Tidal Volume Overdose in Pressure Control Modes: An Observational Study

Abstract

Background:

Studies confirm that excessive tidal volume (V_T) increases the risk of ventilator induced lung injury. A previous model study by Chatburn et al (Comput Biol Med 2016;75:235-242) showed that V_T measurement error may occur during pressure control ventilation (PCV) with preset inspiratory time (T_I). Some ventilators calculate V_T as integral of flow over T_I or expiratory time (T_E). For a passive patient during PCV, this calculation is accurate. However, an inspiratory effort increase then relaxation results in expiratory flow during T_I. Integration of expiratory flow results in volume deducted from the calculated V_T (see figure). Hence the displayed volume is lower than the peak value of the volume waveform, the true V_T. The displayed V_T will be inaccurately low, possibly leading to inadvertent tidal volume over-dosage. The purpose of this study was to determine the prevalence of two main outcome variables: V_T error and V_T over-dosage (ie, V_T >8 mL/kg) during routine mechanical ventilation with PCV modes in our ICUs.

Methods:

All patients in medical and neuro ICUs with active inspiratory efforts, in PCV modes, using Medtronics PB840 or GE Carescape R860 ventilators were included. Volume (V) waveforms were observed for at least 10 breaths per patient. Waveforms with downward V deflection (see figure) during T_I were deemed to have V_T error present. If error was present, error size was calculated by subtracting the digital display of V_T from the peak value of the V waveform.

Results:

We recorded data for 50 patients. A V_T error was present in 32% of patients. Average error was -1.2 mL/kg (range -0.6 to -2.6). However, 75% of the time the error was too small to calculate. V_T over-dosage occurred in one patient whose displayed V_T was 7.9 mL/kg but true V_T was 10.4 mL/kg. This patient also had the longest T_I (1.2 s).

Conclusions:

In this small pilot study, we observed that V_T error is present in a substantial portion of patients during PCV, with the largest error being associated with the longest T_I as expected from the previous study. Results of this study warrant the need to monitor V waveforms for patients receiving PCV with an active exhalation valve feature to ensure patients are receiving V_T in the range of 6-8 mL/kg of ideal body weight