Measuring the lung activity of young children and infants is no simple matter, given the lack of co-operation of the subject. In adults, the most widely used instrument for measuring airflow is the pneumotachograph.

This is a device which is based on the principle that air flowing through an orifice produces a pressure difference across the orifice that is a function of the velocity of the air.

The differential pressure is sensed by a pressure transducer supported by output conditioning electronics that converts the analogue differential pressure signal into a dc voltage for monitoring purposes.

The size and sensitivity of the pneumotachograph when measuring a baby’s breathing becomes crucial. While a typical adult’s tidal volume of breathing air may measure about 500ml, a baby’s tidal volume averages about 8ml/kg of bodyweight. The corresponding monitoring signals are smaller and what would be considered acceptable noise on an adult monitoring system produces an unacceptable noise/signal ratio for an infant’s lung function test data. Also, a baby’s breathing rhythm is naturally faster and more variable than that of an adult, adding complexity to the signal.

One solution is to link a PC with a pressure transducer (such as the MP45 pressure transducer from Hawco) that is capable of measuring such differential pressures. The transducer can be used with pneumotachographs operating in scale ranges from 0.02l/s (for premature infants) up to 14l/s (for athletes).

Software running on the PC can be used to continuously calculate total volume of air. The curves generated for each breath can be reviewed by the physician and saved for review using analysis software which compares the subject’s flow and volume curves with statistically compiled tables.

Figure 1: Typical pneumotech Instrumentation

Figure 2: The MP45 differential pressure transducer

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