Introduction
The standard mode of ventilation used in newborn infants prior to the availability of synchronized ventilation was known as intermittent mandatory ventilation (IMV). This pressure-controlled, time-cycled mode of ventilation provides a set number of “mandatory” mechanical inflations. The patient continues to breathe spontaneously, using the fresh gas flow available in the ventilator circuit. However, without synchronization of the infant’s spontaneous effort, the irregular respiratory pattern of a newborn baby leads to frequent asynchrony between the infant and the ventilator, sometimes resulting in a ventilator inflation that occurs just as the infant is exhaling ( Fig. 18-1 ). High airway pressure, pneumothorax, poor oxygenation, and large fluctuations in intracranial pressures leading to increased risk of intraventricular hemorrhage were the consequences of such asynchrony. Heavy sedation or muscle paralysis was often necessary in the past to prevent the baby from “fighting the ventilator.” These interventions resulted in greater dependence on respiratory support, lack of respiratory muscle training, generalized edema, and inability to assess the infant’s neurologic status. The advantages of synchronizing the infant’s spontaneous effort with the ventilator cycle, rather than using muscle relaxants, are intuitively obvious and supported by a number of short-term physiologic studies and modestly sized randomized clinical trials demonstrating improved gas exchange and other benefits of synchronized ventilation ( Table 18-1 ). Unfortunately, the two largest randomized trials of synchronized ventilation were conducted many years ago using outdated technology (pressure trigger) and had other methodologic issues; both failed to clearly demonstrate benefits of synchronization. A Cochrane meta-analysis demonstrated shorter duration of mechanical ventilation with synchronized vs nonsynchronized ventilation but no effect on other important outcomes. The effect of synchronizing the ventilator inflations with the patient’s own effort is illustrated in Figure 18-2 . This chapter focuses on the commonly used modes of synchronized ventilation. Less widely used modes are discussed in Chapter 21 on Special Ventilation Techniques.
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Trigger Technology
The availability of effective synchronized ventilation for neonatal applications lagged considerably behind its use in adults because of the technological challenges occasioned by the small size, weak respiratory effort, and short time constants of preterm infants. The ideal triggering device for newborn ventilation must be sensitive enough to be activated by a small preterm infant but must also be relatively immune to auto-triggering. A very rapid response time to match the short inspiratory times and rapid respiratory rates seen in small premature infants is also critically important. An additional challenge is the common presence of a variable leak of gas around uncuffed endotracheal tubes (ETT). The types of triggering devices used in clinical care and their relative advantages are listed in Table 18-2 . Flow triggering using a flow sensor at the airway opening has proved to be the best method that is currently widely available. Either a variable orifice differential pressure transducer (pneumotachometer) or a hot-wire anemometer may be used for flow detection, with the latter being the preferred choice. Flow triggering is much more sensitive than pressure triggering and is capable of detecting a patient effort of as little as 0.2 mL/min.
| Method/Technology | Advantages | Disadvantages |
|---|---|---|
| Airway pressure/pressure transducer | Simple, no added dead space | Lacks sensitivity, causes long trigger delay, high WOB; no tidal volume measurement |
| Airflow/hot-wire anemometer or pneumotachograph | Good sensitivity, rapid response, provides tidal volume measurement | Added dead space, prone to auto-triggering with ETT leak |
| Thoracic impedance/ECG leads | No added dead space | Affected by placement, poor electrode adhesion; no tidal volume measurement |
| Abdominal motion/applanation transducer (Graseby capsule) | Rapid response, no added dead space | Susceptible to artifact with incorrect position; affected by change in patient position; limited availability; no tidal volume measurement |
| Electrical activity of the diaphragm/transesophageal electromyography | No added dead space, very rapid response, not affected by leak; ideal for NIV | Costly, somewhat invasive; limited availability; no tidal volume measurement |
An attractive new synchronization technology, available as of this writing only on the Maquet Servo ventilators (Maquet, Wayne, N.J.), uses the electrical activity of the diaphragm, detected by transesophageal electromyography, to trigger ventilator inflation. This approach is attractive because it has the shortest trigger delay and is not affected by ETT leakage, thus being particularly suitable for noninvasive synchronized ventilation. However, it cannot currently be used independent of the neurally adjusted ventilatory assist mode, which has not yet been adequately evaluated in small preterm infants with immature respiratory control.
While flow triggering is the best widely available method of synchronization, it is not without limitations. The interposition of the flow sensor adds approximately 0.8 mL of dead space to the ventilator circuit, a volume that becomes a larger proportion of the tidal volume as the size of the patient decreases. The second limitation is its susceptibility to auto-triggering in the presence of a leak around the ETT. A substantial leakage flow during the expiratory phase will be erroneously interpreted by the ventilator as an inspiratory effort, potentially resulting in an excessively rapid inflation rate, hypocarbia, or air trapping. Auto-triggering is more of a problem with ventilation modes that support every patient breath and should be suspected when the ventilator rate is >70/min with no evidence of patient inspiratory effort, especially when there is water in the expiratory limb of the ventilator circuit. The simplest way to verify that tachypnea is caused by auto-triggering is to briefly switch the ventilator to continuous positive airway pressure (CPAP) mode. If auto-triggering were occurring, the patient’s respiratory rate would immediately be less than the previous rate and usually fall briefly to zero because of the induced respiratory alkalosis. When auto-triggering is recognized, it can be mitigated by making the trigger less sensitive. Unfortunately, the size of the leak can change quite rapidly, requiring constant vigilance and frequent adjustment. Furthermore, when the trigger sensitivity is decreased, increased patient effort is needed to trigger inflation and the trigger delay increases, both of which are highly undesirable ( Fig. 18-3 ). Most devices now allow a fixed amount of leak compensation to mitigate this problem, but a fixed compensation level does not account for the variability of the leak. Specialty neonatal ventilators employ effective leak compensation technology that derives the instantaneous leak flow throughout the ventilator cycle and mathematically subtracts this flow from the raw measurement ( Fig. 18-4 ). This approach eliminates ETT leak-related auto-triggering, but the device may still be affected by water in the expiratory limb of the circuit. The use of heated circuits and modern ventilator circuits with a semipermeable expiratory limb that eliminates water condensation (Evaqua™, Fisher & Paykel, Auckland, New Zealand) has virtually eliminated auto-triggering, allowing trigger sensitivity to remain at the most sensitive value and preserving the rapid response time and minimal work to trigger inflation.
Basic Synchronized Modes
Patient–Ventilator Interactions with Synchronized Ventilation
The key concept in minimizing the need for invasive respiratory support is to avoid heavy sedation and muscle paralysis and to maximally utilize the patient’s spontaneous respiratory effort. While allowing the patient to breathe spontaneously during mechanical ventilation has clear advantages as listed above, it leads to considerable challenges for the clinician, who needs to appreciate the complex interaction between the awake, spontaneously breathing infant and the various modes of synchronized ventilation. A key concept in understanding these interactions is an appreciation of the additive nature of the patient inspiratory effort and the positive pressure generated by the ventilator. As illustrated in Figure 18-5 , the tidal volume entering the infant’s lungs is driven by the transpulmonary pressure, the sum of the negative inspiratory effort of the infant and the positive inflation pressure from the ventilator. Because in a preterm infant, the spontaneous effort is sometimes sporadic and always highly variable, the resulting transpulmonary pressure and tidal volume are typically quite variable. The following paragraphs will attempt to clarify the way an infant’s spontaneous respiratory pattern interacts with the common ventilator modes.
Synchronized Intermittent Mandatory Ventilation
Synchronized intermittent mandatory ventilation (SIMV) provides a preset number of inflations, as in standard IMV, but these are synchronized with the infant’s spontaneous respiratory effort, if present. SIMV may be pressure or volume controlled, but in neonatal applications it is almost always pressure controlled and time cycled. To prevent mandatory inflations during expiration, there is a brief refractory period so that triggering can occur only within a trigger window. If no spontaneous effort is detected during a trigger window, a mandatory inflation will be given. Spontaneous breaths in excess of the set ventilator rate are not supported. This is not a problem with the relatively rapid set rate typically used in the acute phase of the disease but results in uneven tidal volumes (V T s) and high work of breathing during weaning in very small infants owing to the high airway resistance of the narrow ETT. As discussed in Chapter 2 , resistance to flow is inversely proportional to the fourth power of the radius, making it hard for tiny infants to breathe effectively through the small ETT. The high ETT resistance coupled with the limited muscle strength and mechanical disadvantage of the infant’s excessively compliant chest wall results in ineffective spontaneous breathing with a high dead space:V T ratio. Because anatomic and instrumental dead space is fixed, small breaths that largely rebreathe dead-space gas will contribute minimally to effective alveolar ventilation (alveolar ventilation = minute ventilation − dead space ventilation). To maintain adequate alveolar minute ventilation, a relatively large V T , typically around 6 mL/kg, is thus required with the limited number of ventilator inflations ( Fig. 18-6 ). From a practical standpoint, the biggest problem with SIMV is that the operator must adjust both rate and pressure (V T ) to facilitate weaning and overall respiratory support.
Assist Control
Assist control (AC) is a conventional mechanical ventilation mode that supports every spontaneous breath (this is the “assist” part) that is sufficient to trigger a ventilator inflation and provides a minimum rate of ventilator inflations in case of apnea (the “control” part). In Europe the mode is sometimes referred to as synchronized intermittent positive-pressure ventilation. AC is a time-cycled mode that can be pressure or volume controlled, but in neonatal applications it is typically pressure controlled. Because every spontaneous breath is supported, AC provides more uniform V T delivery and lower work of breathing than SIMV ( Fig. 18-7 ). An inspiratory time is set, which may produce an inspiration that is either too long or too short, a problem that is avoided with the use of flow cycling (see below). The clinician sets a ventilator rate for mandatory “backup” inflations that provide a minimum ventilator rate in case of apnea, and the inflations can be triggered only within the trigger window. The backup rate should be set just below the infant’s spontaneous rate, usually 30 to 40 breaths/minute depending on the baby’s size, to allow the infant to trigger the inflations. The goal is to have the infant and the ventilator work together, resulting in lower ventilator pressure. An excessively high backup rate will result in an increased number of untriggered inflations when the ventilator backup rate kicks in before the infant has a chance to breathe ( Fig. 18-8 ). A backup rate that is too low will result in excessive fluctuations in minute ventilation and oxygen saturations during periods of apnea. Because the infant controls the inflation rate, gradual withdrawal of support is accomplished by lowering the peak inflation pressure (V T ) rather than the ventilator rate. In fact, the ventilator rate should never need adjustment once the baby is generating spontaneous respiratory effort. In this fashion, the amount of support provided to each breath is decreased, allowing the infant to gradually take over the work of breathing. This slightly less intuitive weaning strategy and some long-held misconceptions addressed later in this chapter appear to be the reasons for the apparent reluctance of some practitioners to adopt this mode.
