The neonatal but not the mature heart adapts to acute tachycardia by beneficial modification of the force-frequency relationship.

The force-frequency relationship (FFR) reflects alterations in intracellular calcium cycling during changing heart rate (HR). Tachycardia-induced heart failure is associated with depletion of intracellular calcium. We hypothesized (1) that the relative resistance to tachycardia-induced heart failure seen in neonatal pigs is related to differences in calcium cycling, resulting in different FFR responses and (2) that pretreatment with digoxin to increase intracellular calcium would modifies these changes. LV +dP/dt was measured during incremental right atrial pacing in 16 neonatal and 14 adult pigs. FFR was measured as the change in +dP/dt as HR was increased. Animals were randomized to control or intravenous bolus digoxin (n = 8 neonate pigs in the 0.05 mg/kg group and n = 7 adult pigs in the 0.025 mg/kg group) and paced for 90 min at 25 bpm greater than the rate of peak +dP/dt. Repeat FFR was then obtained. The postpacing FFR in neonatal control pigs shifted rightward, with peak force occurring 30 bpm greater than baseline (P < 0.03). There was no vertical shift; thus, force at 150 bpm decreased (P < 0.03) and force at 300 beats/min increased (P < 0.08). In adult control pigs, FFR shifted downward (P < 0.01), with decreased force generation at all HRs. In both neonates and adult pigs, digoxin increased +dP/dt at all HRs; however, in neonate pigs digoxin decreased the contractile reserve by abrogation of the rightward shift of FFR. An adaptive response to tachycardia in the neonate pig leads to improved force generation at greater HRs. Conversely, the response of the mature pig heart is maladaptive with decreased force generation. Pretreatment with digoxin modifies these responses.