Heart-lung preparation Definition.
Heart-lung preparation is an experimental set up, devised by Starling. It is used to demonstrate the effects of various factors on the activities of heart, particularly heart rate and cardiac output. This preparation is also used to record the cardiac function curves.
Procedure of Heart-lung preparation.
Heart-lung preparation is usually done in dogs. After giving anesthesia, neck of the dog is opened and a tracheal cannula is inserted into the trachea. Tracheal cannula is connected to a respiratory pump, so that respiration in the animal is controlled artificially, to avoid any disturbance during the experimental procedure. Then, chest is opened and an arterial cannula is inserted into one of the branches of aorta. All the other branches from arch of aorta and descending aorta are ligated.
Arterial cannula is connected to two instruments:
1. Mercury manometer to measure the arterial blood pressure.
2. Air bottle, which provides elasticity artificially (as in the case of arterial wall).
Thus, the blood ejected from left ventricle passes into air bottle through the arterial cannula and rubber tubes. From the air bottle, the blood is diverted through a tube which provides artificial resistance. Air bottle is also connected to a pressure bottle. Pressure bottle is attached to a pressure pump. This pump is used to maintain the pressure within the set up. Artificial resistance is offered by applying pressure surrounding the resistance tube.
Resistance tube is also connected to a manometer. After passing through the resistance tube, blood is allowed to flow through a warming glass coil, which is kept inside a water bath with a heater. Temperature of water bath is controlled, so that the temperature of blood could be maintained. Warming coil is connected to a venous reservoir through a flowmeter, which determines the amount of blood flow (cardiac output). Venous reservoir is connected to superior vena cava by a rubber tube.
A screw type clamp is fitted to the rubber tube. This clamp is used to adjust the amount of blood returning to heart (venous return). A thermometer is also fitted to the tube to note the temperature of blood. A third mercury manometer is connected to the inferior vena cava. It is used to determine the venous pressure. A cardiometer is fitted to the ventricle. This cardiometer is connected to a recording device like Marey tambour or polygraph, to record the ventricular volume changes. Pulmonary circulation is kept intact for continuous oxygenation of blood.
Uses of Heart-Lung Preparation.
Thus, in this set up, the heart works as an isolated organ. So, the effects of various factors can be demonstrated on the activities of heart, like heart rate, ventricular volume and cardiac output.
Examples
1. When venous return decreases, stroke volume decreases.
2. When venous return increases, stroke volume increases.
3. When resistance increases, cardiac output decreases.
4. When resistance decreases, cardiac output increases.
5. Heart-lung preparation is also used to record two types of cardiac function curves:
a. Cardiac output curves.
b. Venous return curves.
Though the cardiac function curves are obtained in experiments using the animals, these curves represent the functions of the ventricles in human heart also .
Cardiac Function Curves Definition .
Cardiac function curves are Frank-Starling curves, which demonstrate the capacity of ventricles to pump blood and to maintain blood circulation throughout the body. Most of the cardiac function curves are obtained from animal experiments, by using heart-lung preparation. However, these curves are considered to represent the functions of ventricles in human heart.
Cardiac function curves are of two types :
1. Cardiac output curves .
2. Venous return curves.
1. Cardiac output curves .
Cardiac output curves are the curves that show the relationship between cardiac output and right atrial pressure. Right atrial pressure, in turn, depends upon venous return.
Normal Cardiac output curves .
Normally, left ventricular output is 5 L/minute, when the pressure in right atrium is 2 mm Hg. When the atrial pressure rises between 4 and 8 mm Hg, the left ventricular output also increases. It increases to about two and a half times of normal (basal) output, i.e. the output increases to about 13 to 14 L/minute. This is the maximum limit for increase in cardiac output. Further increase in right atrial pressure does not increase the ventricular output and the curve shows a plateau.
Right ventricular output is 5 L/minute, when the right atrial pressure is zero. This reaches the maximum, i.e. 13 to 14 L/minute when the atrial pressure increases between 2 and 4 mm Hg . Thus, the cardiac output curves demonstrate that cardiac output is directly proportional to atrial pressure up to a certain extent . Plateau of the curve shows that the heart can control the output by itself if the atrial pressure rises beyond +8 mm Hg. It is due to the fact that in normal conditions, venous return is decreased when atrial pressure raises above +8 mm Hg.
Factor affecting Cardiac output curves .
Shifting of cardiac output curve to left indicates increase in cardiac output and shifting to right indicates decrease in cardiac output. The conditions which shift the cardiac output curve to left or right are discussed below:
Shift to Left.
When there is an abnormal increase in the functioning of the heart (hypereffective heart), the cardiac output curve is shifted to left, indicating increase in cardiac output.
Conditions when shift to left occurs.
1. Combined stimulation of sympathetic and the parasympathetic nerves supplying the heart: It causes hyperexcitation of the heart, resulting in increased rate and force of contraction. The cardiac output increases up to 25 L/minute (i.e. the plateau is shifted to left). Increase in output is about twice the maximum output in normal conditions (13 to 14 L/minute).
2. Hypertrophy of heart: It increases cardiac output up to 10 to 19 L/minute. It is because of increase in force of contraction.
3. Excitation (by cardiac nerves) of the heart along with hypertrophy of the ventricles: In this condition, the cardiac output is elevated above 35 L/minute. It occurs in Marathon runners. Increase in cardiac output is an important factor for prolonged running
time of Marathon runners.
Shift to Right.
When the functioning of heart decreases (hypoeffective heart), cardiac output curve is shifted to right, indicating decrease in cardiac output.
Conditions when shift to right occurs.
1. Stimulation of parasympathetic nerve fibers of the heart.
2. Inhibition of sympathetic nerves to heart.
3. Myocardial infarction.
4. Diseases of the valves in the heart.
5. Congenital heart diseases.
Effect of Extracardiac Pressure on Cardiac output Curve.
Extracardiac pressure is the pressure outside the heart. Intrapleural pressure is the major extracardiac pressure. When it increases above the normal level, i.e. from –6 to –2 mm Hg or becomes positive, the venous return decreases, resulting in decrease in cardiac output. Cardiac output curve is shifted to right.
It happens in opening of thoracic cage and in positive pressure breathing. When the intrapleural pressure decreases, i.e. when it becomes more negative, the venous return increases and the cardiac output also increases. The curve is shifted towards left. It is common in negative pressure breathing.
Cardiac Tamponade.
Cardiac tamponade is the mechanical compression of heart due to accumulation of fluid in pericardial space. In addition to intrapleural pressure, accumulation of fluid in pericardial space also increases the extracardiac pressure and compresses the heart. In cardiac tamponade, the cardiac output decreases and output curve is shifted to right.
Venous return curves .
Venous return curves are the curves which demonstrate the relationship between venous return (blood flow in vascular system) and right atrial pressure. Venous return curves are also called systemic vascular function curves. Normally, 5 L of blood returns to heart every minute. When right atrial pressure increases, venous return decreases due to backpressure. When venous return decreases, the cardiac output also decreases.
Analysis of Cardiac Function Curves.
Relation of cardiac output and venous return with right atrial pressure is determined when cardiac output curves and venous return curves are merged together .
Coupling of Cardiac & Vascular Functions.
Cardiac output represents cardiac function and venous return represents vascular function. Coupling or merging of cardiac output (cardiac function) curves and venous return (vascular function) curves shows that when venous return is normal (5 L/minute), the cardiac output
as well as the right atrial pressure are normal. Relation between cardiac output and venous return under normal conditions is represented by (A). When the venous return increases (B), the cardiac output also increases along with increase in right atrial pressure. Thus, any factor that alters venous return alters the cardiac output also.
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