To a significant degree, the clinician’s choice of ventilator modes is limited by the equipment available in his or her NICU. Although most modern ventilators are capable of providing the basic modes of synchronised (patient-triggered) ventilation, there are a variety of hybrid modes or combinations that may be unique to each manufacturer and device. For example, some ventilators make possible the combination of synchronised intermittent mandatory ventilation (SIMV) and pressure support ventilation (PSV), and other ventilators employ PSV as a stand-alone mode with a backup rate, similar to assist/control (AC) but flow rather than time cycled. PSV eliminates the inspiratory hold, making for better synchronisation and automatically adjusting the effective inspiratory time (T _I) as time constants change. However, this mode may not be appropriate in extremely small premature infants in the first few days of life, in whom respiratory time constants are so short that the actual T _I may be less than 0.25 s. This duration of inspiration may be inadequate for gas mixing in the terminal air spaces and additionally may lead to excessively rapid respiratory rates.

Ventilators designed primarily for adult/paediatric patients but capable of also supporting neonates tend to have a greater variety of modes (some untested in newborn infants) and to offer volume-controlled ventilation. Unlike on the specialty neonatal ventilators, PSV on these devices does not have a backup rate and thus requires a reliable respiratory effort, not often present in preterm infants. However, when ventilating newborn infants, the clinician must be aware of the pitfalls of applying various modes to this unique population. The major limitation of volume-controlled ventilation is that the device only controls the volume delivered into the proximal (ventilator) end of the circuit, not the volume entering the patient’s lungs. Because of compression of gas and stretching of the circuit, as well as leak around uncuffed endotracheal tubes (ETT), there is only a very indirect relationship between set and delivered V _T. For this reason, time-cycled, pressure-limited ventilation has been the standard ventilation mode in the NICU. There are several volume-targeted modes of pressure-limited ventilation with different operating principles and capabilities that more accurately control delivered V _T in newborn infants. Some of these modalities have been studied in considerable detail and shown to reduce the incidence of hypocapnia, excessively large V _T, duration of mechanical ventilation and to decrease levels of pro-inflammatory cytokines (Keszler 2009). Volume-targeted ventilation is discussed in detail elsewhere in this book. The more conventional modes are discussed below, and suggestions regarding the most appropriate modes for neonatal ventilation are incorporated in Table 42.1.

Despite many years of daily use, no consensus has emerged regarding the relative merits of the commonly used modes of synchronised ventilation. Guidance from large prospective trials with important clinical outcomes, such as incidence of air leak, chronic lung disease or length of ventilation, is lacking. Short-term clinical trials have demonstrated less tachypnoea and less variable V _T (Mrozek et al. 2000), more rapid weaning from mechanical ventilation (Chan and Greenough 1994; Dimitriou et al. 1995) and smaller V _T with smaller fluctuations in blood pressure (Hummler et al. 1996) with AC, compared to SIMV. There are important physiological considerations suggesting that SIMV may not provide optimal support in very premature infants. SIMV supports a user-defined number of breaths synchronised with the infant’s spontaneous respiratory effort. Spontaneous breaths in excess of the set rate are not supported, resulting in uneven tidal volume and high work of breathing during weaning. This is particularly important in extremely small and immature infants intubated with a narrow bore ETT (Fiastro et al. 1988). The high airway resistance of the ETT, limited muscle strength and mechanical disadvantage of the infant’s excessively compliant chest wall result in small, ineffective V _T. Small breaths that largely rebreathe dead space gas will contribute little to effective alveolar ventilation. To maintain adequate alveolar minute ventilation, relatively large V _T is thus required with the limited number of mechanical inflations provided by the ventilator. In a group of ELBW infants in the recovery phase of respiratory distress syndrome (RDS) ventilated with SIMV, the V _T of mechanical inflations was about 6 mL/kg with spontaneous V _T of 2.5 mL/kg, very close to anatomical dead space (Osorio et al. 2005). However, most clinicians continue to use SIMV, particularly during the weaning phase (Sharma and Greenough 2007). This is based on the intuitive assumption that fewer mechanical inflations are less damaging and on the traditional thinking that the ventilator rate must be lowered before extubation. However, because lung injury is primarily caused by excessive V _T, irrespective of the pressure required to generate that V _T (Dreyfuss and Saumon 1993; Hernandez et al. 1989; Dreyfuss and Saumon 1998), the smaller V _T used with AC (Hummler et al. 1996) likely outweighs the faster rate. This concept is supported by the Oxford Region Controlled Trial of Artificial Ventilation study where a ventilator rate of 60 inflations per min compared to 20–40 was shown to result in less air leak with unsynchronised IMV (OCTAVE Study Group 1991). Many clinicians also mistakenly believe that assisting every breath prevents respiratory muscle training. This concern reflects a poor understanding of the patient–ventilator interaction during synchronised ventilation. With synchronised ventilation, the V _T is generated by the combined inspiratory effort of the patient (the baby pulling air in) and the positive pressure generated by the ventilator pushing air in. The resulting transpulmonary pressure, together with the compliance of the respiratory system, determines the V _T. Thus, as ventilator inflation pressure is decreased during weaning, the infant gradually assumes a greater proportion of the work of breathing and achieves respiratory muscle training. Ultimately, the ventilator pressure is decreased to the point when it only overcomes the added resistance of the ETT and circuit, at which point the infant should be extubated.

There is little difference between the modes during the acute phase of the disease when SIMV rate is relatively high. The differences become more meaningful during weaning, especially at rates <20. The less intuitive weaning process with AC appears to be one of the reasons clinicians continue to employ SIMV during weaning from mechanical ventilation. Another reason is the anecdotal observation that sometimes infants become tachypnoeic with AC and may have mild hypocapnia. This tachypnoea may reflect the infant’s response to low mean airway pressure (P _AW) during weaning – shortening the expiratory time (T _E) serves to maintain lung volume at end expiration. Tachypnoea appears to be infrequent when decreasing peak inspiratory pressure (PIP) is balanced by higher PEEP to avoid loss of lung volume. Low rate SIMV, especially in small infants with 2.5-mm-diameter ETT, is likely to impair the weaning process. Some ventilators offer the option of supporting spontaneous breaths with pressure support ventilation. This will compensate for the problems associated with simple SIMV but becomes a more complex process requiring the clinician to select settings for both SIMV and the level of pressure support. Nonetheless, more rapid weaning when SIMV was combined with PSV compared to SIMV alone has been demonstrated (Reyes et al. 2006). Based on these findings, it is recommended that when SIMV is used during the weaning process, PSV should be utilised to support spontaneous breathing. The alternate and less complicated approach is to use AC or PSV as a stand-alone mode to assist all spontaneous efforts and simply wean the inflations pressure, taking care to maintain adequate P _AW. This is most easily accomplished when using volume-targeted modes of ventilation discussed earlier in this book. Finally, the attributes of the particular ventilator may influence the choice of triggered modes. Many flow-triggered devices are susceptible to auto-triggering due to leak around the ETT. This is more problematic with AC than with SIMV, where the mechanical breath rate is limited. Devices that have a very effective leak-adapted trigger and breath termination algorithms, such as the Draeger Babylog, are virtually immune from this problem and thus facilitate the use of AC.

This section focuses on the initial management of the ventilated infant following intubation. Overall rationale for, and evidence supporting, choice of ventilator settings in newborns is set out in Sect. 42.4.

After intubation, a period of ventilation using a portable ventilation device will usually be required until the endotracheal tube is secured, nasogastric tube inserted, and the baby positioned in readiness for ventilation. In preterm infants, bolus surfactant may also be given at this time. If the ventilation device does not allow delivery of PEEP, the infant should be connected to the ventilator as quickly as possible.

The initial ventilator settings should take into account the context in which ventilation is being applied to the neonate (Table 42.1) and the apparent response to the pressure settings being used during ventilation with a portable device. There should be a stepwise approach to setting the ventilator at the time of initiation of mechanical ventilation (Table 42.2), guided in part by a thorough clinical appraisal (Table 42.3). PEEP should be considered first, with suggested PEEP levels for a range of different situations set out in Table 42.1. The T _I should be set at around 0.5 s for a term infant and 0.35 s for a preterm infant, and then further adjusted depending on the analysis of the flow–time curve, which can be displayed on most modern neonatal ventilators. For most applications of time-cycled pressure-limited ventilation, T _I should be set long enough to allow complete or near-complete inflow of gas into the lung during inspiration. If the T _I is too short, there may not be sufficient time for an adequate tidal volume to be delivered nor for alveolar recruitment to occur with each tidal inflation. Setting PIP should be guided by a visual appreciation of chest rise and preferably the measured V _T. Suggested target values for V _T are set out in Table 42.1. Finally, T _E (and thus, respiratory rate) is adjusted, with the aim of producing sufficient minute ventilation to clear CO₂ without causing dynamic hyperinflation related to inadvertent PEEP. An estimate of minute ventilation can be made using the product of measured V _T and respiratory rate, with a value of 200–300 mL/kg/min usually being an adequate starting point before the first pCO₂ reading is obtained. Note that this estimate of minute ventilation does not incorporate dead space volume, which is substantial in some infants, particularly those with hyperinflated lungs (“alveolar dead space”). Additionally, rapid shallow breathing leads to high dead space to V _T ratio and reduced alveolar ventilation. Avoidance of inadvertent PEEP can be difficult when an infant has a rapid spontaneous breathing rate and each breath is being supported by the ventilator. Examination of the flow–time curve during expiration will reveal whether gas outflow is complete before the next inflation begins and thus whether dynamic hyperinflation is likely to occur. If so, further adjustment to the mode of ventilation, respiratory rate and even inspiratory time may be necessary, as discussed below.