Sunday, May 15, 2011

The Intrinsic Diaphragmatic Frequency

Image 1: Missed trigger attempts, notice the flow distortions (purple flow waveform) without associated breaths. Ventilator measured rate 12 bpm, however true intrinsic rate 30 bpm. 

Many patients receiving mechanical have high intrinsic diaphragmatic rates (≥30 breaths per minute) even when very well assisted [1]. This is usually unnoticed because the ventilator only measures machine or patient triggered breaths; however the patient’s true respiratory rate may be higher. This can be problematic when performing a spontaneous breathing trial (SBT) for a couple of reasons.

First, a number of institutions perform SBT’s while the patient is still hooked to the ventilator. The ventilator will not detect the ineffective efforts so when calculating the frequency/tidal volume (F/VT) based on the ventilators measured total frequency one may obtain a false positive. Example a patient’s measured Vt is 300 ml & true intrinsic rate is 35 bpm however, the machine only measures 30, so your calculated f/vt equals 100 when it is actually 116.

Second, when the operator decreases the level of assist the measured respiratory rate may immediately increase. This immediate increase in respiratory rate is not distress, what is happening is there is less dynamic hyper-inflation due to the reduction in pressure support. So the ineffective efforts are eliminated and the intrinsic rate is captured. These sudden increases in respiratory rate may lead the operator to think that the patient is in distress. This may result in placing the patient on a higher level of assist and prolonging the weaning process.

  1. Younes,M. (circa 2006). Lecture on PAV+. Covidien Healthcare. Boulder, Co.