The cardiac conductive system consists of several distinct regions, as follows: the sinoatrial node (SAN), commonly referred to as the heart’s natural pacemaker because it sets the basic rhythm of the heart. Within the SAN, pacemaker cells spontaneously generate regular electrical impulses called action potentials which then travel rapidly over the atria, initiating atrial systole; the atrioventricular node (AVN), a key region of the conductive system located in the lower portion of the right atrium. Action potentials that have spread over the atria rapidly converge on the AVN and are delayed here for around a tenth of a second, allowing time for blood to pass from the atria into the ventricles. Should the SAN be damaged (e.g. following infarction), the AV node is able to take over the role of pacemaker. The AVN is connected to atrioventricular bundle (AVB), frequently referred to as the bundle of His; this consists of a relatively thick bunch of fibres that extends the length of the interventricular septum before splitting into the right and left bundle branches.
Following the delay at the AVN, the AVB and bundle branches rapidly conduct action potentials into the ventricles to initiate ventricular depolarisation and contraction (systole). The right and left bundle branches split into fine extensions called Purkinje fibres which permeate deeply into the myocardium of the left and right ventricles. These allow rapid propagation of action potentials through the ventricles, ensuring that all the muscle fibres of the ventricles contract in synchrony to enable the most efficient ejection of blood from the heart by ensuring that the pumping chambers contract as a single unit (termed a syncytium).
Figure 3.6a and b The electrical conductive tissues and ECG waves
Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
The ECG (electrocardiogram)
The ECG is an important diagnostic tool. ECG machines monitor the conduction of action potentials through the heart’s electrical conductive tissues and the cardiac muscle that forms the myocardium.
Electrical waves and time intervals viewable on an ECG
Ideally when learning the components of an ECG, it is suggested that you should continually refer to the cardiac conductive system, and for this reason the two are presented together (Figure 3.6a and b). The electrical activity shown in Figure 3.6b corresponds to a single heartbeat and consists of:
The P wave: This small wave corresponds to the action potentials generated by the heart’s natural pacemaker, the sinoatrial node (SAN), and their passage across the atria. Therefore, nurses can regard this small initial peak (P) as corresponding exactly to the time when the patient’s atria are undergoing atrial systole. It may be a useful memory aid to think of the P wave as corresponding to activity generated by the pacemaker (p for pacemaker).
The P-R interval: This short time period corresponds to atrial contraction and the short delay in action potentials that occurs at the AV node to allow ventricular filling.
The QRS complex: Since the ventricles are so thick and muscular when action potentials spread through these lower chambers, a much larger electrical signal is generated. This is recorded on an ECG as a large spike termed the QRS complex. It is useful to regard this portion of the ECG as corresponding to the time when the ventricles are undergoing ventricular systole.
The T wave: This final major electrical peak corresponds to the ventricles returning to their resting state (ventricular repolarisation). Nurses can regard the T wave as corresponding to the time when the ventricles are relaxing as they undergo ventricular diastole.
The S-T segment: The period of electrical activity between ventricular depolarisation and repolarisation. Characteristic changes to this segment are frequently seen in patients with CAD, particularly when the patient has their ECG recorded when on a treadmill (stress ECG).
Clinical uses of ECG machines
Nurses routinely monitor ECGs, looking for changes in heart rate and rhythm. Most modern machines will provide an alert if a rhythm disturbance is detected. The P wave, QRS complex and T wave are repeated, with each heart beat resulting in the standard sinus rhythm that nurses observe on an ECG machine. The distance and timing between QRS complexes on an ECG trace provide an accurate measure of the patient’s current heart rate. Most ECG machines calculate this automatically to display a real-time reading of the heart rate.
Recording of an ECG may be deemed necessary in many circumstances including:
irregular heart beat (palpitations);
chest pain (angina pectoris);
suspected myocardial infarction (heart attack);
suspected electrolyte disturbances/imbalances;
loss of consciousness (syncope);
bradycardia (slow heart rate) or tachycardia (rapid heart rate);
to investigate the condition of the myocardium, in conditions such as heart failure, cardiomyopathy or long-standing hypertension.
Cardiac arrhythmias
The term cardiac arrhythmia is used to describe any changes to the transmission of action potentials through the cardiac conductive system that leads to an abnormal heart rhythm or heart rate. Many arrhythmias such as simple ectopic beats are benign with no major physiological effects, while others may result in a heart that pumps blood less effectively, leading to a reduction in cardiac output. In the most severe arrhythmias cardiac output may cease completely, leading to sudden cardiac death.
Changes to the cardiac conductive system
By the age of 50 around 50–75 per cent of the pacemaker cells of the heart’s natural pacemaker SAN have been lost, while the cardiac muscle fibres of the atrioventricular bundle (bundle of His) undergo varying degrees of fibrosis. These changes may reduce the efficiency of cardiac conduction and contribute to the increased risk of ectopic beats (palpitations) seen in older people.
Ectopic beats
Two major types are recognised and are classified according to which chambers are affected. Premature atrial contractions (PACs) occur when there is an early contraction of the atria and can usually be recognised on an ECG as an extra P wave. PACs are minor events and often not even perceived by the patient. Premature ventricular contractions (PVCs) occur as a result of early contraction of the ventricles; these are usually more noticeable to the patient. Often the patient will complain that it feels as if their heart has paused (or lurched) in their chest followed by a more powerful subsequent recovery beat.
Frequently PVCs may be experienced in clusters or in predictable patterns before disappearing and returning hours, days or even months or years later. Since PVCs occur as a result of abnormal conduction in the ventricles, they are more easily recognisable as irregular spikes on an ECG that interrupt the normal sinus rhythm. Most PVCs, while quite scary for the patient, are benign and often related to stress, dehydration or use of stimulants such as caffeine or nicotine. They are also very common in women going through the menopause. When PVCs occur sequentially one after the other,