When thoracic impedance (TI) is monitored continuously, as with an apnea monitor, a small negative deflection in the signal occurs with each heart beat. If this cardiac deflection is enhanced by computer averaging of TI segments synchronized with the ECG R wave, a characteristic waveform called the impedance cardiogram is produced. Since this deflection (ΔZ) may be theoretically explained by the redistribution of blood into and out of the lungs during the cardiac cycle, the technique was evaluated as a possible way to assess the magnitude of ductus flow in preterm infants with symptomatic PDA. 69% of 147 studies of 21 infants with PDA showed a ΔZ greater than 2 standard deviations above the mean of infants without PDA. Serial studies of infants who developed PDA revealed an increasing ΔZ associated with the onset of clinical shunting. During the course of ductus shunting ΔZ correlated significantly with a shunt score based on physical findings of PDA. ΔZ increased 22-31% following blood transfusion, an event likely to produce a momentary increase in ductus shunt. These data demonstrate empirically that IC may be a sensitive indicator of changes in ductus shunt magnitude as well as having objective diagnostic value. As no manipulation of the infant is required except placement of chest electrodes, the technique has the additional potential for continuous monitoring of shunt magnitude in critically ill infants.
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Cotton, R., Lindstrom, D., Catterton, W. et al. NONINVASIVE ASSESSMENT OF PATENT DUCTUS ARTERIOSUS (PDA) SHUNT MARNITUDE WITH IMPEDANCE CARDIOGRAPHY (IC). Pediatr Res 11, 387 (1977). https://doi.org/10.1203/00006450-197704000-00109