Cardiac Output

CARDIAC OUTPUT Cardiac output is defined as the amount of blood pumped out by each ventricle per minute into the circulation. Stroke volume – The amount of blood pumped out by each ventricle per beat. Cardiac output = Heart rate x

Stroke volume

= 5-6 L/min It means cardiac output is controlled by heart rate and stroke volume. Factors affecting heart rate Cardiac innervations Cardiovascular centers----- vasomotor centre and vagal centre

Factors affecting stroke volume Stroke volume is affected by contractibility, after load and pre load

FRANK STARLING LAW Force of ventricular contraction is directly proportional muscle fibers.

to initial length of

Pre Load is directly proportional to End Diastolic Volume. (End Diastolic Volume is the amount of blood remaining in ventricles at the end of diastole). Greater is preload; more is length of muscle fibers. EDV is increased by greater venous return. Therefore more is pre load more is force of myocardial contraction.

FACTORS AFFECTING MYOCARDIAL CONTRACTIBILITY

z z z z

INCREASED BY Catecholamines. Increase in heart rate. Change in heart rhythm. Drugs--- theophylline, caffeine, digitalis. DECREASED BY • Parasympathetic stimulation. • Heart failure. • Hypoxia, acidosis, hypercapnia. • Drugs — barbiturates, quinidine.

FACTORS INCREASING END DIASTOLIC VOLUME • • • • •

Increased venous return. Increased total blood volume. Atrial contractions. Decrease in intra thoracic pressure. Increase pumping action of skeletal muscles.

FACTORS DECREASING END DIASTOLIC VOLUME • • • •

Posture change (standing). Decreased total blood volume. Decrease in compliance of ventricles Increase in intra thoracic pressure.

Methods for determining cardiac output 1. Fick principle Amount of substance taken up by an organ/ unit time = Blood flow x Difference between arterial and venous level Cardiac output ( F ) =

O2 consumption / min AO2 - VO2 =

250 ml/min 5ml%

= 5L/min 2. ECHOCARDIOGRAPHY 3. Indicator Dilution Method