Measurement of Cardiac Function

Introduction

Heart is a muscular pump which is mainly involved in pumping the life fluid "Blood" throughout the body. It is a dynamic organ which alters its pumping mechanism to ensure that the body receives sufficient oxygen, nutrients, and hormones. It receives deoxygenated blood from tissues, pumps it to the lungs to oxygenate it and sends it back to circulate. This function can be quantified by various parameters. The purpose of this paper is to present the various parameters involved in measurement of the cardiovascular function.

Background

The human circulatory system is analogous to two non-stop pumps. It is a closed elastic circuit which is comprised of the muscle pump - the heart, the tubes - the arteries and the veins and the fluid - the blood. Out of the two pumps where one continuously pumps out blood to the lungs to recharge i.e. oxygenation and the other one sends blood to discharge oxygen to the tissues.

The human circulatory system is analogous to two non-stop pumps in series where one continuously pumps out blood to the lungs to recharge i.e. oxygenation and the other one sends blood to discharge oxygen to the tissues. It is a closed circuit which is comprised of the muscle pump - the heart, the tubes - the arteries and the veins and the fluid - the blood.

The functional design of the Heart

The heart is comprised of four chambers - two atria and two ventricles both divides as right and left. It is conical shaped organ where the base is at the top and the apex is pointed at the bottom. The top portion of the heart which receives blood is called the atria and the bottom portion is the ventricles which pumps blood out of the heart. The atrioventricular septum separates the atria from the ventricles; the interatrial and interventricular septum separates right and left atrium and ventricles respectively. The heart has 4 valves - two atrioventricular namely bicuspid, tricuspid, and two semilunar valves aortic and pulmonary. Both the atria act as a reservoir of blood and ventricles pumps blood to the lungs and the systems. The right atrium receives deoxygenated blood from the tissues through the veins, then it drains the blood to right ventricle which is guarded by the tricuspid valve. The right ventricle pumps blood to the pulmonary system to oxygenate the blood and send sends back to the left atrium which in turn drains blood through the bicuspid or mitral valve to the left ventricle pumps it out to the system. This whole process is called a cardiac cycle .

The Impulse network system of the heart

For the efficacy of the cardiac function, the cycle should be continuous and rhythmic. This is carried out by a specialized tissue located in the junction of superior vena cava and right atrium called Sinoatrial Node (SAN). This is called the pacemaker center of the heart; from here the impulses travel to the rest of the heart via Atriventricular Node (AVN), bundle of His and the Purkinje network so that it can function effectively and meet the supply - demand system of the of the body .

Parameters involved in determining the cardiovascular function

Cardiac output.

The first and foremost functional parameter of the heart to determine is Cardiac Output (CO). It is crucial because it determines amount of the blood needed for adequate perfusion in order to meet the requirements of the body . It is defined as the volume of blood ejected from the left ventricle per minute, this equals to 5.0 L. According to Brown and Smallwood , It is "the index of well being and performance of the heart" . In turn this is dependent on two factors viz. one Stroke Volume (SV), which is the volume of blood ejected out of the left ventricle per stroke/beat of the heart, which is equal to 0.07L. The other factor is Heart Rate (HR), which is the amount of beats per minute which is around 72 beats per minute.

This value is normal for an average man. This value may vary due to certain physiological and/or pathological conditions. For example, according to during exercise this value may increase up to 20-25 L/min . Under pathological conditions like congestive cardiac failure (CCF) the CO is significantly reduced .

There are a wide range of methods available for evaluating the CO, but they are categorized into two broad categories one the invasive methods and the other is the non Invasive methods. The invasive methods include the Fick's method, indicator dilution methods and the transesophageal echocardiography. The invasive methods involve catheterization i.e., insertion of a catheter into the artery/vein which is generally performed by using Swan-Ganz catheter. The catheter is inserted into the pulmonary artery via the subclavian or jugular veins to the right atrium and right ventricle and the pulmonary valve. The non invasive methods include cardiac ultrasound, transthoracic Doppler echocardiography, and the ICG technologies .

Fick's Method.

If the following parameters were known i.e. total oxygen consumption by the person and the oxygen content in the arterial and venous blood, then the CO can be calculated using the formula

Where VO2 is the systemic volume consumption, CaO2 and CvO2 is the arterial and venous blood oxygen concentration respectively . The total oxygen consumption is measured using the spirometer. The blood is drawn from the pulmonary artery and arterial blood from any peripheral artery for deoxygenated and oxygenated blood concentrations respectively .

Indicator dilution methods.

This method includes the dye dilution method, and the thermal dilution method.

Dye dilution method.

This method is also based on the Fick's method i.e., instead measuring oxygen content in the blood, the dye content is measured. In this dye dilution method, a known amount of the dye - indocyanine (green dye) or lithium chloride injected into the pulmonary artery using the catheter. The dye concentration is measured using a densitometer from the systemic artery. This data from the densitometer gives the dye concentration over a period of time. The cardiac output is measured using the given formula.

Where k is the densitometer calibration constant, D is the amount of the dye injected and A is the area under the curve which is obtained by the dye concentration vs. time .

Thermodilution (TD)

This method is also same as the dye dilution method, but the difference is the temperature is the indicator. A known temperature of the saline solution injected into the pulmonary artery as well as recording the body's core temperature is recorded using a multichannel pulmonary artery catheter (PAC) tipped with a thermistor . The cardiac output is measured using the formula below.

Where Vi is the injectate volume; Si and Sb are the respective specific gravities of the injectates and blood; Ci and Cb are the respective specific heats of the injectate and the blood; Ti and Tb are the respective temperatures of the injectate and the blood .