Response of Ventilator-dependent Patients to Different Levels of Pressure Support and Proportional Assist

Abstract

The ventilator's response to the patient's effort is quite different in proportional assist ventilation (PAV) and pressure support ventilation (PSV). We wished to determine whether this results in different ventilatory and breathing pattern responses to alterations in level of support and, if so, whether there are any gas exchange consequences. Fourteen patients were studied. Average elastance (E) was 22.8 (range, 14 -36) cm H2O/L and average resistance (R) was 15. 7 (range, 9-21) cm H2O/L/s. The highest PSV support (PSVmax) was that associated with a tidal volume (VT) of 10 ml/kg (20.4 +/- 3.2 cm H2O), and the highest level of PAV assist (PAVmax) was 78 +/- 7% of E and 76 +/- 7% of R. Level of assist was decreased in steps to the lowest tolerable level (PSVmin, PAVmin). Minute ventilation, VT, ventilator rate (RRvent), and arterial gas tensions were measured at each level. We also determined the patient's respiratory rate (RRpat) by adding the number of ineffective efforts (DeltaRR) to RRvent. There was no difference between PSVmin and PAVmin in any of the variables. At PSVmax, VT was significantly higher (0.90 +/- 0.30 versus 0.51 +/- 0.16 L) and RRvent was significantly lower (13.2 +/- 3.9 versus 27.6 +/- 10.5 min-1) than at PAVmax. The difference in RRvent was largely related to a progressive increase in ineffective efforts on PSV as level increased (DeltaRR 12.1 +/- 10.1 vs 1.4 +/- 2.1 with PAVmax); there was no significant difference in RRpat. The differences in breathing pattern had no consequence on arterial blood gas tensions. We conclude that substantial differences in breathing pattern may occur between PSV and PAV and that these are largely artifactual and related to different patient-ventilator interactions.

Improved Synchrony and Respiratory Unloading by Neurally Adjusted Ventilatory Assist (NAVA) in Lung-Injured Rabbits

“…Leung et al showed that the pressure-time product of the diaphragm increased during the wasted efforts, but different from the present study, they still found a decrease in the pressure-time product when all breaths (triggered and not triggered) were considered. Work by other groups has since demonstrated that the asynchrony during PSV is due to dynamic hyperinflation (20). Our current results in rabbits of increased diaphragm energy expenditure at high levels of PSV are different from our previous study in ventilated adult patients, where increasing PSV reduced the EAdi and Pdi consistently.…”

“…As there were three to four steps tested with either mode, to allow for comparisons, the data were divided into three levels of assist in each mode (NAVAlo, NAVAmed, and NAVAhi and PSVlo, PSVmed, and PSVhi), as described by others (20). All variables were analyzed for NAVAlo, NAVAmed, NAVAhi and PSVlo, PSVmed, and PSVhi with nonparametric tests for repeated-measures analysis of variance (ANOVA) (Friedman repeated-measures ANOVA on ranks) (Sigmastat, Jandel Scientific, San Rafael, CA).…”

Proportional Assist Ventilation and Neurally Adjusted Ventilatory Assist

“…[41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] As with the evaluation of PAV in other settings, most of the comparisons have focused on the physiologic response when PSV is changed to PAV. In general, during invasive ventilation the change from PSV to PAV results in lower tidal volume, more rapid respiratory rate, lower peak airway pressure, and lower mean airway pressure, without significant changes in gas exchange or hemodynamics.…”

Monitoring of Patient-Ventilator Interaction at the Bedside

“…Use of an esophageal balloon, which permits determination of pleural pressure, and respiratory muscle electromyograms have been used to measure a variety of patient-ventilator interactions and to compute WOB. [5][6][7][8][9] However, these devices are not used during routine patient care, and clinicians must rely on physical examination of the patient as well as visual inspection of waveforms to assess for patient-ventilator syn-chrony and asynchrony. Visual inspection of waveforms has been shown to correlate well with esophageal-balloon readings, but is not without error.…”

“…Visual inspection of waveforms has been shown to correlate well with esophageal-balloon readings, but is not without error. 5,6 Artifacts such as cardiac oscillation may mimic asynchronies, and clinicians must learn to distinguish between these and true asynchronies. 10 There are times when clinicians standing at the bedside are unable to distinguish between asynchrony and artifact with certainty, in which case he/she has to use clinical judgment to determine optimal patient treatment.…”