Cardiac Cycle and Cardiac Output MCQs

Phase 1 - Initial repolarization Mainly due to efflux of potassium ions through transient outward K + channels Also due to inactivation of fast Na + channels Reference: Ganong, 25 th edition page no. 522

Sympathetic stimulation to the heart results in, Increase in heart rate (positive chronotropy) Increase in myocardial contractility (positive inotropy Increase in cardiac conduction velocity (positive Dromotropy) Increase in cardiac excitability (positive Bathmotropy) Augments cardiac relaxation (positive lusitropy) Decrease in refractory period Reference: Ganong, 25th edition page. 592

Explanation: In the jugular venous pulse, the "a" wave corresponds to right atrial contraction . In atrial fibrillation , there is no effective atrial contraction because the atria fibrillate instead of contracting in a coordinated manner. As a result, a wave is absent from the JVP tracing. This absence of "a" waves is a classic diagnostic clue in atrial fibrillation. Explanation of other options: Steep a waves (Option B): Seen in tricuspid stenosis or pulmonary hypertension when right atrial contraction occurs against resistance. Absent c waves (Option C): The c wave represents bulging of the tricuspid valve into the right atrium during ventricular systole; it is not typically absent in atrial fibrillation. Absent x descent (Option D): Loss of x descent is seen in conditions like tricuspid regurgitation and constrictive pericarditis , not in atrial fibrillation.

Ans: D. 60% (Ref Gunong 25/e p540 24/e p542) Ejection fraction calculated from the given volume pressure curve is 60%. Pressure Volume Loop: ab: isovolumetric contraction bc: Ventricular contraction during systole cd: Isovolumetric relaxation. Calculation: End-Diastolic Volume (EDV) (Point a) = 130 mL End Systolic Volume (ESV) (Point d) = 50 mL Stroke Volume (SV) = EDV–ESV = 80 mL Ejection Fraction = SV/EDV = 80/130 = 0.6 i.e. % EF = 60%.

Isovolumetric relaxation. At the end of systole both the AV valves and semilunar valves are closed, pressure inside the ventricles falls even more without any change in volume(isovolumetric) Reference: Ganong, 25th edition /page no. 541

Cardiac index = 5/1.7 = 3 liter/min/m² Reference: Ganong, 25th edition page no .545

Normal cardiac output values Men-5.6 L/min Women 4.9 L/min Reference: Ganong, 25th edition /page no .544

Experiments were performed in which subjects stay in in isolation in specially constructed rooms without any clues regarding the time of day in the outside world-no clocks, windows, radios, televisions Results of such studies the circadian rhythm persists, but its duration varies among subjects and is typically slightly longer (e.g. 25 hours) When they were reexposed to the external day-night cycle, the normal circadian rhythm resumes Reference: Ganong, 25th edition page no.309