DYNAMIC PROPERTIES OF BODY PLETHYSMOGRAPHS AND EFFECTS ON PHYSIOLOGICAL-PARAMETERS

A model incorporating compliance, resistance, inertia, and the thermal time constant of plethysmographs is used to describe the effect of it s dynamic properties on measured respiratory parameters. Using numeric al simulation we studied the effect of distortion of flow signals from 13 infants in whom flow and esophageal pressure had been recorded. Th e distortion in amplitude, shape, and timing of the recorded flow patt erns was dependent on the dynamic properties of the plethysmograph. Fo r constant-volume ''pressure'' plethysmographs, errors of derived para meters such as compliance and resistance are very important if the the rmal time constant is short. These errors are not corrected by calibra ting the plethysmograph at the breathing frequency. Time correction of the flow signals in volume-displacement plethysmographs gives accurat e results when the plethysmograph resistance and compliance are low. O verall, a volume-displacement plethysmograph with moderately high resi stance of the flowmeter, corrected for internal pressure and inertia, gives the best possible results. A model incorporating compliance, res istance, inertia, and the thermal time constant of plethysmographs is used to describe the effect of its dynamic properties on measured resp iratory parameters. Using numerical simulation we studied the effect o f distortion of flow signals from 13 infants in whom flow and esophage al pressure had been recorded. The distortion in amplitude, shape, and timing of the recorded flow patterns was dependent on the dynamic pro perties of the plethysmograph. For constant-volume ''pressure'' plethy smographs, errors of derived parameters such as compliance and resista nce are very important if the thermal time constant is short. These er rors are not corrected by calibrating the plethysmograph at the breath ing frequency. Time correction of the flow signals in volume-displacem ent plethysmographs gives accurate results when the plethysmograph res istance and compliance are low. Overall, a volume-displacement plethys mograph with moderately high resistance of the flowmeter, corrected fo r internal pressure and inertia, gives the best possible results.