McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 4 2 Introduction to the cardiovascular system

The P wave immediately precedes the contraction of the atria. The QRS complex immediately precedes the contraction of the ventricles and then relaxation of the ventricles during the T wave. The repolarisation of the atria (the Ta wave) occurs during the QRS complex and usually is not seen on an ECG. In certain conditions of atrial hypertrophy, the Ta wave may appear around the QRS complex. In addition to the five waves, several areas represent critical points on the ECG. These include the following: PR interval : Reflects the normal delay of conduction at the AV node QT interval : Reflects the critical timing of repolari­ sation of the ventricles ST segment : Reflects important information about the repolarisation of the ventricles A person with a normal ECG pattern and a heart rate within the normal range for that person’s age group is said to be in normal sinus rhythm. However,

abnormalities in the shape or timing of each part of an ECG tracing help to reveal the presence of particular cardiac disorders. Arrhythmias A disruption in cardiac rate or rhythm is called an arrhythmia or dysrhythmia . Various factors, such as drugs, acidosis, decreased oxygen levels, changes in the electrolytes in the area and build-up of waste products, can change the cardiac rate and rhythm. Arrhythmias can arise because of changes in the automaticity or con­ ductivity of the heart cells. They are significant because they interfere with the work of the heart and can disrupt the cardiac output, which eventually will affect every cell in the body. Several different types of arrhythmias may occur. Sinus arrhythmias The SA node is influenced by the autonomic nervous system to change the rate of firing to meet the body’s

Time

R

R

P

P

T

T

Q

Q

Millivolts

S

S

PQ (PR) QRS

ST

QT

Key: Each vertical square represents 0.1 mV of electrical charge. Each horizontal square equals 0.04 sec of time.

Approximate values for normal intervals: PQ (PR) interval—0.16 sec QT interval—0.3 sec QRS interval—0.08 sec P wave—0.08 sec ST interval—0.1 sec

P wave = Electrical changes associated with atrial depolarisation QRS complex = Electrical changes associated with ventricular depolarisation T wave = Electrical changes associated with ventricular repolarisation The electrical changes associated with atrial repolarisation normally coincide with the QRS complex and are obscured by it.

FIGURE 42.5  The normal electrocardiogram waveform.