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CARDIAC OUTPUT Venous Return Heart Failure By Dr. Eman EL Eter Associate Prof. Physiology Dep.

CARDIAC OUTPUT Venous Return Heart Failure By Dr. Eman EL Eter Associate Prof. Physiology Dep. Lecture 7: Objectives. By the end of this lecture the students are expected to: Define cardiac output, stroke volume, end-diastolic and end-systolic volumes.

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CARDIAC OUTPUT Venous Return Heart Failure By Dr. Eman EL Eter Associate Prof. Physiology Dep.

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  1. CARDIAC OUTPUTVenous ReturnHeart Failure ByDr. Eman EL EterAssociate Prof.Physiology Dep.

  2. Lecture 7: Objectives • By the end of this lecture the students are expected to: • Define cardiac output, stroke volume, end-diastolic and end-systolic volumes. • Define physiological conditions affecting CO • List causes of high and low output pathological states. • Define venous return and describe factors controlling venous return

  3. Lecture outline • Cardiac output is the amount of blood pumped by each ventricle per minute (5L/min). It varies physiologically with age, body mass index, physical activity, sleep, meals, pregnancy, etc.. But there are pathological conditions that lead to a significant increase in CO including hyperthyroidism, anemia and conditions decreasing CO as myocardial infarction. CO is well controlled and regulated by Many Factors: venous return, ABP , blood volume and nervous regulation. This lecture will focus on Venous return as an important factor determining CO. Venous return represents the amount of blood returning to the heart per minute. Venous return is controlled by many factors:1) Frank-Starling’s mechanism, 2) mean systemic filling pressure, 3) tissue metabolism, 4) thoracic pump, 5) Gravity, 6) Muscle pump, 7) blood volume.

  4. Definitions • Cardiac output (CO): Amount of blood pumped by each ventricle per minute. It equals 5L/min. • Stroke volume (SV): Volume of blood is ejected by each ventricle/beat.(70 ml/beat). S V times the number of beats per minute(heart rate,HR), equals the cardiac output (CO). • CO = SV · HR • Stroke volume is expressed in ml/beat and heart rate in beats/minute. Therefore, cardiac output is in ml/minute. Cardiac output may also be expressed in liters/minute.

  5. Definitions……….. End-diastolic volume (EDV): Amount of blood remaining in the heart by the end of diastole.(140 mL) End-systolic volume (ESV): Amount of blood remaining in the heart by the end of systole. =70 mL SV = EDV-ESV Cardiac index: =Cardiac output/BSA: This allows direct comparison of the cardiac index of large and small patients.

  6. Ejection fraction

  7. Ejection fraction The percentage of blood that is pumped out of a filled ventricle as a result of a heartbeat. The heart does not eject all the blood in the ventricle. Only about 2/3 of the blood is normally pumped out with each beat, and that fraction is referred to as the ejection fraction.  It decreases when the cardiac muscle becomes diseased.

  8. Stroke volume • SV = EDV - ESV. • This measurement can be affected by changes in the heart's ability to contract, the force of contraction, the volume of blood available to be pumped or other variables such as resistance within the circulatory system that can affect or alter these factors. Severe hemorrhage or shock, heart damage or extreme infections can change the heart's ability to pump effectively. • It is affected by cardiac contractility, preload, and afterload.

  9. Heart Rate • An increase in heart rate due to stimulation or excitation by activity, drugs, medications or other sources, will result in an increase in cardiac output. A decrease in the heart rate due to an electrical abnormality and some medications can result in a decreased cardiac output.

  10. Heart Rate • When heart rates become excessively fast, the heart may not have enough time to adequately fill with blood between beats; this can result in a decreased cardiac output. • When heart rates are too slow the cardiac output can also decrease significantly, and interventions may be required to increase the heart rate. Implanted pacemakers may be surgically placed to ensure a consistent adequate heart rate.

  11. Physiological variations Exercise (up to 700%) Eating ( 30%) High environmental temperature ( ) Pregnancy ( ) Anxiety ( 50-100%) Sympathomimitics, epinephrine ( ) Sitting or standing from lying position ( 20-30%)

  12. Pathological variations Increasing CO: • Fever • Hyperthyroidism • Anemia Decreasing CO: • Hypothermia. • Hypothyroidism • Myocardial diseases e.g. infarction, failure… • Rapid arrhythmias

  13. Factors affecting CO ABP VR

  14. Factors affecting Cardiac output: Preload (Venous return): Itrepresents the amount of blood returning to the heart per minute. Venous return is controlled by many factors: 1) Frank-Starling’s mechanism, 2) mean systemic filling pressure, 3) tissue metabolism, 4) thoracic pump, 5) Gravity, 6) Muscle pump, 7) blood volume. 8) Valsalva maneuvour

  15. 1.Venous Return (Preload) • Venous return (VR) is the flow of blood back to the heart. Under steady-state conditions, venous return must equal cardiac output (CO) when averaged over time because the cardiovascular system is essentially a closed loop. Otherwise, blood would accumulate in either the systemic or pulmonary circulations. Although cardiac output and venous return are interdependent, each can be independently regulated.

  16. Skeletal muscle pump

  17. Thoracic pump

  18. Valsalva

  19. Factors affecting venous return, cont… Mean systemic filling pressure: • Sympathetic fibers supply veins (sympathetic venoconstrictor tone). • This is important to maintain venous pressure and hence creates gradient to push blood to the right atrium. • Acute venous dilation can lead to significant drop in VR and so Cardiac output.

  20. Definition of Frank-Starling Law(Intrinsic regulation of CO) • Ability of the heart to change its force of contraction and therefore stroke volume in response to changes in venous return. • Also defined as the ability of the heart to pump all blood coming to it without allowing systemic stasis, within limits.

  21. Factors affecting preload

  22. Measurement of CO Echocardiography • Record real-time changes in ventricular dimensions, thus computing stroke volume, which when multiplied by heart rate, gives cardiac output.

  23. Measurement of CO, cont….. Fick Principle : • An old technique. • Equation used: • CO = VO2/(O2art – O2ven) Compute cardiac output (CO) indirectly from: • body oxygen consumption (VO2), • the mixed venous (O2ven), and • arterial oxygen contents (O2arterial blood) • To calculate CO, the oxygen contents of arterial and venous blood samples are measured, and at the same time, whole body oxygen consumption is measured by analyzing expired air. • The blood contents of oxygen are expressed as ml O2/ml blood, and the VO2 is expressed in units of ml O2/min.

  24. Example: If O2art and O2ven contents are 0.2 ml and 0.15 ml O2/ml blood, respectively, and VO2 is 250 ml O2/minute, then , how much will be the CO? Then , CO= 250 mlO2/min /(0.2-0.15) mlO2/ml blood= 5000 ml/min, or 5 L/min. Ventricular stroke volume = CO/HR.

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