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cardiovascular conditioning occurs (Marieb and Hoehn 2010). This results in hypertrophy of the heart with an increase in its stroke volume, leading to a lower resting heart rate but with the same cardiac output (Marieb and Hoehn 2010). If persistent bradycardia occurs in an individual as a result of ill health, this may result in inadequate blood circulation to body tissues. Bradycardia is often a warning of brain oedema after head trauma (Marieb and Hoehn 2010) and is one of the indications of raised intracranial pressure.

Rhythm

The pulse rhythm is the sequence of beats. In health, these are regular. The coordinated action of the muscles of the heart in producing a regular heart rhythm is due to the ability of cardiac muscle to contract inherently without nervous control (Marieb and Hoehn 2010). The coordinated action of the muscles in the heart results from two physiological factors.

Gap junctions in the cardiac muscles which form interconnections between adjacent cardiac muscles and allow transmission of nervous impulses from cell to cell (Marieb and Hoehn 2010).

Specialized nervelike cardiac cells that form the nodal system. These initiate and distribute impulses throughout the heart, so that the heart beats as one unit (Marieb and Hoehn 2010). The nodal system is composed of the sinoatrial node, atrioventricular node, atrioventricular bundle and the Purkinje fibres.

The sinoatrial node is the pacemaker, initiating each wave of contraction. This sets the rhythm for the heart as a whole (see Figure 12.5). Its characteristic rhythm is called sinus rhythm.

Figure 12.5 Conduction system of the heart. Autorhythmic fibres in the SA node, located in the right atrial wall, act as the heart’s pacemaker, initiating cardiac action potentials that cause contraction of the heart’s chambers. The conduction system ensures that the chambers of the heart contract in a coordinated manner. Redrawn from Tortora and Derrickson (2009) with permission.

In patients younger than 40 years, irregularity may be linked to breathing, when the heart rate increases on inspiration and decreases on expiration. Although this is rarely noticeable in adults (Higgins 2008), it is normal and is known as sinus arrhythmia (Woods et al. 2005). Defects in the conduction system of the heart can cause irregular heart rhythms, or arrhythmias, resulting in uncoordinated contraction of the heart.

Fibrillation

Fibrillation is a condition of rapid and irregular contractions. A fibrillating heart is ineffective as a pump (Marieb and Hoehn 2010).

Atrial fibrillation is a disruption of rhythm in the atrial areas of the heart occurring at extremely rapid and uncoordinated intervals. The rapid impulses result in the ventricles not being able to respond to every atrial beat and, therefore, the ventricles contract irregularly (Adam and Osborne 2005). The incidence of atrial fibrillation in the general population is approximately 1%, rising to 10% in people aged over 70 years (Goodacre and Irons 2002).

There are many causes of this condition, but the following are the most common:

ischaemic heart disease

acute illness

electrolyte abnormality

thyrotoxicosis.

Atrial fibrillation can complicate or cause many other medical conditions, including stroke and heart failure (Navas 2003a). If poorly managed, patients are at increased risk of arterial thromboembolism and ischaemic stroke (Jevon 2007). If the patient has a fast ventricular rate some contractions may not be strong enough to transmit a pulse wave that is detectable at the radial artery. In this instance, checking the radial pulse is an unreliable method of assessing ventricular rate. Simultaneous monitoring of the apex beat and radial pulse is advisable in patients with atrial fibrillation, because it will determine the ventricular rate more reliably and identify whether there is an apex beatradial pulse deficit (Jevon 2007).

This procedure requires two nurses and is described in Assessing gross pulse irregularity.

Ventricular fibrillation is an irregular heart rhythm characterized by chaotic