Slope of the Pressure-Time Waveform Predicts Resistance and Compliance in Mechanically Ventilated Subjects
Condition: Respiration Disorders
MeSH terms: Respiration Disorders
Study type: Observational

Summary: 
      There are two fundamentally different ways to ventilate critically ill patients: constant
      flow, volume-preset modes (such as volume assist-control) and pressure-preset modes (such as
      pressure-control and pressure-support). Critically ill patients suffer mechanical
      derangements of the respiratory system that raise the work of breathing. Knowledge of these
      mechanical properties is useful diagnostically and as a measure of response to treatment over
      time. It has been proposed that only constant flow, volume-preset modes are able to offer
      diagnostic information about the changes in the subject's lungs in terms of resistance and
      elastance properties. This study proposes to examine if similar information can be extracted
      from pressure-preset modes by comparing information from both modes of ventilation.
    

Detailed description: 
      Aim 1: To compare the respiratory system resistance and elastance obtained during
      constant-flow, volume-preset ventilation (using conventional means) and during
      pressure-preset ventilation (by analyzing the slope of the flow versus time waveform, as
      described below).

      Aim 2: To determine whether patient effort and level of alertness impair the accuracy of
      resistance and elastance measurements during pressure-preset ventilation.

      Hypothesis 1: Our primary hypothesis is that the flow versus time waveform contains
      information sufficient to calculate the respiratory system resistance and elastance. To test
      the primary hypothesis, we propose to measure resistance and elastance of subjects ventilated
      in the ICU during assist-control ventilation (a standard constant flow, volume-preset mode).
      Then we will record the flow versus time waveform during pressure-preset ventilation. By
      extrapolating the flow versus time waveform (which is generally linear) to the time axis, one
      can calculate elastance since at zero flow, the alveolar pressure equals the ventilator
      inspiratory pressure. Then Ers = (Pinsp - Total PEEP)/Extrapolated VT, where Pinsp is the set
      inspiratory pressure and extrapolated VT is the tidal volume if inspiratory time had been
      sufficient to allow equilibration between patient and ventilator (using trigonometry).
      Similarly, by extrapolating the flow versus time waveform to the flow axis (to find the
      maximal flow), one can calculate the resistance, assuming that flow depends on the pressure
      difference between ventilator and patient and the square of the resistance. We will compare
      the values derived during pressure-preset ventilation with those determined during
      assist-control (taken as the true values).

      Hypothesis 2: We hypothesize that inspiratory effort will be sufficient in some subjects to
      distort the flow versus time waveform from that which would be seen if the patient were
      passive, leading to erroneous values for resistance and elastance. We will estimate the
      respiratory drive using a standard measure, the fall in Pao during a brief inspiratory
      occlusion 100ms following the onset of inspiration (P0.1). Further, we will measure each
      subject's alertness on the Richmond Agitation-Sedation Scale (RASS). We expect our
      estimations of resistance and elastance to less accurate (during pressure-preset ventilation
      compared with assist-control) in subjects with greater respiratory drive and higher levels of
      alertness.
    

Observational model: Cohort
Time perspective: Prospective
Sampling method: Probability Sample
Study population: 
        Inclusion Criteria

        Subjects may be included in the study if they meet the following inclusion criterion and no
        exclusion criteria:

        [1] Adult medical or surgical ICU patients who are mechanically ventilated. Using a random
        number generator each subject will be assigned to pressure control or volume control for
        initial set of measurements.
      
Keywords: mechanical ventilation