PURPOSE: The purpose of this study is to calculate the mechanical properties of the respiratory system noninvasively during pressure-preset ventilation (pressure-control; PCV). During a PCV breath, flow declines gradually as alveolar pressure rises. The pressure-time waveform is quite linear and contains quantitative information regarding resistance and compliance of the respiratory system. We use a novel method to calculate resistance and compliance during PCV and compare with those obtained during volume-preset ventilation (constant flow volume assist control; ACV).
METHODS: Twelve mechanically ventilated patients were enrolled. Initial application of ACV or PCV was determined by randomization. During ACV, a 0.4 second end-inspiratory pause was set and the following measured: peak pressure (Ppeak); plateau pressure (Pplat), end-expiratory pressure (PEEP or autoPEEP using the end-expiratory port occlusion technique), tidal volume (VT) and inspiratory flow rate. During PCV, inspiratory pressure, PEEP (or autoPEEP), and tidal volume were recorded and the flow versus time waveform was printed for analysis. The linear component of the pressure-time waveform was extrapolated to time and flow axes (visual best fit). Using the equation of motion for the respiratory system, assuming a non-linear resistance (resistance proportional to the square of flow), we calculated resistance and compliance. During ACV, compliance was calculated as VT/(Pplat-PEEP) and resistance as (Ppeak –Pplat)/Flow.
RESULTS: The mean compliance during PCV was 37.1 mL/cm H2O (S.D. 15.7) and during ACV was 36.9 mL/cm H2O (S.D. 12.2). The mean resistance during PCV was 15.3 cmH2O/(l/s) (S.D. 5.5) and during ACV was 12.9 cmH2O/(l/s) (S.D 3.6).Comparing ACV and PCV, a strong correlation is evident between compliance (r2 = 0.72, p-value 0.0005) and resistance (r2 = 0.51, p-value 0.0088).
CONCLUSION: During PCV the inspiratory flow waveform is linear and its slope contains information regarding resistance and compliance. These values correlate strongly to those obtained during ACV.
CLINICAL IMPLICATIONS: This demonstrates a noninvasive method of assessment of respiratory mechanics during PCV which could be used to assess patients’ respiratory function over time and in response to interventions.
DISCLOSURE: Nicole Collett, No Financial Disclosure Information; No Product/Research Disclosure Information