, may be an applicable strategy during anesthesia, although some anesthesia ventilators are capable of rates only to 100 breaths/min. The conceptual advantage of HFV is a lower peak airway pressure combined with nonbulk flow of gas to provide a motionless surgical field. This technique has not proven clinically advantageous in patients with respiratory failure, but it is helpful in patients with large pulmonary air leaks. In addition, pulmonary complications in neonates may be reduced with this strategy.

Classification, Special Features, and Modes of Ventilation

Ventilator Classification

Historically, many different anesthesia ventilators have been produced over the years. Many of them incorporated features that are no longer considered valuable, but these serve in the general description and understanding of ventilator function. Commonly, such things as ventilator mechanisms, cycling parameters, and special clinical features were used to classify anesthesia ventilators.

Power Source

Most ventilators today function in an environment where electricity and compressed gases are readily available. In the past, however, some ventilators were designed to function solely on pneumatic gases. Currently, the only ventilators that function solely on pneumatic gases are not anesthesia delivery systems; these pneumatically powered ventilators are used in patient transport and in magnetic resonance imaging suites. The Omni-Vent (Allied Healthcare Products, Inc., St. Louis, MO) is one example; it also illustrates one of the simplest control mechanisms: I-time, E-time, and flow rate are the only set variables ( Fig. 6.16 ). Ventilators that use the bag-in-a-bottle design commonly have a double circuit in which a high-pressure driving gas, electrical circuits, and solenoids are used to achieve ventilator function. Some newer ventilators use electricity exclusively to drive a piston ventilator, which spares gas use for the patient.

Fig. 6.16, The Omni-Vent battery powered transport ventilator. It has three simple controls: inspiratory time, expiratory time, and inspiratory flow rate.

Drive Mechanism

Classification of ventilators has historically been reduced to those that push or drive the patient gas to the patient, which is done by either a bellows or a piston. An alternate design uses a turbine to replace the bellows or the piston to drive gas to the patient. In addition, technologic improvements in computer software have made direct gas control via finely controlled solenoids and servos a reality.

Bellows Ventilators

Bellows ventilators have used two main varieties of bellows designs over the years: the ascending, or standing, bellows and the descending, or hanging, bellows. The designation of ascending or descending was based on bellows movement on exhalation; an ascending bellows rises during exhalation ( Fig. 6.17 ), and a descending bellows falls ( Fig. 6.18 ).

Fig. 6.17, Bag-in-a-bottle bellows design, ascending bellows variety. (A) Ohmeda 7000 ventilator; (B) illustration of Ohmeda 7800 ventilator (GE Healthcare, Waukesha, WI). The bellows is contained within a clear housing, and a drive gas pushes the gas inside the bellows through the circuit. The bellows displayed in the images are deflated.

Fig. 6.18, Bag-in-a-bottle bellows design, descending bellows variety. Shown is the Datascope Patient Monitoring Anestar anesthesia machine (Mindray North America, Mahwah, NJ) with descending bellows. The bellows (arrow) are in the inflated (end of exhalation) position.

In the event of a circuit disconnect or significant leak, an ascending bellows would not fill or would improperly fill during exhalation. This provides clinicians a visible monitor of ventilator function. Because of the improved patient safety, this type of bellows generally is preferred, but does not represent a standard according to the latest document by the International Organization for Standardization (ISO 80601-2-13). The ECRI Institute (Plymouth Meeting, PA) has published a commentary and recommendations for hanging bellows in its Health Devices Alerts (1996-A40).

The hanging bellows, which typically is weighted, could fill whether a circuit disconnect, leak, or normal exhalation was present. Room air could enter the circuit and allow the bellows the ability to return to its filled position. In association with fresh gas decoupling, hanging bellows ventilators must rely on a separate reservoir bag to detect leaks and inadequate FGF.

Bellows designs are not without inherent problems. Because a high-pressure gas typically is used to drive the bellows, a hole or perforation in the bellows can subject the patient to driving gas pressures and result in barotrauma. In addition, mixing of the patient circuit gas with the driving circuit gas may cause unpredictable concentrations in oxygen. Typically, 100% oxygen is used as the driving circuit gas. When high oxygen concentrations are undesirable or the potential for an airway fire exists, leaks into the patient circuit gas may have disastrous consequences. When air is used as the driving circuit gas, unexpectedly low concentrations of oxygen may be observed. In both circumstances, patient awareness may occur because the driving gas may dilute the intended gas concentrations of inhaled anesthetics to be delivered to the patient. In addition, hypoventilation may occur if the bellows is not properly seated inside the bellows housing assembly.

Piston Ventilators

Piston ventilators rely on a piston-cylinder configuration, in which an electric motor is used to drive or displace the piston within the cylinder to cause gas flow. Tidal volume accuracy is believed to improve because the precise position of the piston during inspiration is monitored from start to finish; the motor returns the piston to the filled position prior to the next delivered breath. The drive motor requires maintenance and usage monitoring for good function because ventilator failure has been reported from worn motor parts. A leak occurring at the piston diaphragm could cause a loss of circuit gases to the room with hypoventilation during inspiration. Entrainment of room air into the patient circuit could occur as the piston returns to the filled position. Some recent designs have placed the piston ventilator within the workstation housing to make visible operation impossible. Piston-driven ventilators currently manufactured by Dräger Medical (Telford, PA) have placed the piston in both vertical and horizontal positions in different models. Such systems use fresh gas decoupling with a separate reservoir bag, which is monitored for circuit leaks or inadequate FGF.

Turbine Ventilator

A turbine ventilator relies on a spinning turbine, similar to a fan, to produce a drive gas pressure. An electric motor is used to spin the turbine. Higher revolutions per minute (RPMs) are associated with higher pressures, while lower RPMs are associated with lower circuit pressures. Accurate sensors and fine control of the turbine and circuit mechanisms allow good control of the circuit pressures, volumes, and flows. Constant flow of the turbine keeps circuit gases flowing allowing circuit dead space reduction, improved gas mixing, and rapid establishment of desired gas concentrations. Currently, Dräger Medical is the only manufacturer of a turbine ventilator.

Modern Ventilator Solenoids and Servos

As electronic opportunities for ventilator control have evolved, solenoids and servos have been a mainstay of the control of circuit pressures and gas flow ( Fig. 6.19A ). When applied in a ventilator, modern solenoids and servos can be used to finely control circuit pressures and gas flows. A solenoid mechanism is composed of a magnet wrapped by a coil of wire. When an electric current is applied to the coil, the magnet can be pulled into or pushed out of the coil. This is similar in function to an electric lock in a car door, which has an “on/off” type of action. Gas valves controlled by a solenoid require known/fixed pressures upstream and downstream of the controlled valve. Time is the key element in placing the valve in the open or closed position, which then regulates the gas flow.

Fig. 6.19, Solenoid and Servo Mechanisms. A solenoid (A) is a magnet contained within a wire coil. When an electric current is applied, the magnet slides in or out of the coils depending on orientation of the charge flow and magnet orientation. A solenoid is more often used to control other devices in an “on/off” fashion. A servo (B) is an electric motor controlled by a digital device (computer or software). Servos are typically joined to control other devices that have fine control between on and off states. The example here is a servo for a radio-controlled airplane.

A servo mechanism uses an electric motor to rotate a shaft that controls some other mechanism. The motor is under fine control of a computer-software system. Specialized gearing also contributes to precise control. Servos can provide “on/off” settings but are better utilized when finely controlling valves between the “on/off” settings. Hobbyists use radio-controlled servos in toy planes and cars to throttle up and down the engines or motors that control speed ( Fig. 6.19B ). As a departure from traditional ventilator drive systems, Maquet Critical Care AB, Solna, Sweden has produced a system that combines the gas modules and vaporizers using servos under computer-software control to directly generate circuit pressures and flows. This system does not have a bellows, piston, or turbine.

Cycling Behavior

Cycling behavior is believed to be one of the most complicated concepts in ventilator classification and description. In a normal ventilator breath, two significant events occur: inspiration and exhalation. However, cycling behavior describes the event that transitions the ventilator from exhalation to inspiration and from inspiration to exhalation. In sequence, the ventilator cycles from exhalation to inspiration, inspiration occurs, the ventilator cycles from inspiration to exhalation, and exhalation occurs. For most modern anesthesia ventilators, respiratory rate and I:E ratio are set in controlled modes, either by volume control ventilation (VCV) or pressure control ventilation (PCV); time cycles the breath from exhalation to inspiration and from inspiration to exhalation. A sensor can be used to initiate inspiration on spontaneous patient effort in pressure support ventilation (PSV). The trigger, an observed parameter to allow inspiration to occur, can use pressure, volume, or flow values. As will be seen, the method of delivering the inspired breath also can be manipulated. Cycling from inspiration to exhalation can occur as a result of achieving a set volume, pressure, flow, or time.

Exhalation typically is a passive event. Pressures in the airway can be manipulated during exhalation by the addition of PEEP. More recent developments in ventilator cycling allow the cycling from exhalation to inspiration to include uses of airway pressure, volume, and flows in the ICU. However, most anesthesia ventilators still use time as the determinant to cycle from exhalation to inspiration.

Many machines can function in at least two modes; the most common example is the ventilator that normally cycles when a given time or tidal volume is reached. In the GE-Datex-Ohmeda 7800 series, reducing the pressure limit makes the ventilator cycle when a given pressure is reached. By contrast, in the Dräger AV-2+, the inspiratory phase is not ended; the bellows is held in its compressed state by the continuing flow of drive gas, which escapes to the atmosphere through the pressure-limiting valve. These mechanisms afford a poor basis for classification because, as demonstrated, they may not indicate functional behavior.

Inspiratory Flow

The inspiratory flow classification describes the method used to make a breath: either a flow generator or pressure generator. A flow generator is used in ventilation when the inspiratory flow waveform is controlled around the use of a high-pressure source; the source in this case is typically greater than five times the airway pressure. A pressure generator is used when the pressure waveform is controlled around the use of a low-pressure source, usually near or moderately higher than the airway pressure.

When a high-pressure source is used, changes in patient compliance and resistance have little effect on the inspiratory flow waveform. As such, a fixed or constant flow can be established to generate a reliable tidal volume breath for a given inspiratory time. This type of ventilator is called a constant flow generator .

With a low-pressure source, changes in compliance and resistance affect the inspiratory flow waveform. As a result, the pressure waveform is controlled because it is not affected by patient changes and the use of a low-pressure source. Mechanisms used to achieve this behavior of constant airway pressure during the inspiratory period include the use of a spring or weight on the bellows. This is known as a constant-pressure generator . On inspiration, a large pressure gradient exists from the ventilator to the alveoli that results in high initial gas flows. Rising gas volumes in the lung slowly equilibrate pressures between the ventilator and lung to cause a diminishing inspiratory flow; the delivered tidal volume is the result of the variable flow related to the equilibrating pressures between the ventilator and lung and the time allotted for inspiration to occur.

Some flow generators vary the inspiratory flow to provide different flow waveforms; that is, increasing flow (ascending ramp), decreasing flow (descending ramp), or sinusoidal flow may be selected. This type of ventilator is called a variable-flow generator. Modern flow generators can provide pressure generator behavior by using negative feedback to vary the orifice of the inspiratory flow control valve during inspiration (e.g., the GE 7900 Smart Vent).

Classification by inspiratory flow should not be confused with the volume- or pressure-control modes of ventilation. Modern ventilators can now switch between flow and pressure generator methods. In addition, both flow and pressure generators can achieve the same modes of ventilation by advances in sensor application and electronic feedback control.

Control Of The Pressure And Flow Waveform During Ventilation

The pattern of ventilation used occasionally has a critical influence on lung function. The most common example in the OR is the hypotension that accompanies high-pressure ventilation. In critical care, greater attention is given to optimizing lung function by appropriate selection of the inspiratory waveform. For example, several advantages have been proposed for using a decelerating waveform: the maximum pressure is minimized, the risk of barotrauma is diminished, alveoli are kept expanded, V/Q ratios are more uniform, and distribution of gas within the lung is facilitated.

Controlled ventilation also tends to affect venous return and cardiac output. To a large extent, the best strategy to optimize cardiac output is the reverse of that which promotes improved lung function. A relatively short inspiratory period, with no time for distribution and no plateau, will have the least effect on the mean intrathoracic pressure and therefore on venous return.

Debate about the potential benefits of variation in the inspiratory waveform has persisted for at least 35 years. The expense and complexity required to achieve such waveforms have been critically discussed since the early 1960s. In the OR, fine control of the inspiratory waveform usually is not provided or needed. To this day, many anesthesia ventilators offer no control of the inspiratory waveform, and the anesthesiologist must occasionally approximate a desired pattern with the equipment available.

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