Developmental changes in myocardial contractile reserve in the lamb.

Area of Science:

• Cardiovascular Physiology
• Neonatal Development
• Cardiac Contractility

Background:

• Assessing serial changes in myocardial contractility and reserve in newborn lambs is crucial for understanding cardiovascular adaptation.
• The endsystolic pressure-volume relationship provides a robust method for evaluating cardiac function in vivo.

Purpose of the Study:

• To evaluate serial changes in myocardial contractility and reserve in normal lambs during the first month of life.
• To investigate the role of beta-adrenergic pathways in modulating cardiac function during early development.

Main Methods:

• Utilized an in vivo adaptation of the endsystolic pressure-volume relationship in lambs.
• Involved left thoracotomy, insertion of pressure and echo transducers, and placement of flow transducers.
• Measured contractility via left ventricular wall stress-volume index curves, with afterload manipulation using phenylephrine.

Main Results:

• Demonstrated a progressive decrease in resting myocardial contractility over the first four weeks of life.
• Observed no significant change in maximal contractility during isoproterenol infusion, but reserve increased with age.
• beta-Adrenergic blockade with propranolol initially decreased contractility, with this effect diminishing over time.

Conclusions:

• Newborn lambs exhibit a limited myocardial contractility reserve that improves progressively with age.
• The initial limited reserve is attributed to a high resting beta-adrenergic state in the neonatal period.