Inotropic responses to digoxin during hypoxia and autonomic blockade

Area of Science:

• Cardiology
• Pharmacology

Background:

• Digoxin is a cardiac glycoside used to treat heart failure.
• Hypoxemia can impair myocardial contractility.
• The role of the autonomic nervous system in digoxin's effects under hypoxemia is not fully understood.

Purpose of the Study:

• To investigate the inotropic effects of digoxin in dogs.
• To compare digoxin's effects during hypoxemia and autonomic blockade.
• To elucidate the protective role of digoxin against hypoxia-induced cardiac dysfunction.

Main Methods:

• Left ventricular contractility (VC) assessed using function curves relating (dP/dt)max and stroke volume to end-diastolic pressure.
• Experiments conducted in dogs with and without autonomic blockade (propranolol, τεαc, atropine).
• Hypoxia induced by reducing partial pressure of oxygen (Pao2) to 28 mmHg.
• Serum digoxin levels measured by radioimmunoassay.

Main Results:

• Digoxin significantly augmented VC in dogs.
• The increase in VC with digoxin was greater in autonomically blocked dogs.
• Blocked dogs under hypoxia showed substantial VC increases with digoxin.
• Unblocked dogs under hypoxia experienced a fall in VC despite digoxin, though less severe than controls.
• Serum digoxin levels were comparable across experimental groups.

Conclusions:

• Digoxin protects myocardial contractility from the detrimental effects of prolonged hypoxia.
• This protective effect is more pronounced in the absence of autonomic nervous system influence.
• Autonomic blockade may unmask or enhance digoxin's cardioprotective effects during hypoxia, potentially by preventing sympathetic withdrawal.