DESCRIBE THE FACTORS AFFECTING CARDIAC OUTPUT.

Transcription

1 DESCRIBE THE FACTORS AFFECTING CARDIAC OUTPUT. BY: DISHA PRAKASH I MBBS, ROLL NO: 16M069

2 OBJECTIVES OF LEARNING Terminology and conceptual understanding of Cardiac Output. Factors regulating Cardiac Output. Measurement of Cardiac Output. ABSTRACT The goal of cardiac system is to maintain adequate blood flow to all tissues of the body. The heart works in conjugation with cardiovascular centres and peripheral blood vessels to achieve the goals. Figure 1: Cardiovascular System

3 TERMINOLOGIES CARDIAC OUTPUT Cardiac output is the volume of blood pumped by heart each minute. It is product of heart rate and stroke volume and averages about 5.25L/min. Cardiac output = Heart Rate X Stroke Volume Heart Rate = number of beats per minute. Stroke volume =amount of blood ejected in each beat. VENOUS RETURN Venous Return is defined as quantity of blood flowing from the veins into the right atrium per minute. In steady state, the heart pumps blood at the same rate at which blood enters the heart. Cardiac Output (CO) =Venous Return (VR) For the normal heart, CO is largely determined by events in peripheral circulation thus venous return derives cardiac output. Figure 2: Venous Return

4 FACTORS REGULATING CARDIAC OUTPUT 1. The key factor regulating stroke volume is the amount of stretching that occurs to ventricular wall prior to ventricular contraction. The more cardiac muscle stretches, the more forcefully it contracts. These stronger contractions increase stroke volume. Since, Cardiac Output (CO) Stroke Volume (SV) 2. Increase in Heart Rate also increases the cardiac output as; Cardiac output is directly proportional to the Heart rate. Various factors like exercise, rise in body temperature, drugs like Caffeine, emotional or physical exertion, dehydration increases heart rate, indirectly increasing the cardiac output. Figure 3: Factors affecting cardiac output 3. Increase in Sympathetic response in case of Fight-and-Flight situation increases the heart rate hence increases the cardiac output also decrease in parasympathetic response decreases the cardiac output. 4. Increase in Venous Return increases the End Diastolic Volume thus increasing stroke volume with the cardiac output. Factors affecting EDV: * The duration of ventricular diastole. * Venous return When HR increases, the duration of diastole is shorter. Less filling means a smaller EDV. When venous return increases, a greater volume of blood flows into the ventricles and EDV is increased. SV is directly proportional to EDV thus cardiac output increases.

5 Figure 4: Controlling factors of Cardiac Output FRANK-STARLING MECHANISM Increased venous return increases the ventricular filling (end-diastolic volume) and therefore preload. Myocyte stretching increases the sarcomere length. The myocardial fibres are stretched; the force of contraction is increased. The length of the fibre is determined primarily by the volume of blood in the ventricle, thus EDV is the primary determinant of preload This mechanism enables the heart to eject the additional venous return, thereby increasing stroke volume, thus cardiac output. Figure 5: Frank-Starling Mechanism Increase in venous return to the left ventricle And increases LVEDP thereby increases the volume and stroke volume. Figure 5.1: Frank-Starling Curves Changes in the afterload and inotropy shift the curve up or down.

6 FICKS PRINCPLE: MEASUREMENT OF CARDIAC OUTPUT Figure 6: Ficks Principle CO = O 2 absorbed per minute by the lung (ml/min) Arteriovenous O 2 difference (ml/l of Blood) REFERENCES Cardiac Output, Cardiac Output and Its Regulation, Google Images (Source of Images) Medical Physiology, Guyton & Hall, 12 th Edition, Page Thank You.