Mechanical ventilation strategies

1

Why might a patient require intubation and mechanical ventilation?

There are three main indications:

• 1)

  Hypoxic respiratory failure

• 2)

  Hypercarbic respiratory failure

• 3)

  Airway protection

These three indications may be caused by primary respiratory pathology (e.g., pneumonia, chronic obstructive pulmonary disease [COPD], acute respiratory distress syndrome [ARDS]), systemic disease or impairment (e.g., Glasgow coma scale < 8, Guillain-Barré syndrome, drug intoxication), or airway compromise (e.g., retropharyngeal abscess, head and neck cancer, tracheal stenosis). The decision to intubate and provide mechanical ventilation (is based on both qualitative data (i.e., clinical examination, patient’s wishes, and goals) and quantitative data (i.e., oxygen saturation, respiratory rate [RR], arterial blood gas analysis). The decision must be individualized because arbitrary cutoff values for partial pressure of oxygen, partial pressure of carbon dioxide, or pH as indicators of respiratory failure may not be germane to all patients. The principal goal of mechanical ventilation is to support gas exchange and minimize ventilator-induced lung injury until the underlying indication for it is resolved.

2

Why are patients intubated and placed on MV for general anesthesia?

In most surgical operations, patients are intubated and placed on mechanical ventilation for two reasons: (1) airway protection, and (2) hypercarbic respiratory failure because of the neuro and respiratory depressant effects of anesthetic agents and paralytics. Mechanical ventilation may be discontinued and the patient extubated following emergence from general anesthesia, once these two indications are resolved. Specifically, the patient needs to demonstrate that they can protect their airway (e.g., stick out tongue, follow commands, evidence of coughing or gagging from the endotracheal tube [ETT]) and can breathe spontaneously without assistance.

3

Define tidal volume, respiratory rate, minute ventilation, I:E ratio, PEEP, FiO ₂ .

• Tidal volume (TV) : The volume of gas delivered to the lungs on inhalation (e.g., 500 mL)

• Respiratory rate (RR) : The number of breaths per minute. Sometimes referred to as frequency on some ventilators (e.g., 12 breaths per minute)

• Minute ventilation (MV ): The amount of gas exchanged with the lung per minute where MV = RR × TV (e.g., 6 LPM)

• Inspired to expired (I:E) ratio : The ratio of time spent on inhalation versus exhalation (e.g., 1:2). Some ventilators may use inspiratory flow rate or inspiratory time (T _i ) as a surrogate for this parameter

• Positive end-expiratory pressure (PEEP) : Positive pressure delivered to the lungs to prevent atelectasis during exhalation

• Fraction of inspired oxygen (FiO ₂ ) : The percent oxygen delivered to the patient with each TV (e.g., 50%)

4

Define peak inspiratory pressure, plateau pressure, pulmonary compliance, airway resistance.

• Peak inspiratory pressure (PIP) : The peak pressure during inhalation is measured on the inspiratory limb proximal to the ETT. This measurement is affected by the pulmonary compliance and pulmonary resistance ( Fig. 27.1 ).

  

• Plateau pressure (P _plat ) : Reflects the pressure within the alveoli and is affected by the pulmonary compliance, but not resistance. Also measured on the inspiratory limb proximal to the ETT, it is done by performing a breath hold immediately following inhalation, when flow = 0 (see Fig. 27.1 )

• Pulmonary complianc e: This measures the overall compliance of the entire pulmonary system, including both the compliance of the lungs and extrinsic contributions from the abdomen and chest wall

• Airway resistanc e: This measures the resistance of the pulmonary system and reflects the difference between the PIP and P _plat .

5

How does the plateau pressure (P _plat ) measure the alveolar pressure (P _alv )?

Recall Ohm’s law, Δ V = I × R, where Δ V is a measure of the voltage drop (or gradient) across a resistor (R) for a given current (I). This equation can be applied to the pulmonary system where Δ P = Flow × R, with resistors in series between the inspiratory limb of the circuit and alveoli. These resistors are represented by the ETT (R _ett ) and the distal airways (R _airway ), each causing a pressure gradient (Δ P  ) for a given flow. During a breath hold, flow = 0, and the pressure gradient, Δ P across R _ett and R _airway is also 0. Therefore the inspiratory pressure is the following: P _insp = P _trach = P _alv , where P _insp is termed P _plat when flow = 0.

6

List causes for an elevated PIP. How could you differentiate between them with a P _plat ?

See Table 27.1 .

7

What is the difference between volume control and pressure control ventilation?

Volume control ventilation provides breaths that are volume constant and pressure variable. This means that the “volume” of the delivered breath is set, or “controlled,” by the clinician. The resultant airway pressures with this form of ventilation will depend on the compliance and resistance of the patient's respiratory system, which includes the ETT. Conversely, pressure-control provides breaths that are pressure constant and volume variable. In this mode, the amount of positive “pressure” delivered with each breath is set, or “controlled,” by the clinician, and the TV that results from that pressure is again dependent on the resistance and compliance of the patient's respiratory system. Please see Fig. 27.2 .

Remember that in the operating room, events that cause a sudden change in respiratory compliance (e.g., prone positioning, laparoscopy, or paralytics) or airway resistance (e.g., bronchospasm, secretions, mucous plugs, kinked ETT) are common and can cause a dramatic change in the delivered TV, with pressure-control modes of ventilation.

8

How does a volume-controlled breath deliver the prescribed volume?

Volume-controlled ventilation strategies could arguably be termed flow-control based on the mechanism of how the ventilator delivers the TV. The clinician sets the TV, RR, I:E, and the ventilator uses this data to calculate the necessary flow needed to deliver the prescribed TV. For example, assuming a TV of 500 mL, an RR of 10 breaths per minute, and an I:E of 1:2, the ventilator would first calculate the period (time) for one complete breath cycle (inhalation and exhalation) using the RR. In this example, 10 breaths per minute equals 6 seconds per breath. Then it calculates the T _i using the I:E ratio, which equals 2 seconds. The inspiratory flow rate is then calculated, which equals 500 mL/2 sec = 250 mL/sec. Traditionally, this breathe is given at a constant flow (e.g., 250 mL/sec) over the T _i as a “square waveform,” but may be given as a “decelerating waveform” to mimic physiological breathing and improve patient comfort.

9

What are the advantages and disadvantages of a “volume-controlled” breath?

The primary advantage is that TVs are constant, minimizing the risk of hypoventilation or hyperventilation. The disadvantage is the nonphysiologic character of the delivered breaths, leading to patient discomfort or patient-ventilator dyssynchrony. This is because of two reasons: (1) flow is constant, and (2) TV is constant (see flow pattern in Fig. 27.2 ). Normal physiologic breathing is characterized by a high initial flow rate that downslopes toward zero (i.e., exponential decay) with variable TVs. Although most intensive care unit (ICU) ventilators have the ability to also deliver a synthetic “decelerating” flow patterns with volume-controlled breaths to mimic physiological breathing, the TVs are fixed and the decelerating flow pattern is linear (analogous to a right triangle) and not a true exponential decay pattern. In general, volume-controlled breaths are generally used just after induction and during maintenance of anesthesia.

10

What are the advantages and disadvantages of a “pressure-controlled” breath?

The primary advantage is better patient comfort and a higher average inspired airway pressure. Because only the pressure is constant, the patient can determine the delivered inspiratory flow rate and TV which is more comfortable than a volume-controlled breath. Further, the inspiratory flow pattern is not forced but rather governed by the resistance and compliance of the patient’s pulmonary system yielding an exponential decay flow pattern (see Fig. 27.2 ). The “pressure controlled” breath also delivers a higher average airway pressure for a given TV on inspiration compared with a volume-controlled breath. This facilitates alveolar recruitment from atelectasis and marginally increases the arterial partial pressure of oxygen (PaO ₂ ):FiO ₂ ratio. The disadvantage is because the TVs are variable, any changes to pulmonary compliance or resistance may lead to problems with hyperventilation or hypoventilation. In general, “pressure-controlled’ breaths are most often used just before extubation on awake spontaneously breathing patients.

11

How does a controlled mandatory ventilation mode, such as strict volume or pressure-controlled ventilation, interact with the patient?

Controlled mandatory ventilation modes do not allow the ventilator to interact or synchronize with the patient’s respiratory effort. A mandatory mode of ventilation is set to deliver a fixed RR and TV (volume controlled ventilation [VCV]) or inspiratory pressure (pressure controlled ventilation [PCV]), regardless of the patient’s efforts. This can be a source of significant distress in an awake patient, as they may attempt to initiate a breath but the ventilator will not allow them to breathe. Therefore these modes of ventilation should be reserved for those who are deeply sedated and not initiating any respiratory effort, such as during general anesthesia.

Because of historical reasons, VCV and PCV imply controlled mandatory ventilation modes that do not synchronize with patient effort. However, these terms are still used to describe the “control variable” that determines the volume and pressure characteristics for ventilator modes that do synchronize with the patient. This is often a source of confusion. For example, Volume control synchronized intermittent mandatory ventilation (VC-SIMV) delivers a synchronized breath, but the clinician “controls” the delivered “volume.”