Cardiac Output&Venous ReturnDr Mazlyn MustaphaMB BCh BAO (Trinity College Dublin), MRCP (Ireland)Physiology DepartmentUniversiti Sains Malaysia12th November 2008

Definition The volume of blood pumped by either ventricle in one minute

The output of the two ventricles are equal over a period of time (not necessarily during each minute)

Cardiac output Cardiac output = stroke volume x heart rate

Cardiac output of a healthy adult is approximately 5 L/min

It does not remain constant, varies depending on needs of the body

Cardiac Index Cardiac output - depends on body buildOutput per unit body surface area - similar in all individuals irrespective of body sizeThus cardiac index is used

Cardiac index =

Cardiac index is best used to compare cardiac output in different individuals. This is done in research & not clinical practice. Cardiac outputBody surface area (m2)

Significance Cardiac output total amount of blood available for adequate tissue perfusionMust match needs of the body

IF NOT:

Lower than needs CVS has to adjust distribution of available outputGreater than required unnecessary workload on heart & increases the pressure within arteries (causing damage in the long term)

SignificanceDetermines pressure in the arteriesLow output low pressure & reduced force available to push blood through the bodyHigh output high pressure damages blood vessels

Measurement of Cardiac Output Classical methods Fick principleDye dilution method

Measurement of Cardiac OutputEchocardiography Ultrasound waves - used to measure blood flow in aorta Cardiac output is calculated

Cardiac output is not usually measured in clinical practice (more relevant in research)

Determinants of Cardiac Output CO = SV x HR= (EDV ESV) x HR

Factors that determine cardiac output are those that influence:End Diastolic VolumeEnd Systolic VolumeHeart Rate

Determinants of Cardiac Output

HeartInotropyChronotropy12341243PreloadAfterloadContractilityHeart rate

Determinants of Cardiac OutputPreload - defined as initial stretching of the cardiac myocytes before contraction & is related to sarcomere lengthIn normal heart, determined by volume of blood that fills the ventricle at the end of passive filling & atrial contraction - the end-diastolic volume

Determinants of Cardiac OutputThe effect of preload Through the Frank-Starling mechanism Heterometric autoregulation (Autoregulation of strength of ventricular contraction that occurs in direct relation to end diastolic fiber length, as in law of the heart.)

Determinants of Cardiac OutputPreload A major determinant - amount of blood returning to heart through the veinsVenous returnVenous return to the heart is the sum of all local blood flows from individual segments of peripheral circulationUsually applied to return to the right heartA concept, not a measurement

Determinants of Cardiac OutputFactors which determine preload by influencing venous return: venous blood pressure venous compliance & tone blood volume gravity muscle pump respiratory pump Venous return will determine filling pressure - the pressure in atria at onset of ventricular fillingClinically central venous pressure is measured as an indicator of adequacy of venous return

Determinants of Cardiac OutputMuscle pumpRespiratory pump

Determinants of Cardiac OutputFactors other than venous return that affect vent preload 1. Ventricular compliance distensibility of ventricle, determines end-diastolic volume of intraventricular filling pressure2. Filling time - Increase in heart rate will cause proportionate, reciprocal decrease in stroke volume due to reduced filling time (with increased heart rates, diastole is reduced more than systole)3. Atrial contraction - at rest, heart rate has little influence on preload because most ventricular filling occurs in rapid & reduced filling phases. At high heart rates, atrial systole plays significant role in ventricular filling & determining preload

Determinants of Cardiac OutputAfterload-Load" that heart must pump blood against. Higher the afterload, higher the ventricular wall tension. Normally afterload is closely related to aortic pressure. The pressure that the ventricle generates during systolic ejection is very close to aortic pressure

Determinants of Cardiac OutputAfterload (on the left ventricle) is affected byAortic pressureSystemic vascular resistance Aortic valve stenosis Ventricular dilation

Determinants of Cardiac OutputEffect of afterloadWhen afterload increasesEnd-systolic volume increases Stroke volume decreasesFrank starling curve shifts down and to the right

Increased afterloadDecreased afterload

Determinants of Cardiac OutputInotropyChanges in stroke volume can be caused by changes in ventricular inotropy (contractility)B decreased inotropyC increased inotropy

Determinants of Cardiac OutputInotropyChanges in inotropy produce significant changes in ejection fraction (EF) [EF = SV/EDV %]Increasing inotropy leads to an increase in EF, while decreasing inotropy decreases EFTherefore, EF is often used as a clinical index for evaluating the inotropic state of the heartEF may be determined by echocardiography

Determinants of Cardiac OutputContractility of the myocardium (Inotropy)Autonomic nervous system activityCirculating catecholaminesOxygen & CO2 levelsH+ ion concentration Plasma electrolyte levelsLoss of myocardium Drugs

Determinants of Cardiac OutputHeart rate is influenced byAutonomic nervous system activityCirculating catecholaminesOxygen & CO2 levelsH+ ion concentration Plasma electrolyte levelsDrugs

Determinants of Cardiac OutputHeart rate & autonomic supply At rest, vagal effect predominatesAt high heart rates sympathetic input predominates

Determinants of Cardiac OutputEffect of increased heart rate on cardiac output Direct effect increase in CO due to increased number of times heart pumpsIndirect effect shortening of ventricular diastole causing reduced ventricular filling tends to reduce EDV & therefore COThe net effect on CO depends on which of the above predominates

Determinants of Cardiac OutputCardiac output increases with increasing heart rate until a limiting rate - around 200 for adultsThis is due to an increase in inotropy of the heart Beyond this the CO falls

Determinants of Cardiac OutputPreloadVenous return filling pressure (affected by many factors)ventricular distensibilityfilling timeatrial contractionAfterloadaortic pressuresystemic arteriolar resistanceInotropyautonomic nervous systemcirculating catecholaminesfunctioning myocardiumoxygen supplyHeart rateautonomic nervous system

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