Cardiac output factors responsible and effecting

Cardiac output

1. CARDIAC OUTPUT By Dr. AUBRY Department of Anaesthesiology Government Medical College Kadapa

2. Cardiac output Definition It is the volume of blood pumped systematically by the ventricle of the heart each minute Cardiac output depends on 2 variables Heart rate and Stroke volume CO = HR x SV

3. CO = HR x SV 72 x 70 = 5 Litres approx Cardiac index = CO BSA

4. Heart rate Autonomic nervous system controls automaticity and rate of spontaneous depolarization of SA Node Sympathetic system increases heart rate Parasympathetic system decreases heart rate

5. Stroke volume It is the difference between the ventricular End Diastolic Volume (EDV) and the End Systolic Volume (ESV) SV = EDV – ESV = 120 – 50ml =70 ml of blood EDV is the volume of blood in the ventricle at the beginning of contraction and ESV is the volume of blood in the ventricle at the end of contraction

6. What are the factors affecting EDV: Increase: - Stronger atrial contraction. -Increased total blood volume -Increased venous tone. -Increased sk m pump. -Increased negative intrathoracic pressure. Decrease: Standing Increased intrapericardial pressure. Decreased ventricular compliance

7. Stroke volume Pre load After load Contractility Wall motion abnormalities

8. 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.

9. Starling Law, cont….. Mechanism of Starling Law: An increase in preload lead to an increase in the sarcomere length and increases troponin C calcium sensitivity, which increases the rate of cross-bridge attachment and detachment, and the amount of tension developed by the muscle fiber. This increases SV.

10. Afterload Afterload can be defined as the "load" that the heart must eject blood against. The afterload is closely related to the aortic pressure .

11. Afterload When arterial pressure is reduced, the ventricle can eject blood more rapidly, which increases the stroke volume and thereby decreases the end-systolic volume.

12. Effect of an increase in afterload on SV An increase in afterload , lead to an increase in end-systolic volume and a decrease in stroke volume . An increase in afterload shifts the Frank-Starling curve d own and to the right (from A to B). Explanation:, an increase in afterload decreases the velocity of fiber shortening. This reduces the rate of volume ejection so that more blood is left within the ventricle at the end of systole (increase end-systolic volume) . A decrease in afterload shifts the Frank-Starling curve up and to the left (A to C).

13. Factors affecting afterload Vascular tone (and therefore blood pressure) Aortic stiffness Myocardial tension (affected by hypoxia, volume overload) Preload Valvular regurgitation

14. Afterload is increased Aortic stenosis. Arterial hypertension

15. Relevance to General anesthesia Almost all anaesthetic drugs have actions on cardiovascular system They act as vasodialators and decrease both pre load and afterload They are negatively inotropic , reduce contractility This is the reason for a fall in Blood pressure during induction and maintenance of anesthesia Management?