Lecture 9: Ventricular Function Flashcards
Define cardiac output and write an equation for its calculation
Cardiac output = volume of blood ejected by one ventricle in one minute
CO = HR * SV
Define cardiac index (CI) and write an equation for its calculation
Cardiac index = relates resting CO to body size
CI = CO / BSA
Briefly explain importance of cardiac index (CI) and state approx. normal value
Important because it reflects metabolic rate - continually changes to meet changes in demand
Normal value = 3.4 +/- 0.8 L/min/m2
List four determinants of cardiac performance (AKA four determinants of ventricular function)
Preload
Afterload
Contractility
Heart rate
Identify 3 determinants that also affect isolated cardiac muscle performance
SV depends on interplay of:
Diastolic stretch
Contractility
Arterial pressure
State the most precise definition of ventricular PL
Diastolic filling pressure
List two parameters that can be used to estimate preload
End diastolic volume
End diastolic pressure
Describe 3 potential mechanisms that explain Starling’s law
Increased fraction of crossbridges formed between actin + myosin
Filament lattice spacing
Muscle stretch influences tension development and calcium sensitivity (stretching the muscle requires less calcium to get same amount of tension)
Which potential mechanism is the most important to explain Starling’s law
Increased fraction of crossbridges formed between actin and myosin
How does a ventricular function curve illustrate Starling’s law?
Depicts how with increased muscle fiber length, the stroke volume increases
List examples of conditions or interventions that can alter ventricular PL
Hemorrhage (or severe dehydration)
Posture (move from supine to upright)
Neural tone
Muscle compression
Respiration
Define afterload
Wall stress produced by properties of ventricle itself combined with additional properties of the vasculature during systole
Explain why AL changes throughout ventricular ejection
SV varies inversely with wall stress during ejection
Wall stress itself varies during the rapid and slow ejection phases of the cardiac cycle
Define contractility
Intrinsic force-generating capacity of the cardiac cell under fixed PL and AL
Describe the normal regulation of ventricular IS
Contractility and relaxation can be enhanced by catecholamines (act via cAMP/PKA)
Most important inotropic = sympathetic noradrenaline
Illustrate the effect of an increase in IS on ventricular function curve
Greater AL = the less it shortens during systole (reduced shortening also reduces contractile force)
Increase in sympathetic stimulation (inc. BP) = decreased SV
Illustrate the mechanism for the effect of cardiac glycosides on IS
Digitalis inhibits sodium-potassium ATPase = intracellular sodium accumulates
Calcium extrusion via Na-Ca exchanger is inhibited = accumulation of calcium in cytoplasm and SR
= increase in contractility because of extra calcium hanging around
List three estimates of IS
Echocardiography (estimate most commonly used clinically)
Using dP/dt (estimate obtained from Wiggers diagram)
Calculation of ejection fraction from EDV and ESV (estimate from PV loop)
Define the end systolic pressure volume relationship (ESPVR) and explain significance
Can be used to estimate contractility
At IVC, the ventricle is filled = higher volume means higher pressure
Higher volume decreases contractility because then it is harder for ventricle to contract as adequately
Compare relative systolic and diastolic times at a HR of 60 bum and 180 bpm
Diastolic times decrease by 1/3
What influences SV?
Energy of contraction
Aortic pressure
How can energy of contraction be increased?
Raising EDP to stretch myocardium = Sterling’s law
Inc. in contractility = inc. strength of contraction by sympathetic nerves and adrenaline
What factors regulate stroke volume?
Filling pressure (preload) = Starling’s law of heart
Arterial pressure opposing ejection (afterload)
Contractility: sympathetic nerves + circulating agents
Total peripheral resistance
What is Starlings law of the heart?
Increase in contractile energy with stretch
Where stretch of muscle fibers set by diastolic filling pressure (PL)
What is a ventricular function curve?
Plot of SV versus filling pressure
Or any plot where
X-axis = index of resting fiber length
Y-axis = measure of contractile energy
What does a ventricular function curve depict via the descending limb?
SV declines in over-distended hearts
