Evolution of Left ventricular Function and Its Tolerance of Tachycardia in Early Infancy: a Simultaneous Invasive and Noninvasive Assessment in Neonatal and Young Infant Piglets

Background: The literature examining ventricular function of the neonate and young infant is fragmented. Furthermore, the clinical implications of the observed differences have been largely extrapolated from adult studies. Clinical echocardiography-based investigations at rest, for instance, have suggested the neonatal left ventricle (LV) rapidly improves its ability to relax within weeks of delivery. There is a paucity of data, however, on the functional reserve of the neonatal and young infant heart when facing hemodynamic stress. As reduced ventricular relaxation in the diseased adult heart is associated with poor tolerance to atrial tachycardia due in part to reduced filling time, it has been thought that the neonatal heart may be less tolerant to tachycardia than that of the infant. We sought to explore the impact of early LV maturation on its ability to tolerate atrial tachycardia. We hypothesized that the neonatal LV would be less tolerant of atrial tachycardia than that of the young infant equivalent. Methods: Under general anesthesia (propofol, isoflurane), neonatal (1-3days, NP) and infant (14-17days, YP) Landrace cross piglets (n=7 each) were instrumented intravascularly with Millar® high-fidelity and pacing catheters in the left ventricle and right atrium, respectively. Invasive hemodynamic and basic echocardiography parameters were acquired at baseline, and at 200, 230 and 260bpm. Speckle tracking technique was then used for different left ventricular deformation studies (i.e. strain, strain rate and twist). Results: At all rates, NP maintained their LV output and blood pressure, whereas YPs did not. Negative dP/dt was lower in NPs at baseline (-1599±83 vs. -2470±226mmHg/s, p=0.007) but increased with pacing (p=0.002). In contrast, YP baseline negative dp/dt was greater and tended to decrease with pacing (p=0.056). All other invasive measures of diastolic function (left ventricular end-diastolic pressure, tau and isovolumic relaxation time) did not differ between groups. Except for fractional shortening which was reduced in YP at 260bpm; all invasive and noninvasive measures of contractility did not differ between groups. Twist studies demonstrated a similar increase in peak LV twist and untwisting rate in response to tachycardia (combined data: baseline -259±22; at 260bpm -498±59 deg/s, p=0.003); however, overall twist patterns differed between groups: NP tended to increase apical positive rotation (p=0.1) while YPs increased basal negative rotation (p=0.009) to augment LV twist. Finally, as it permitted testing of the effects of tachycardia on different noninvasive LV function markers, it was shown that echo-measured strain rate is not independent of heart rate in the immature heart. This is in contrast to what has been demonstrated for the more mature heart. Conclusion: The neonatal piglet LV appears more tolerant of atrial tachycardia than that of the young piglet heart which could relate to better diastolic performance and differences in twist mechanics. Use of strain rate in the immature heart should take into consideration changes in heart rate.