Resistance Measurements
For patients receiving mechanical ventilation, airway resistance (Raw) is the measure of the total resistance of the ventilator circuit, airway adapters, endotracheal/trach tube, and the patient’s airways. Raw equals the change in pressure divided by flow: Resistance = Pressure/Flow. An electrical circuit analogy is Ohm’s Law: Resistance = Voltage/Current.
The Pressure in cm H2O is peak pressure minus plateau pressure. The Flow in liters per second (LPS) needs to be a square waveform and not a decelerating waveform. The units of Raw are cm H2O/L/s-1, with normal values equal or below 10 cm H2O/L/s-1. LPM/60 converts to LPS, so a square flow waveform at 60 LPM equals 1 LPS. Raw calculations at Flow at 1 LPS simplify to Raw = Peak pressure – Plateau pressure. Raw is measured by the respiratory care practitioner noting peak pressure and then engaging the inspiratory hold momentarily to measure plateau pressure. If the ventilator offers an automated measurement of these pressures, confirmation should be performed using the graphics screen.
There are applications for Raw that could be integrated into hospital guidelines. Is bronchodilator therapy indicated? Baseline Raw should be elevated. What is the efficacy/duration of bronchodilator therapy? Efficacy: Raw should be lowered after treatment; duration: the time to re-elevation of Raw. Is there loss of patency of the endotracheal tube? Serial increases of Raw may be noted (Shah and Kollef. Crit Care Med. 2004;32[1]:120; Wilson et al. Chest. 2009;136[4]:1006.)
Dr. Herbert Patrick, MSEE, FCCP
NetWork Steering Committee Member
