Cardiac Physiology

Question 1

What are the determinants of cardiac output?

Answer 1

HR x SV

Stroke volume is determined by preload, afterload and contractility

Question 2

What is preload?
How is it assessed?
How is it increased and decreased?

Answer 2

The blood returning to the RA and LA.
Volume in the ventricle stretches the fibers, and thus predicts the SV. A full ventricle provide a good stroke volume.

• RV assessed by CVP, JVD
• LV assessed by PWP, lung sounds
• If preload needs to be increased, fluid volume given (IVF, blood products, volume expanders)
• If preload needs to be decreased, diuretics given & venous vasodilators (NTG, ACEI, ARBs)

Question 3

What is afterload?
How is it assessed?
How is it increased and decreased?

Answer 3

The resistance that the ventricle must overcome to eject volume.

• RV assessed by PVR
• LV assessed by SVR
• Increased by compensatory mechanism of SNS, vasopressors (norepinephrine, phenylephrine, dopamine, vasopressin)
• Decreased by vasodilator drugs (nitroprusside, milrinone, ACEI, ARBs)

Question 4

What is contractility?
How is it assessed?
How is it increased and decreased?

Answer 4

The efficiency of the fiber shortening regardless of length.

• evaluated by EF%
• Increased by positive inotropic drugs (dopamine, dobutamine, epinephrine, norepinephrine)
• decreased by negative inotropic drugs (Ca++ blockers, Beta blockers, others)

Question 5

Where are baroreceptors located and what do they do?

Answer 5

• located in carotid sinus (the bifurcation of carotid artery) and aortic arch
• the baroreceptors senses low or high BP
• -when BP is low, the receptors stimulate the vasomotor center to induce vasoconstriction and inhibit the vagus nerve which allows HR to increase
• -when BP is high, the receptors inhibit the vasomotor center which causes vasodilation and stimulates the vagus nerve to slow HR

Question 6

What are side effects of beta blockers?

Answer 6

• Bradycardia, AV block, HF, Hypotension
• Mask signs hypoglycemia (tachycardia, sweating, flush)
• Cause hypoglycemia d/t inhibition of glycogen converting to glucose in the liver (glycogenolysis)
• Bronchoconstriction, pulmonary edema (r/t HF)
• Peripheral vascular vasoconstriction
• depression, sexual dysfxn, wt gain, sleep disturbance, fatigue

Question 7

What are symptoms and treatment for Beta Blocker overdose?

Answer 7

Symptoms:

• bradycardia/heart block & hypotension (most common)
• myocardial depression, cardiogenic shock (severe cases)
• seizures, hypoglycemia, bronchospasm, ventricular arrhythmias (torsades)

Treatment:

• Glucagon (↑glucose, HR, contractility)
• Atropine & IVF
• IV calcium ↑contractility
• Catecholamines don’t work well b/c of beta blockade (dopamine and dobutamine rely on stimulating Beta)

Question 8

What do Calcium channel blockers effect?

Answer 8

• depolarization of SA and AV node cells (“slow current” calcium dependent cells
• facilitates contraction of heart and smooth muscle layer of blood vessels (actin-myosin interaction in muscle)
• most decrease HR, AV conduction velocity, and contractility (especially verapamil)
• coronary vasodilation (prevent vasospasm)
• peripheral vasodilation (afterload reduction)