Monitoring of PARDS

It is obvious that patients with PARDS need the minimal monitoring routinely used in children admitted to the paediatric intensive care unit (PICU), including continuous monitoring of cardiac and respiratory rates, pulse oximetry and blood pressure [45]. Ventilatory monitoring should at least include respiratory system mechanics such as exhaled Vt and inspiratory pressures delivered to the patient. However, the peak inspiratory pressure is largely influenced by the resistance of endotracheal tube and airways and is therefore a poor surrogate for pressures applied to the alveoli in the context of using a ventilatory mode that does not incorporate a zero-flow state. Adult data has shown that there is a poor correlation between the plateau pressure and the transpulmonary pressure (Ptp). Ptp (i.e. the difference between the alveolar and the pleural pressure) reflects lung strain, but this requires the use of an oesophageal catheter [4]. So far, no paediatric data has shown improved outcome when oesophageal manometry is routinely applied in ventilated PARDS patients. In contrast, oesophageal manometry-guided PEEP setting resulted in improved oxygenation and respiratory compliance over the first 72 hours in adult ARDS patients compared with PEEP setting according to the ARDSnet PEEP/FiO₂ protocol [135].

Ventilators should also display flow-time, pressure-time and volume-time curves as they provide simple and useful information [45]. The expiratory phase examination allows for the detection of dynamic hyperinflation (inspiration occurring while expiration flow is not null) and expiratory flow limitation, a condition that may occur especially during ventilation with high respiratory rate for compensation of low tidal volumes and relatively long inspiratory time. Also, high expiratory flow during early expiration is a sign of poor compliance, with a fast time constant. Asymmetry between inspiratory and expiratory volume may suggests air leak from the respiratory system. Noisy expiratory and inspiratory flow may suggest the presence of tracheal secretions [88]. Finally, patient ventilator asynchrony may be identified by observing ventilator waveforms, although this is not the most superior method of detecting PVA [18, 26]. The location of measurement of flow and tidal volume is important. Vt measured at the proximal airway with a pneumotachograph was found to be different from those measured at the ventilator, especially in infants and small children [5, 25, 69].

Quasi-static pressure-volume (PV) loop is the reference method to evaluate the compliance of the respiratory system; monitoring this loop has been proposed as tool to identify the lower inflection point (LIP) of the PV loop and to set the level of PEEP. However, the technique is complex and the patient needs to be deeply sedated or paralyzed, the interpretation of the loop is difficult and there are no clinical benefits of this monitoring reported in adult ARDS. Also, data in PARDS is lacking. Alternatively, the dynamic pressure-volume loop may be used for patient monitoring, but these loops are very much influenced by the resistance of the respiratory system including endotracheal tube [67]. Dynamic pressure-volume loops should therefore be interpreted with caution. Furthermore, evaluating the dynamic compliance (i.e. the ratio of exhaled tidal volume divided by the driving pressure) mandates a zero-flow state, the absence of patient effort, precise measurement of the Vt and no or minimal leakage around the ETT. The suitability of other various parameters for PARDS has not been evaluated, including the ‘stress index’ (i.e. the shape of the airway pressure curve during inspiration), measuring of intrinsic PEEP (PEEPi) when expiratory flow is limited, oesophageal manometry to calculate transpulmonary pressure at the end of in- and expiration or calculate work of breathing, measuring respiratory drive by P0.1 and the electrical activity of the diaphragm (EADi) [62]. Although continuous monitoring of end-tidal CO₂ is recommended to assess the accuracy of ventilation support, it is unclear if incorporating volumetric capnog- raphy improves PARDS outcome. Furthermore, end-tidal CO₂ cannot be used as a reliable surrogate for arterial or capillary CO2 until a correction method is validated

[75]. Peripheral venous blood gas also does not accurately predict arterial gas.

Chest radiography is initially important to establish the diagnosis of PARDS; however, the optimal frequency of radiograph control in children with PARDS is not established. Chest CT scan may be necessary in PARDS to help with the aetiologi- cal diagnosis or to diagnose certain complications but not as routine monitoring tool. Lung ultrasonography is a simple, radiation-free, easily accessible and validated method to evaluate pleural effusion in adults and children [145]. It is increasingly used in adults with ARDS to detect lung complications and to evaluate lung recruitment [93, 145]. Data in children with PARDS are not available.