The effect of heart rate on systolic and diastolic time intervals and cardiac function in stable and sick preterm infants

Diastolic function includes relaxation and filling of the heart and is dependent on heart rate (HR) and time spent in either systole (St) or diastole (Dt). The St/Dt ratio increases as HR increased and is a marker of overall cardiac function in children with heart failure. The aim of this study is to describe cardiac cycle events in preterm infants. Cardiac ultrasounds of preterm infants <32 weeks were retrospectively reviewed for cardiac cycle events from Tissue Doppler images and grouped by clinical indication of the scan as stable or as clinical deterioration with significant illness. 412 scans in 201 preterm infants were analyzed. St/Dt ratio increased with increasing HR. St increased from 50 to 54 % of the cardiac cycle length with shortened diastolic duration. Isovolumetric contraction and relaxation remained unchanged at 9 and 12 % of the cardiac cycle respectively. Mean St, Dt and St/Dt ratio in stable infants were 195(19) msec, Dt 175(24) msec and 1.13 (0.16) respectively. Sick infants without cardiovascular medications showed a significantly shorter St and lower St/Dt ratio. Cardiovascular medications increased isovolumetric times, Dt, and early diastolic duration. Reference values for cardiac cycle durations and St/Dt ratio are presented. Preterm hearts adapt to higher HR by shifting towards systole and shortened early diastole by optimising its force frequency relationship and enhanced relaxation. The St/Dt ratio was altered in preterm infants with significant illness. This simple ultrasound marker could be tested in further studies that investigate cardiovascular medications in preterm infants • Filling of the heart is dependent on heart rate, and time spent in either systole (St) or diastole (Dt). • Preterm infants increase their St/Dt ratio with higher heart rate, similar to infants and children. • The cardiac cycle shifts towards systole by optimising its force frequency relationship and enhancing myocardial relaxation. • St/Dt ratio was altered with significant illness and with vasoactive medications, and could assist studying preterm infants with cardiovascular compromise.