Chapter 33

INTRODUCTION

Mechanical ventilation monitors are designed to continuously measure the characteristics of the inspiration and expiration cycle (ie, respiration). These monitors typically use sensors and electronic circuits to measure and display: (1) volume of air moved (eg, tidal and minute volume); (2) inspiratory and expiratory pressures (eg, mean airway pressure and positive and expiratory pressure); (3) the respiratory rate; and (4) to detect cessation of breathing (apnea). Monitors of mechanical ventilation also test system integrity, such as the presence of system leaks, patient disconnections, and operational verification of chosen setting and alarms.

Ventilatory support begins with pressure, which drives flow, which after integration with time yields volume. These primary variables, along with their transduced signals, generate additional variables, resistance and compliance. By convention, specific variables are tracked and displayed as functions of time (eg, pressure, flow, volume, minute ventilation, end-tidal carbon dioxide). Specific combinations of variables, each of which are time dependent (eg, pressure and volume, and flow and volume), are processed and displayed as loops and displayed breath by breath. The key steps in this process of data management are the transduction of a variable into its electrical equivalent and, then, digitization of that electrical signal. Once a variable (eg, pressure at a specific moment) is transduced and digitized it becomes similar to a picture that can be copied, filed, shared, compared, and manipulated in myriad other ways.

MEASURING GAS FLOW, VOLUME, AND PRESSURE

There are several methods to set or measure gas flow. Flow is actually not easy to measure. Flowing gases in tubes generate velocity and pressure, which can be used to measure the flow indirectly. These spirometers or respirometers are prone to errors caused by inertia, friction, and water condensation. Typically the spirometers are placed proximal or distal to the inspiratory and expiratory valves or at the Y-connector that attaches to the patient’s airway.

Velocity and Pressure Flowmeters

Flow may be described as laminar or turbulent. The velocity at which flow turns from laminar to turbulent flow is the critical velocity and is dependent on the radius (r) of the tube, as well as, the viscosity (η), density (ρ), and Reynolds number (κ), a constant specific to the gas. Critical velocity = κη/ρr. Volume can be directly measured.

A. Fixed-Orifice Flowmeters

They channel gas through a narrowed conduit. This narrowing increases the resistance to flow dropping the pressure of the gas as it exits. Using Poiseuille’s law, the flow of the gas can then be calculated. Flow = (πr4ΔP)/(8ηL), where r and L is the radius and length of the resistor, respectively; η is the viscosity of the gas; and ΔP is the difference in pressure across the resistor. The pressure drop across this resistance is sensed ...

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