Slope of the Pressure-Time Waveform Predicts Resistance and Compliance in Mechanically Ventilated Subjects
| Status: | Recruiting |
|---|---|
| Conditions: | Pulmonary |
| Therapuetic Areas: | Pulmonary / Respiratory Diseases |
| Healthy: | No |
| Age Range: | 18 - Any |
| Updated: | 4/2/2016 |
| Start Date: | November 2007 |
| Contact: | Greg A Schmidt, MD |
| Email: | gregory-a-schmidt@uiowa.edu |
| Phone: | 319-384-6746 |
Slope of the Pressure-Time Waveform Predicts Respiratory System Resistance and Elastance in Mechanically Ventilated Subjects
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.
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.
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.
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.
Inclusion Criteria:
- Subjects may be included in the study if they meet the following inclusion criterion
and no exclusion criteria:
- Adult medical or surgical ICU patients who are mechanically ventilated.
Exclusion Criteria:
- Subjects with planned extubation at time of screening
- Subjects who are judged too hemodynamically unstable by their attending physician to
participate in the study
- Subjects on high-frequency ventilation
- Subjects whose oxyhemoglobin saturation cannot be maintained at 88% or greater
- Subjects with large air leaks around the endotracheal or tracheostomy tube
- Subjects younger than 18 years old
- Inability to obtain informed consent from the subject or the subject's authorized
representative
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