Realistic human muscle pressure for driving a mechanical lung

Abstract

Background: An important issue in noninvasive mechanical ventilation consists in understanding the origins of patient-ventilator asynchrony for reducing their incidence by adjusting ventilator settings to the intrinsic ventilatory dynamics of each patient. One of the possible ways for doing this is to evaluate the performances of the domiciliary mechanical ventilators using a test bench. Such a procedure requires to model the evolution of the pressure imposed by respiratory muscles, but for which there is no standard recommendations. Methods:In this paper we propose a mathematical model for simulating the muscular pressure developed by the inspiratory muscles and corresponding to different patient ventilatory dynamics to drive the ASL 5000 mechanical lung. Our model is based on the charge and discharge of a capacitor through a resistor, simulating contraction and relaxation phases of the inspiratory muscles.Results: Our resulting equations were used to produce 420 time series of the muscle pressure with various contraction velocities, amplitudes and shapes, in order to represent the inter-patient variability clinically observed. All these dynamics depend on two parameters, the ventilatory frequency and the mouth occlusion pressure. Conclusion:Based on the equation of the respiratory movement and its electrical analogy, the respiratory muscle pressure was simulated with more consistency in regards of physiological evidences than those provided by the ASL 5000 software. The great variability in the so-produced inspiratory efforts can cover most of realistic patho-physiological conditions.

Performances of domiciliary ventilators compared by using a parametric procedure

“…Based on the (rare) physiological data available, we thus developed a more realistic evolution of the muscular pressure ( Fig. 1) [13].…”

“…The respiratory muscle pressure was simulated by using two exponential functions, one for inspiration and one for expiration (see [13] for details). The whole breathing cycle Fig.…”

“…1 Realistic muscular pressure P mus . Muscular pressure model used for driving the ASL 5000 as developed by Fresnel et al [13] thus simulated only depends on two clinical parameters that are i) the breathing frequency f b and ii) the mouth occlusion pressure P 0.1 at 0.1s representing the stiffness of the inspiratory effort.…”

“…From these two parameters and the formula governing the exponential evolution of the muscular pressure, the maximum amplitude P max and the duration τ ins of the contraction (corresponding to the inspiratory phase) and the duration τ exp of the relaxation (corresponding to the expiratory phase) are automatically computed by using [13] …”

“…We therefore focused our attention in elaborating a protocol providing reliable and reproducible results. First, we based our tests by simulating patient breathing cycles with the help of a realistic muscular pressure as developed in [13] for driving our mechanical lung (an ASL 5000, Ingmar Medical, Pittsburgh, USA). In order to avoid the situation where two different ventilators are tested in two different operating conditions, each ventilator was parametrized in a systematic way and was tested with a large set of lung dynamics; from our point of view, this is a very key point since testing a ventilator with a single lung dynamics can lead to two opposite biased situations; i) the lung dynamics was an example chosen because the ventilator manages it in an optimal way and ii) the lung dynamics is unfortunately one of those the ventilator is not able to synchronize with.…”