Clinical evaluation of a computer-controlled pressure support mode

Abstract

We have designed a computerized system providing closed-loop control of the level of pressure support ventilation (PSV). The system sets itself at the lowest level of PSV that maintains respiratory rate (RR), tidal volume (VT), and end-tidal CO(2) pressure (PET(CO(2))) within predetermined ranges defining acceptable ventilation (i.e., 12 < RR < 28 cycles/min, VT > 300 ml [> 250 if weight < 55 kg], and PET(CO(2)) < 55 mm Hg [< 65 mm Hg if chronic CO(2) retention]). Ten patients received computer-controlled (automatic) PSV and physician-controlled (standard) PSV, in random order, during 24 h for each mode. An estimation of occlusion pressure (P(0.1)) was recorded continuously. The average time spent with acceptable ventilation as previously defined was 66 +/- 24% of the total ventilation time with standard PSV versus 93 +/- 8% with automatic PSV (p < 0.05), whereas the level of PSV was similar during the two periods (17 +/- 4 cm H(2)O versus 19 +/- 6 cm H(2)O). The time spent with an estimated P(0.1) above 4 cm H(2)O was 34 +/- 35% of the standard PSV time versus only 11 +/- 17% of the automatic PSV time (p < 0.01). Automatic PSV increased the time spent within desired ventilation parameter ranges and apparently reduced periods of excessive workload.

Figure 1

Contributions to inadequate ventilation of Intermediate RR (28< RR · 35 breaths/min,) High RR: (RR >35 cycles/min), Low RR: (RR <12 breaths/min), Low Vt: (Vt<300 ml or 250 ml if weight >55 Kg), and High P_etCO₂ (P_etCO₂ < 55 mmHg or 65 if COPD) during 24h of standard PSV in the 10 patients studied. For ventilation in standard PSV, inadequate ventilation represented 36% of the total duration of ventilation in this mode whose 24% were spent with critical ventilation. Definition of abbreviations: SPS: standard pressure support ventilation (physician-controlled), APS: automatic pressure support ventilation (computer-controlled).

Figure 2

Contributions to inadequate ventilation of Intermediate RR (28< RR ·35 breaths/min,) High RR: (RR >35 cycles/min), Low RR: (RR <12 breaths/min), Low Vt: (Vt<300 ml or 250 ml if weight >55 Kg), and High P_etCO₂ (P_etCO₂ < 55 mmHg or 65 if COPD) during 24h of automatic PSV in the 10 patients studied. For ventilation in automatic PSV, inadequate ventilation represented 9% of the total duration of ventilation in this mode whose 5% were spent with critical ventilation. Definition of abbreviations: SPS: standard pressure support ventilation (physician-controlled). APS: automatic pressure support ventilation (computer-controlled).

Figure 3

These two figures show for patient #7 the evolution of both the PSV and the RR levels (left panel), and the evolution of RR/Vt (right panel) over the two periods of 24 hours of ventilation either with (APS) or without (SPS) the automated system. Note that the very high values of the rapid shallow breathing index (f/Vt) during SPS were no more present during APS.