SV determined by ventriculography, nuclear scans (MUGA) and echocardiography
However, prone to errors, costly, and difficult to obtain after an "intervention"
Useful alternative techniques for measuring CO:
Fick Principle
: the total uptake or release of any subst. by an organ is the product of the blood flow to the organ and the arteriovenous conc. difference of the subst.
Pulmonic blood flow = O2 consumption / AV O2 difference across the lungs
O2 consump determined by measuring the metabolic rate (assumed 250ml/min)
O2 difference measured by determining oxygen content of the arterial blood and of a mixed venous sample (central venous site like the pulmonary artery)
Cardiac Function Curve
CF curve relates the CO to the central venous press or mean atrial pressure (venous press – preload): see fig 2
increase in contractility (inotrophy), will shift curve upward
or the same CVP or preload will result in a greater CO
decrease in contractility (neg. inotrophy and/or heart failure) will shift curve downward
or the same CVP or preload will result in a lower CO
remember, increase in preload
Þ increase in CO, but no change in contractility
Drug actions and Mechanisms
Digitalis
Inhibits Na/K ATPase
Þ Ý Na+ inside Þ ß of Na/Ca antiport Þ Ý Ca++ inside Þ Ý myocardial contractile force
Drug is also vasolytic
b
-agonists
b
agonists activate adenylate cyclase Þ Ý cAMP which activates PKA
PKA phosphorylates:
L-type Ca++ channels:
Ý amt of Ca++ entering the cell Þ Ý contractile force
SR protein phospholambam:
Ý rate of Ca++ reuptake by the SR which Ý rate of relaxation (Ý in lusitrophy)
TNI subunit of troponin:
ß affinity of TNC for Ca++ which Ý rate of relaxation (Ý in lusitrophy)
also increases the heart rate (chronotrophy)
Phosphodiesterase inhibition
PDE catalyzes the breakdown of cAMP, thus inhibiting PDE will increase cAMP which activates PKA leading to the positive inotrophic response above
Ca++ channel antagonists (Ca++ blockers)
Ca++ channel antagonists are negative inotrophic agents
decreased Ca++ entry
Þ decreased contractility
also reduces conduction in the SA and AV nodes (neg chronotrophic agent) and accelerates the rate of ventricular relaxation
Heart Failure
causes downward shift of the cardiac function curve