Respiratory Muscle Function

By adjusting the pressure support level it is possible to modulate the relative contribution of the patient and of the ventilator to the total WOB [33]. This is certainly a major advantage of PSV: indeed, aside from the mandatory presence of an intact neural drive, an effective muscular function is necessary to initiate the PSV breath and at least participate in inspiratory WOB. A relatively simple method to assess muscular strength is the maximal absolute pressure that is generated against an occluded airway [34].

Respiratory Mechanics Respiratory system compliance (Crs)

The widespread notion that complete relaxation of respiratory muscles is not reliably achievable in spontaneously breathing patients has limited the measurement of respiratory mechanics during partial ventilatory support.

A clinicallyreliable measurement ofrespiratorysystem compliance maybe most commonly obtained by performing the classic occlusion maneuvers [35, 36], according to the simple equation:

Respiratory Compliance Measurement

Fig. 1. Tracing of flow, volume, airway pressure (Paw), and esophageal pressure (Pes), of an end-expiratory (left side), and end-inspiratory (right side) airway occlusion during pressure support ventilation (PSV). Main parameters obtained during occlusions are displayed. Vt: tidal volume; Pel,rse: elastic recoil pressure of respiratory system at the end of expiration; Pel,rsi: elastic recoil pressure of respiratory system at the end of inspiration; PMI: pressure muscle index; Crs, respiratory system compliance. See text for details.

Fig. 1. Tracing of flow, volume, airway pressure (Paw), and esophageal pressure (Pes), of an end-expiratory (left side), and end-inspiratory (right side) airway occlusion during pressure support ventilation (PSV). Main parameters obtained during occlusions are displayed. Vt: tidal volume; Pel,rse: elastic recoil pressure of respiratory system at the end of expiration; Pel,rsi: elastic recoil pressure of respiratory system at the end of inspiration; PMI: pressure muscle index; Crs, respiratory system compliance. See text for details.

where Pel,rsi = elastic recoilpressure of respiratorysystem at the end of inspiration, and Pel,rse = elastic recoil pressure of respiratory system at the end of expiration, which amounts to the sum of PEEP plus auto-PEEP (Fig. 1).

In most modern ventilators, end-inspiratory and end-expiratory occlusion maneuvers are possible also during partial ventilatory support modes. We have shown that, duringPSV, the plateau pressure relaxation following an inspiratoryocclusion is a clinically acceptable estimate of the elastic recoil pressure of the respiratory system [36]. Assessment of the occlusion method in ALI patients who had been shifted from continuous positive pressure ventilation (CPPV) to PSV, showed a good correlation between Crs measured during PSV and during CPPV (CrsPSV = 1.4+0.98*CplCPPV, r=0.945, p<0.001) [37]. However, it is important to underline that in patients with a very high respiratory drive the occlusion method could be limited by insufficient time to achieve a reliable muscle relaxation, and, thus, the measurement should not be trusted.

Respiratory system resistance (Rrs)

Measurement of Rrs during PSV is particularly critical, since it requires the use of esophageal pressure [38]. A method to estimate Rrs, avoiding esophageal pressure measurement, is based on the use of the interrupter technique [39] that has been successfully applied in patients undergoing to PSV [35]. Alternatively, Iotti et al. have proposed an approach based on a least square fitting technique, which allows contemporaneous measurement of Rrs and Crs, avoiding any occlusion maneuver [40]. It is important to note however, that, while airway resistance evaluation is most important in chronically obstructed patients, its clinical relevance in the management of ALI /ARDS is certainly more limited.

Auto-PEEP

The measurement of auto-PEEP is of great importance, since aside from the well known detrimental effects on hemodynamics and dynamic hyperinflation, it may profoundly affect the feasibility and effectiveness of PSV. Auto-PEEP represents a threshold load the patient must overcome to trigger the ventilator, and constitutes an adjunctive ineffective effort. Auto-PEEP can be easily measured by means of the end-expiratory occlusion technique (Fig. 1) [41, 42]. It is important to underline that, at variance with the measurement of P0.1 and maximal inspiratory pressure (MIP), the measurement of auto-PEEP requires that the patient relaxes during the occlusion. Patients with very high respiratory drive may not allow enough time for muscle relaxation to reach a plateau.

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