identical.
Phase four is known as ventricular ejection; as the pressure in the ventricles increases, the aortic and pulmonary valves are forced open and blood is ejected into the pulmonary and systemic circuits.
The final fifth phase is isovolumetric relaxation; here the ventricles undergo diastole and blood begins to flow back against the aortic and pulmonary valves, snapping them shut. Pressure continues to fall within the ventricles until it is below that of the atria. The elastic recoil of the atria and effects of gravity push blood onto the atrioventricular valves which open, returning the heart back to phase one, passive filling.
Cardiac muscle
Unlike skeletal muscle which consists of parallel running fibres, the cardiac muscle, which forms the myocardium, has a branched structure consisting of individual cells joined together by tight junctions termed intercalated discs. These allow efficient and rapid movement of electrical signals through the myocardium, allowing the component fibres to contract in synchrony to ensure the myocardium contracts as a single unit. Cardiac muscle has a unique feature called intrinsic rhythm which is an inbuilt ability to contract at a regular rate, allowing the heart to beat at a relatively regular rhythm even if its natural pacemaker is damaged.
The cardiac conductive system of the heart
As we have seen above, each heartbeat involves precise and accurately timed contraction and relaxation of the heart’s chambers during the cardiac cycle. The five phases of the cardiac cycle are coordinated and timed to split-second accuracy via the cardiac conductive system. This consists of a series of specialised interconnected cardiac muscle fibres which originate in the atria before permeating deep into the ventricles (Figure 3.6a). You may find it useful to think of this system as behaving in a similar way to the cells of the nervous system in that it conducts electrical signals. As these electrical impulses travel through the heart, a coordinated wave of muscular contraction is initiated within the myocardium which corresponds to a single heartbeat. Since the heart is continually beating, this electrical activity within the conductive tissues is continuous and is readily recordable on an ECG.
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
