Test: Cardiac Cycle, Cardiac Output And Regulation - NEET PG MCQ

After the valves are closed (Aortic and pulmonary), pressure continues to drop rapidly during the period of isovolumetric ventricular relaxation.

Preload is EDV at the end of ventricular diastole. A preload leads to onset of ventricular contraction.

Second heart sound duration is 0.1 to 0.14 sec (average 0.12 sec).

A soft, low-pitched third sound is heard about one third of the way through diastole in many normal young individuals.

Fourth heart sound is due to rapid filling of ventricle, which occurs due to atrial contraction.

A fourth sound can sometimes be heard immediately before the first sound when atrial pressure is high or the ventricle is stiff in conditions such as ventricular hypertrophy. It is due to ventricular filling and is rarely heard in normal adults.

“This period of isovolumetric (isovolumic, isometric) ventricular contraction lasts about 0.05 s, until the pressures in the left and right ventricles exceed the pressures in the aorta (80 mm Hg) and pulmonary artery (10 mm Hg) and the aortic and pulmonary valves open.”

“Isovolumetric relaxation ends when the ventricular pressure falls below the atrial pressure and the AV valves open.”

“Total electromechanical systole (QS2), the pre-ejection period (PEP), and the left ventricular ejection time (LVET) is done by recording the ECG, phonocardiogram, and carotid pulse simultaneously. QS2 is the period from the onset of the QRS complex to the closure of the aortic valves, as determined by the onset of the second heart sound.”

QS2: ECG and phonocardiogram are required.
LVET: Carotid pressure transducer is required.
PEP = (QS2 – LVET)
So, without carotid pressure transducer, LVET and PEP calculation would not be possible.

An increased S3 may be heard during late pregnancy. This heart sound is heard in early diastole.

“Ejection fraction, the percent of the end-diastolic ventricular volume that is ejected with each stroke, is about 65%.”

The cardiac output per minute per square meter of body surface (the cardiac index) averages 3.2 L.

Preload increased by increased venous return (VR) to heart. Increase blood volume increases VR to heart due to increase venous pressure.
Standing, sitting or, increase total peripheral resistant decreases VR to heart.

“Energy or, force of contraction is proportional to the initial length of the cardiac muscle fiber” (Starling's law of the heart or the Frank-Starling law). For the heart, the length of the muscle fibers (i.e. the extent of the preload, which depends on VR) is proportional to the end-diastolic volume. The relation between ventricular stroke volume and enddiastolic volume is called the Frank-Starling curve. So, increase VR will lead to increase SV and CO.

CO decreases in sitting or standing from lying position (20–30%).

In pregnancy there is ~40% increase in CO. This is due to increase in both SV (30%) and HR (15%).

Afterload is the force against which the ventricle ejects its contents.
The afterload of the left ventricle is the mean systemic arterial pressure; the afterload of the right ventricle is the mean pulmonary arterial pressure.
So, best index of LV afterload is arterial pressure (mean) in the absence of aortic stenosis.
In the clinical setting, the most commonly used measure of ventricular afterload is systemic vascular resistance (SVR/TPR).
However, SVR is a measure of vasomotor tone that reflects only the non-pulsatile component of peripheral load. In contrast, left ventricular systolic wall stress reflects the combined effects of peripheral loading conditions and factors internal to the heart.

The maximum percentage that the cardiac output can increase above normal is called the cardiac reserve.
Cardiac reserve: 300 to 400% in the healthy young adult, 500 to 600% in athletic, 200–250% in old age. Maximum cardiac reserve is reached during exercise and minimum during heart failure.

For Fick Principle:

• Mixed venous blood is obtained from pulmonary artery.
• Systemic arterial blood obtained from any systemic artery (Brachial/Radial).
• Rate of oxygen absorption is measured by oxygen meter or, spirometer.

Cardiac Output = Oxygen Consumption/Arteriovenous O₂ difference
Here, CO = 500/(0.24 – 0.16) = 500/0.08 = 6250 mL/min

Amputation of a limb or surgical removal of a kidney removes a parallel circuit and reduces the total vascular conductance and total blood flow (i.e. cardiac output) while increasing total peripheral vascular resistance.

MR: Right and down
MS: Left and narrow
AR: Right and tall
AS: Upward and left.

There are two basic types of ET-1 receptors: ET_A and ET_B. Both of these receptors are coupled to a Gq protein and the formation of IP₃.
Increased IP₃ causes calcium release by the sarcoplasmic reticulum which causes smooth muscle contraction.

In a resting, supine man, it averages about 5.0 L/min (70 mL × 72 beats/min).