CVP Physiological and Pathophysiological Concepts 2 — Questions and Answers
Question 1: During cardiopulmonary bypass, which mechanism primarily accounts for the decrease in systemic vascular resistance seen with hemodilution?
- Reduced blood viscosity lowering frictional resistance (Correct answer)
- Increased cardiac output from volume loading
- Vasodilation from hypothermia
- Release of atrial natriuretic peptide
Correct answer: Reduced blood viscosity lowering frictional resistance
Hemodilution reduces blood viscosity, which directly decreases frictional resistance in the vasculature and lowers SVR.
Question 2: A patient on CPB develops a mixed venous oxygen saturation (SvO2) of 55%. Which physiologic condition best explains this finding?
- Hyperdynamic circulation with excessive oxygen delivery
- Inadequate perfusion flow relative to metabolic demand (Correct answer)
- Left-to-right intracardiac shunt
- Hypothermia reducing oxygen consumption
Correct answer: Inadequate perfusion flow relative to metabolic demand
An SvO2 of 55% indicates tissues are extracting more oxygen than normal, reflecting inadequate oxygen delivery relative to demand.
Question 3: Which ion channel is primarily responsible for the rapid depolarization phase (phase 0) of the cardiac action potential in ventricular myocytes?
- L-type calcium channels
- Fast sodium channels (Correct answer)
- Potassium rectifier channels
- Funny (HCN) channels
Correct answer: Fast sodium channels
Fast voltage-gated sodium channels open rapidly during phase 0, producing the steep upstroke of the ventricular action potential.
Question 4: During deep hypothermic circulatory arrest, cerebral protection is enhanced by hypothermia primarily because it:
- Increases cerebral blood flow to wash out metabolites
- Reduces cerebral metabolic rate for oxygen (CMRO2) (Correct answer)
- Prevents cerebral vasoconstriction
- Alkalizes cerebrospinal fluid
Correct answer: Reduces cerebral metabolic rate for oxygen (CMRO2)
Hypothermia markedly reduces CMRO2, extending the safe period of ischemia during circulatory arrest.
Question 5: Starling's law of the heart describes the relationship between:
- Heart rate and stroke volume
- End-diastolic volume and stroke volume (Correct answer)
- Afterload and ejection fraction
- Contractility and cardiac output
Correct answer: End-diastolic volume and stroke volume
Frank-Starling law states that stroke volume increases with greater end-diastolic volume (preload) due to enhanced myofilament overlap.
Question 6: In a patient with severe aortic stenosis undergoing cardiac surgery, the perfusionist must consider that elevated left ventricular end-diastolic pressure (LVEDP) predisposes the heart to:
- Subendocardial ischemia due to impaired coronary perfusion pressure (Correct answer)
- Pulmonary venous hypertension only
- Right ventricular dilation from increased preload
- Decreased oxygen demand from hypertrophy
Correct answer: Subendocardial ischemia due to impaired coronary perfusion pressure
Elevated LVEDP reduces the coronary perfusion pressure gradient, increasing subendocardial ischemia risk in hypertrophied ventricles.
Question 7: Which buffer system provides the most immediate response to acute changes in blood pH during cardiopulmonary bypass?
- Phosphate buffer system
- Bicarbonate-carbonic acid system (Correct answer)
- Protein (hemoglobin) buffer system
- Renal ammonium excretion
Correct answer: Bicarbonate-carbonic acid system
The bicarbonate-carbonic acid system is the principal extracellular buffer and responds within seconds to pH changes during CPB.
CVP Physiological and Pathophysiological Concepts 2
During cardiopulmonary bypass, which mechanism primarily accounts for the decrease in systemic vascular resistance seen with hemodilution?