Correction of Error in Respiratory Resistance Measurements Made With the Flow-Interruption Technique During Mechanical Ventilation: Evaluation of the Puritan Bennett 7200 and 840 Ventilators

Abstract

BACKGROUND:

Calculation of total inspiratory resistance (R_tot) for patients on ventilatory support is typically based on measurement of airflow velocity and airway opening pressure during end-inspiratory occlusion by the inspiratory valve in the ventilator. Systematic error is introduced into R_tot measurements because the inspiratory valve closes over a period of time (not instantaneously, so gas continues to flow into the circuit while the valve is shutting) and because the circuit tubing is a distensible compartment between the occluding valve and the respiratory system. The R_tot-measurement error can be minimized with a rapidly-shutting occlusion valve positioned at the airway opening, or, alternatively, by mathematical correction that accounts for the valve-closure period and circuit tubing characteristics.

METHODS:

In a bench study we measured R_tot with the Puritan Bennett 7200 and 840 ventilators (using the inspiratory valves that are built into those ventilators) and compared those measurements to measurements made with a rapidly-shutting valve at the airway opening. We deemed the rapid-occlusion-valve measurements the best available (benchmark) values. We also studied the closure characteristics of the ventilators' inspiratory occlusion valves and created equations for mathematical correction of R_tot values measured with those valves.

RESULTS:

Compared to the benchmark measurements, the measurements from the Puritan Bennett 7200 averaged 23.2% relative error and 2.6 cm H₂O/L/s absolute error. Measurements from the Puritan Bennett 840 averaged 7.3% relative error and 1.0 cm H₂O/L/s absolute error. Mathematical correction for the circuit tubing and valve-closure time reduced the average relative and absolute error to 3.0% and 0.4 cm H₂O/L/s, respectively, for the Puritan Bennett 7200, and to 4.5% and 0.3 cm H₂O/L/s, respectively, for the Puritan Bennett 840.

CONCLUSIONS:

The Puritan Bennett 840 measures R_tot more accurately than the Puritan Bennett 7200. Our equations to mathematically correct R_tot measurements made with the PB7200 and PB840 are useful in settings where very accurate R_tot measurements are necessary.