1442 COMPENSATORY ALTERATIONS IN RESTING HEART RATE (HR): AN INDICATOR OF TRUE ANEMIA IN PRETERM INFANTS

Abstract

The clinical correlation of signs consistent with anemia as defined by a low Hb alone are poor (Wardrop, C., et al, Arch D. Child 53:855, 1978). In order to define whether resting HR can be used as an indicator of inadequate tissue oxygenation, those factors considered important in oxygen delivery were investigated among a group of 36 infants with birthweights <1500g. Serial determinations included Hb, P₅₀, arterial blood gas (PaO₂) and oxygen consumption (V_O2). Additionally, in 21 of these infants, serial cardiac outputs (Q) were obtained by echocardiography with calculation of the central venous oxygen tension (PvO₂). HR was recorded 1 hour after feeding, during sleep, at the time the V_O2 and ECHO studies were performed. The Hb values ranged from 6.6 g/dl to 18.8 g/dl while the HR varied from 79 to 170 beats/min. The cardiac stroke volume within each infant's data set varied little (avg. change=19% from low to high) in comparison to a wide variation in Q.indicating that the change in HR is the major determinant of Q at this age. No significant correlations existed between HR and Hb, PaO₂, or P₅₀. The HR appeared to be related to the V_O2 (r=+0.31, p<0.001, n=124) and inversely to the PvO₂ (r=-0.42, p<0.05, n=49). These data indicate that HR is dependent on the metabolic needs (V_O2) of the infant. Compensatory changes in HR do not simply reflect the level of Hb but rather the combined effect of Hb, P₅₀, PaO₂, and Q as reflected in alterations of the central venous oxygen tension.

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