Respiratory Monitoring System for Underwater Use

Abstract

Monitoring a diver's respiratory parameters is a challenge. A respiratory monitoring system (RMS) should allow measurements of gas flows, gas composition (O2 and CO2, breath by breath) and pressure swings at the mouth. For physiologic research, breathing resistance and gas composition should be independently adjustable. The pressure of the breathing gas relative to the diver's lung centroid (static lung loading, hydrostatic imbalance) should be controlled. Ideally, an RMS has no depth limitations.In the RMS described here, the diver breathes through a full face mask and large bore hoses to an inspiratory and an expiratory side. Each side has a flow meter and temperature and pressure (relative to the mask) sensors. The inspiratory side is supplied with a constant, surface‐adjustable gas flow to a bag from which the diver breathes. This flow is adjusted to keep the bag full at the beginning of inhalation. Excess gas bubbles into the water. Expiration is buffered by another bag and bubbles into the water.Gas from the mask is sampled to a mass spectrometer for continuous analysis. The flow meter, mass spectrometer, pressure and temperature signals are displayed in real time and recorded.The prone diver is positioned on a cycle ergometer that keeps the chest in a known position relative to the RMS. The depth of the exhaust ports of the RMS relative to the diver's lung centroid controls hydrostatic imbalance. Resistance elements can be added to either breathing hose. The size and shape of the breathing bags set elastic loads.Breathing gas with selected concentrations of O2 and CO2 permits assessment of gas composition effects on respiratory performance when a diver is challenged by experimenter‐imposed respiratory impediments. The RMS has been used in a pool at a depth of about 3 m in close to 100 experiments without incident.