4. MEASUREMENT OF CARDIAC OUTPUT 1. THE FICK METHOD: VO 2 = ([O 2 ] a - [O 2 ] v ) x Flow Flow = VO 2 [O 2 ] a - [O 2 ] v Spirometry (250 ml/min) Arterial Blood (20 ml%) Pulmonary Artery Blood (15 ml%) CARDIAC OUTPUT PERIPHERAL BLOOD FLOW VENOUS RETURN PULMONARY BLOOD FLOW

5. CARDIAC OUTPUT (Q) = VO 2 [O 2 ] a - [O 2 ] v 250 ml/min 20 ml% - 15 ml% = = 5 L/min . Q = HR x SV . SV = Q HR . = 5 L/min 70 beats/min = 0.0714 L or 71.4 ml CARDIAC INDEX = Q m 2 body surface area . 5 L/min 1.6 m 2 = = 3.1 L/min/m 2

6. Measurement of cardiac output 2. Indicator dilution method 3. Doppler Echocardiography

7. Factors Affecting Cardiac Output

17. Factors influencing Venous return

21. Unbalanced Ventricular Output

22. Unbalanced Ventricular Output

23. Factors Involved in Regulation of Cardiac Output

24. RIGHT ATRIAL PRESSURE AND VENOUS RETURN Venous return RA pressure Venous return from the periphery depends on the pressure difference between the peripheral pressure and right atrial pressure. Thus as right atrial pressure (central venous pressure) rises venous return will fall Venules Right atrium 25 mm Hg ∆P = 25 mm Hg 0 mm Hg 25 mm Hg ∆P = 20 mm Hg 5 mm Hg

25. GUYTON’S ANALYSIS OF THE INTACT CIRCULATION Two relationships have been established: A rise of right atrial pressure will more effectively fill the heart in diastole, increase stroke work and thus increase flow in the circulation (assume afterload and heart rate are constant) A rise of right atrial pressure will hinder venous return and thus decrease flow in the circulation. But in the intact circulation venous return must equal cardiac output Venous return RA pressure Cardiac output RA pressure

26. GUYTON’S ANALYSIS OF THE INTACT CIRCULATION Thus right atrial pressure exerts conflicting effects on flow in the intact circulation. But: cardiac out put = venous return. Thus: the two curves can be superimposed to yield an equilibrium point for the circulation. Cardiac output = venous return RA pressure cardiac function curve equilibrium point vascular function curve