The main function of the heart is to pump blood through two circuits:

 
1. Pulmonary circuit: through the lungs to oxygenate the blood and remove carbon dioxide; and

2. Systemic circuit: to deliver oxygen and nutrients to tissues.

In order to beat, the heart needs three types of cells:
 

1. Rhythm generators, which produce an electrical signal (SA node or normal
 pacemaker);

2. Conductors to spread the pacemaker signal; and

3. Contractile cells (myocardium) to mechanically pump blood.

The heart has specialized pacemaker cells that start the electrical sequence of depolarization and repolarization. This property of cardiac tissue is called inherent rhythmicity or automaticity. The electrical signal is generated by the sinoatrial node (SA node) and spreads to the ventricular muscle via particular conducting pathways: internodal pathways and atrial fibers, the atrioventricular node (AV node), the bundle of His, the right and left bundle branches, Purkinje fibers.

When the electrical signal of a depolarization reaches the contractile cells, they contract. When the repolarization signal reaches the myocardial cells, they relax. Thus, the electrical signals cause the mechanical pumping action of the heart.

The SA node is the normal pacemaker of the heart, initiating each electrical and mechanical cycle. When the SA node depolarizes, the electrical stimulus spreads through atrial muscle causing the muscle to contract. Thus, the SA node depolarization is followed by atrial contraction.

The SA node impulse also spreads to the atrioventricular node (AV node) via the internodal fibers. (The wave of depolarization does not spread to the ventricles right away because there is nonconducting tissue separating the atria and ventricles.) The electrical signal is delayed in the AV node for approximately 0.20 seconds when the atria contract, and then the signal is relayed to the ventricles via the bundle of His, right and left bundle branches, and Purkinje fibers. The Purkinje fibers, relay the electrical impulse directly to ventricular muscle, stimulating the ventricles to contract (ventricular systole). Repolarization of the SA node is also spread throughout the atria, and then the ventricles, starting the relaxation phase (ventricular diastole).

Although the heart generates its own beat, the heart rate (!2eats I2er_inute or BPM) and strength of contraction of the heart are modified by the sympathetic and parasympathetic divisions of the autonomic nervous system.

 The sympathetic system acts as an accelerator, speeding up and increasing the contractile force of the heart. Whe-never oxygen demands increase, e.g., during exercise or if blood pressure drops, the sympathetic input increases, causing heart rate and strength of contraction to increase. Sympathetic influence increases during inhalation.

 The parasympathetic input acts like a brake, slowing down the heart. When you relax, the parasympathetic input becomes dominant and the heart rate slows. Parasympathetic influence increases during exhalation.
 

Just as the electrical activity of the pacemaker is communicated to the cardiac muscle, "echoes" of the depolarization and repolarization of the heart are sent through the rest of the body. By placing a pair of very sensitive receivers (electrodes) on other parts of the body, the echoes of the heart's electrical activity can be detected. The record of the electrical signal is called an electrocardiogram (ECG).  You can infer the heartís mechanical activity from the ECG.

The electrical events of the heart are usually recorded on the ECG as a pattern of a baseline broken by a P wave, a QRS complex, and a T wave.

The baseline (isoelectric line) is a straight line on the ECG. It is the point of departure for the electrical activity of depolarizations and repolarizations of the cardiac cycles.

 The P wave results from atrial depolarization.

 The QRS complex is a result of ventricular depolarization and indicates the start of ventricular contraction.

 The T wave results from ventricular repolarization and signals the beginning of ventricular relaxation.

In addition to the wave components of the ECG, there are intervals and segments.
 

An interval is part of the ECG containing at least one wave and a straight line. For example, the PR interval includes the P wave and the connecting line before the QRS complex. The PR interval represents the time it takes for the impulse sent from the SA node to travel to the ventricles.

Segments only refer to a period of time from the end of one wave to the beginning of the next wave. For example, the PR segment represents the time of AV nodal delay and transmission to the ventricles.

The particular arrangement of two electrodes (one positive, one negative) with respect to a third electrode (the ground) is called a lead. The positions of electrodes for the different leads have been standardized. For this lesson, you will record from Lead II, which has a positive electrode on the left ankle, a negative electrode on the right wrist, and the ground electrode on the right ankle. Typical Lead II values are given in Table 5.1.

PHASE	DURATION  (second)	Amplitude (millivolt)
Pwave	0.06-0.11	<0.25
P-R interval	0.12-0.20
P-R segment	0.08
QRS complex (R)	<0.12	0.8 - 1.2
S-T segment	0.12
Q-T interval	0.36-0.44
Twave	0.16	<0.5

 

The average resting heart rate for adults is approximately 70 beats/min Slower heart rates are typically found in individuals who regularly exercise. Athletes are able to pump enough blood to meet the demands of the body with resting heart rates as low as 50  beats/min. Athletes tend to develop larger hearts, especially the muscle in the left ventricle a condition known as "left ventricular hypertrophy." Because of their larger and more efficient hearts, athletes also exhibit other differences in their ECGs. For instance, low heart rate and hypertrophy exhibited in sedentary individuals can be an indication of failing hearts but these changes are "normal" for well trained athletes.

In this lesson, you will record the ECG under four conditions. Because ECGs are widely used, basic elements have been standardized to simplify reading ECGs.   ECGs have standardized grids of lighter, smaller squares and, superimposed on the first grid, a second grid of darker and larger squares. The smaller grid always has time units of 0.04 seconds on the x-axis and the darker vertical lines are spaced 0.2 seconds apart. The horizontal lines represent amplitude in mV. The lighter horizontal lines are 0.1 mV apart and the darker grid lines represent 0.5 mV (Fig. 5.2).

II. EXPERIMENTAL OBJECTIVES

1 ) To become familiar with the electrocardiograph as a primary tool for evaluating electrical events within the heart.

2) To correlate electrical events as displayed on the ECG with the mechanical events that occur during the cardiac cycle.

3) To observe rate and rhythm changes in the ECG associated with body position and breathing.