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Assisted Ventilation
But that life may, in a manner of speaking, be restored to the animal, an opening must be attempted in the trunk of the trachea, into which a tube or reed or cane should be put; you will then blow into this so that the lung may rise again and the animal take in air. Indeed, with a single breath in the case of this living animal, the lung will swell to the full extent of the thoracic cavity and the heart become strong and exhibit a wondrous variety of motions…when the lung long flaccid has collapsed, the beat of the heart and arteries appears wavy, creepy, twisting, but when the lung is inflated, it becomes strong again and swift and displays wondrous variations…as I do this, and take care that the lung is inflated at intervals, the motion of the heart and arteries does not stop.
Andreas Vesalius
De Humani Corporis Fabrica (1543)
The primary objective of assisted ventilation is to support gas exchange until the patient’s ventilatory efforts are sufficient. Ventilation may be required during immediate care of the depressed or apneic infant, before evaluation and during treatment of an acute respiratory disorder, or for prolonged periods of treatment for respiratory failure. Trained personnel and equipment for emergency ventilation should be available in every delivery room and newborn nursery. Positive-pressure ventilation effectively stabilizes most infants who require resuscitation.
This chapter is an introduction to assisted ventilation. Before undertaking assisted ventilation of any form, it must be recognized that the techniques demand time, resources, and experienced personnel. Prolonged ventilation should only be used in units where trained nurses, respiratory therapists, and medical personnel are continuously available.
Respiratory Failure
Hypercapnic respiratory failure is the inability to remove CO 2 by spontaneous respiratory efforts, which results in an increasing arterial P CO2 (PaCO 2 ) and a decreasing pH. Assisted ventilation is most commonly needed to treat hypercapnic respiratory failure. Hypoxemia is usually (but not invariably) present; in many instances, arterial oxygenation can be normalized if the inspired oxygen is increased. Infants with hypoxemic respiratory failure have a predominant problem of oxygenation, usually the result of right-to-left shunt or severe ventilation–perfusion (V̇/Q̇) mismatch. Respiratory failure can occur because of disease in the lungs or in other organs and systems ( Fig. 10.1 ). Assisted ventilation is usually required when severe respiratory failure ensues ( Box 10.1 ).
BOX 10.1
General Indications for Assisted Ventilation
-
Respiratory acidosis with pH <7.20–7.25
-
Hypoxemia while on 100% oxygen or continuous positive airway pressure with 60%–100% oxygen
-
Severe apnea
Clinical Manifestations of Respiratory Failure in the Newborn
The following are findings that should make the clinician suspect respiratory failure:
-
worsening hypercapnia and/or hypoxemia;
-
increase or decrease in respiratory rate;
-
increase or decrease in respiratory efforts (grunting, flaring, retractions);
-
periodic breathing with increasing prolongation of respiratory pauses;
-
apnea; and
-
decreasing blood pressure with tachycardia associated with pallor, circulatory failure, and ultimately bradycardia.
Cardiac Versus Pulmonary Disease
The clinician frequently needs to distinguish between cardiac and pulmonary disease in the sick newborn infant. Cyanotic heart disease may mimic respiratory disease. One possible way to differentiate between the two is to perform a hyperoxia test: place the infant in 100% oxygen for 10 minutes and then obtain an arterial Po 2 (Pa O 2 ), or a pulse oximeter can be used, which is a less invasive means of performing a hyperoxia screening test.
In infants with pulmonary disease, Pa O 2 usually increases to more than 100 mm Hg, whereas infants with cyanotic heart disease show little change in Pa O 2 . The hyperoxia test, although useful diagnostically, may also be misleading. In infants with severe pulmonary hypertension and right-to-left shunt, Pa O 2 may not elevate with 100% oxygen. Alternatively, Pa O 2 may increase more than 100 mm Hg early in life in infants with forms of cyanotic heart disease with high pulmonary blood flow (e.g., total anomalous pulmonary venous return). Echocardiography should be used to distinguish between cardiac and pulmonary disease when hypoxemia is unresponsive to ventilatory support.
Endotracheal Intubation
Most infants should receive positive-pressure ventilation using bag and mask before attempting endotracheal intubation. This improves oxygenation and decreases PaCO 2 , thereby decreasing the likelihood of bradycardia during endotracheal intubation. Positive-pressure ventilation using bag and mask is impractical for prolonged periods but can be used for the following:
-
immediate resuscitation;
-
stabilization before and after endotracheal intubation;
-
ventilation in infants whose condition is deteriorating without obvious cause; and
-
ventilation during transport to intensive care facilities when mechanical ventilation is unavailable.
Mechanical ventilation is an invasive therapy and is indicated only when the benefits outweigh the burdens.
Endotracheal Tube Size
It is preferable to use relatively small endotracheal tubes to prevent tracheal damage. The endotracheal tube should fit loosely enough to allow a leak of gas between tube and trachea when more than 10 cm H 2 O inspired pressure is generated. Tube size can be related to infant size or gestational age. Recommended sizes are as follows in Table 10.1 .
Editorial Comment:
The guideline of assuring a leak between the tube and the trachea is a good principal in larger infants and children, but because the resistance to airflow is inversely proportional to the fourth power of the inside radius of the tube, resistance becomes very great with very small tubes and the risk of obstruction is large. Therefore even though very small tubes may be somewhat easier to insert, we try to avoid tube diameters less than 2.5 mm.
–John Kattwinkel
TABLE 10.1
Endotracheal Tube Size by Gestational Age and Birth Weight
| Gestational Age (week) | Birth Weight (g) | Endotracheal Tube Size (mm internal diameter) |
|---|---|---|
| <24 | <500 | 2.0 |
| 24–28 | 500–1000 | 2.5 |
| 28–34 | 1000–2000 | 3.0 |
| 35–38 | 2000–3000 | 3.5 |
| >38 | >3000 | 3.5–4.0 |
Intubation
Insertion of an endotracheal tube should be performed with universal precautions under a radiant heat source to keep the infant warm. Free-flow oxygen should be administered as necessary.
Intubation can be a painful procedure, and so, premedication with an analgesic agent (morphine, fentanyl, or remifentanil) should be used for nonemergent intubations in neonates. A muscle relaxant (paralytic agent) should only be used with analgesia. Other agents that may be considered include sedatives (midazolam) and vagolytic agents (atropine). The ideal combination and sequence of premedications in neonates has not yet been established. Each unit should have protocols and lists of preferred medications to maximize safety.
The infant should receive positive-pressure ventilation and oxygen as needed between intubation attempts. The tip of the tube should be placed midway between the carina and the glottis. The following measurements in Table 10.2 can be used for endotracheal tube placement.
TABLE 10.2
Endotracheal Tube Insertion Length by Infant Weight
| Infant Weight (g) | Endotracheal Tube Insertion Length (tip to lip, cm) |
|---|---|
| 500 | 6.5 |
| 1000 | 7 |
| 2000 | 8 |
| 3000 | 9 |
| 4000 | 10 |
At these lengths, the distal end of the endotracheal tube should be at the midtrachea. It is easy to inadvertently pass the tube into the right mainstem bronchus, in which case the tube should be pulled back slowly until breath sounds are equal. Breath sounds should be equal bilaterally. A CO 2 detector should be used to confirm endotracheal placement. The tube should be secured well so that movement of the head and neck will not dislodge it. Lightweight plastic connectors can be used to prevent kinking the tube. The endotracheal tube position should be checked radiographically.
Editorial Comment:
I find that visual positioning of the distal stripe on the tube to be very valuable in initially assessing the appropriate depth of endotracheal tube placement. The stripe should be visible through the laryngoscope and should be place just proximal to the vocal cords. Also, if emergency access to the airway is required and endotracheal intubation expertise is not readily available, placement of a laryngeal mask airway (LMA) may be useful (see J. Wyllie, Perlman J, Kattwinkel J, et al. Resuscitation. 2015;95:e169–e201). The limitations are that even the smallest LMA version available may not be appropriate for babies weighing less than approximately 1800 g.
–John Kattwinkel
Oral Intubation
The advantages of oral intubation are the relative ease of insertion and that if necessary, a stylet can be used to aid insertion. Oral tubes should always be used in emergencies. The disadvantages of oral intubation are the increased tube mobility if the tube is inadequately secured and more difficulty in keeping the tube in position.
A laryngoscope with a Miller number 00 (small preterm), 0 (preterm), or 1 (term) blade inserted in the vallecula is used to pull upward to visualize the glottis while leaving the head in a neutral position. It is important not to traumatize the gums and tooth buds. The heart rate should be monitored continuously with auditory and visual signals during attempts at intubation. Continuous O 2 saturation or transcutaneous Po 2 monitoring is helpful because oxygenation can worsen abruptly. It is also helpful if the tube has been previously curved. To stiffen the tube for orotracheal intubation, it may be cooled or a stylet may be used.
Nasal Intubation
The advantage of nasal intubation is the improved stability with the reduced likelihood of slippage into the right mainstem bronchus or accidental extubation. The disadvantages are trauma to the nares and nasal septum, greater difficulty in insertion of the tube, and potential trauma to the developing eustachian tubes and sinuses. Nasotracheal intubation should always be performed as an elective procedure and should not be done in emergencies.
Using a laryngoscope blade, the lubricated endotracheal tube is inserted through the nares until it is visualized in the oropharynx. McGill forceps are used to guide the tube into the glottis. It is helpful if the endotracheal tube has been previously lubricated with a nontoxic, water-soluble lubricant. A stylet is never used for nasotracheal intubation.
Suctioning
Suctioning can be done if there are copious amounts of secretions or suspicion of endotracheal tube occlusion by secretions, but routine suctioning is not necessary. A strict sterile technique with disposable gloves and suction tubes is necessary. Closed systems for suction are available. The suction catheter should not be advanced past the distal end of the endotracheal tube. The infant should be allowed to recover between episodes of suctioning by maintaining stable O 2 saturations with increases in inspired oxygen concentrations and by reexpanding the lung with 10% to 20% more pressure than used for routine ventilation as needed. Saline can be instilled to facilitate removal of secretions when secretions are thick.
Although sometimes necessary, suctioning is potentially dangerous; it may cause a hypoxic episode owing to discontinuation of ventilation, extraction of gas from small airways, or atelectasis. Suctioning beyond the endotracheal tube may produce lesions in the trachea at the site of the suction catheter tip. Use of a special endotracheal tube connector allows mechanical ventilation during suctioning and prevents the catheter tip from going beyond the endotracheal tube.
Editorial Comment:
Suctioning of the endotracheal tube will decrease oxygenation and pulmonary function. I would advocate being guided by pulse oximetry during the procedure, which may require transiently increasing inspired oxygen concentration by 10% to 15% immediately before and following the period of suctioning. I would also advocate avoiding the practice of “routine” suctioning except as secretions warrant.
–John Kattwinkel
Changing an Endotracheal Tube
An endotracheal tube change is required only if the tube becomes dislodged or occluded or if the infant outgrows it. Routine change is not indicated.
Applied Pulmonary Mechanics
The following principles are helpful in understanding mechanical ventilation. A pressure gradient between the airway opening and alveoli must exist to drive the flow of gases during both inspiration and expiration. The pressure gradient required to inflate the lungs is determined largely by the compliance and the resistance of the lungs.
Compliance
Compliance is a property of distensibility (i.e., of the lungs and chest wall) and is calculated from the change in volume per unit change in pressure:
The higher the compliance is, the larger the delivered volume per unit of pressure will be. Compliance in babies with normal lungs ranges from 3 to 6 mL/cm H 2 O. Compliance in infants with respiratory distress syndrome (RDS) ranges from 0.1 to 1 mL/cm H 2 O.
Resistance
Resistance is a property of the inherent capacity of the gas-conducting system (i.e., airways, endotracheal tube, and lung tissue) to oppose airflow and is expressed as the change in pressure per unit change in flow:
Resistance in babies with normal lungs ranges from 25 to 50 cm H 2 O/L/second. Resistance is not dramatically altered in infants with RDS but is increased in intubated infants and ranges from 50 to 150 cm H 2 O/L/second.
Time Constant
Time constant is a measure of the time (expressed in seconds) necessary for 63% of a step change (e.g., airway pressure gradient) toward equilibration. A step change in airway pressure occurs between the beginning and the end of a machine-delivered inspiration (during pressure-limited, time-cycled ventilation). The product of compliance and resistance determines the time constant of the respiratory system:
Time constant = complex × resistance
For example, in an infant with normal lungs:
One time constant = 0.005 L/cm H 2 O
× 25 cm H 2 O/L/second
= 0.125 second
In an intubated infant with RDS:
One time constant = 0.001 L/cm H 2 O
× 50 cm H 2 O/L/second
= 0.050 second
It takes three time constants to achieve 95% of the pressure change to be equilibrated throughout the lungs; it takes five time constants for 99% equilibration. Thus, to allow for a fairly complete inspiration and expiration, inspiratory and expiratory times set on the ventilator should last about three to five time constants. In this example of an intubated infant with RDS, the duration of three to five time constants is 0.150 to 0.250 seconds. A very short inspiratory time can lead to inadequate tidal volume because ventilatory pressures may not equilibrate throughout the lungs ( Fig. 10.2 ). A very short expiratory time can lead to gas trapping because exhalation may not be completed. Very long inspiratory or expiratory times are also not beneficial.
Continuous Positive Airway Pressure
Spontaneous breathing can also be assisted by expansion of the lungs with continuous distending pressure. This technique is valuable when respiratory drive is normal and pulmonary disease is not overwhelming. Continuous distending pressure can be applied with continuous positive airway pressure (CPAP) or continuous negative pressure around the chest wall. Because of the ease of delivery, CPAP is the usual mode of delivery of continuous distending pressure.
Surfactant deficiency in infants with RDS predisposes to alveolar collapse. The resulting atelectatic areas of the lungs are the sites of right-to-left shunting. Functional residual capacity increases when alveoli are prevented from closing by maintaining a continuous positive transpulmonary pressure throughout the respiratory cycle. In addition, ventilation of perfused areas of the lung increases, which reduces intrapulmonary shunt.
A simple system for CPAP was described by Gregory et al in 1971 ( Fig. 10.3 ). A suitable air–oxygen mixture passes through a humidifier. Gas passes through the tubing, which is attached to an endotracheal tube. The screw clamp on the reservoir controls the flow of gas and maintains a constant positive pressure within the system, as indicated on the pressure manometer. The side arm acts as an underwater safety valve by ending under a column of water. Nasal CPAP is simple and effective; it is usually applied with nasal or nasopharyngeal prongs, although other techniques for delivery can be used ( Table 10.3 ). Problems with CPAP generally revolve around feeding difficulties, maintaining a good seal, and nasal trauma. Nursing and medical care are similar to those undertaken during mechanical ventilation.
TABLE 10.3
Techniques of Applying Continuous Positive Airway Pressure
| Method | Advantages | Disadvantages |
|---|---|---|
| Nasal prongs | Simple | Trauma to turbinates and septum; excessive crying; variation in Fi O 2 ; increased work of breathing |
| Nasopharyngeal prongs | Relatively simple, fixation easy | May become blocked or kinked |
| Endotracheal | Effective | Requires intubation; nursing and medical skills as for ventilator |
| Head box | Noninvasive | Neck seal a problem; suction difficult; nerve palsies |
| Face mask | Simple, inexpensive | Abdominal distention, pressure on face and eyes; CO 2 retention; cerebellar hemorrhage |
| Face chamber | Good seal, minimal trauma to face | Expensive; baby inaccessible |
| High-flow cannula | Easy to maintain in place; less trauma | Inconsistent delivery of pressure; leads to more failure compared to Nasal Prong CPAP |
Trials performed before the era of surfactant showed that CPAP decreased death and the need for mechanical ventilation, but increased the risk for pneumothorax. Subsequent surfactant trials showed that early surfactant treatment in the first 2 hours after birth compared with delayed treatment decreased air leaks and death or bronchopulmonary dysplasia (BPD). However, the control infants in these trials received mechanical ventilation, and it is possible that mechanical ventilation without surfactant causes lung injury. More recent trials have compared the effects of CPAP to mechanical ventilation with surfactant administration. Early CPAP, as compared with early intubation with surfactant, decreased BPD and/or death. In bigger preterm infants, CPAP results in benefits comparable to that of surfactant. Together, these studies indicate that CPAP is an effective alternative to intubation and surfactant in the treatment of RDS in most preterm infants.
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