ECG monitoring looks at the electrical impulses in the cardiac muscle (myocardium). This is generated through placing three electrodes in designated positions. Myocardial cells are striated like skeletal muscles, the action of myocardial cells differs in that movement is involuntary like a smooth muscle and each cell communicates via a ‘calcium channel’. This communication causes a contraction or relaxation via a slide-like mechanism. This intrinsic function is controlled by the autonomic nervous system and is influenced by some hormones, including adrenaline and thyroxine. The myocardial cells contract in response to this electrical activity, therefore, having an understanding of what the electrical activity is helps us to understand what the heart is doing and informs a fuller assessment of the infant or child. Electrical impulses, when measured this way, create deviations upwards or downwards from the isoelectric (flat) line. These ‘deviations’ help us map out the electrical activity and by understanding what normal looks like, we can then identify abnormal electrical impulses. This summary will gave an overview of basic, normal electrocardiology. One heart beat is generally represented by a collection of letters, P, QRS, T and U (rarely recognized). By understanding the normal, the abnormal becomes easier to follow, therefore only the ‘normal’ will be discussed in this chapter.

One single impulse: Conduction begins in the SA, the sinoatrial node, otherwise known as the ‘natural pacemaker’. This is found within the wall of the right atrium, near the superior vena cava. The impulse discharges across both atria, resulting in contraction and depolarization of the AV, the atrioventricular node. The AV node is found in the atrial septum (the dividing wall) near the atrioventricular valves. The impulse then passes down the conduction fibres (Bundle of His) which divide into the left and right bundle branches at the top of the intraventricular septum. These branches terminate in complex fibres called Purkinje fibres, forcing the impulse to spread rapidly across the inner surface of the ventricles causing ventricular contraction. The impulse ends as it moves upwards along the ventricular walls.

One complete cycle passes through the following impulses:

• P wave: This represents atrial depolarization or the passage of electricity from the SA node to both atria. The normal P wave in children and infants is 0.04–0.07 seconds, it is slightly longer in adults.
  
• P-R interval: This is the interval from the beginning of the P wave to the beginning of the QRS complex or the time taken from atrial depolarization to ventricular depolarization. This is usually flat due to a brief delay in conduction. This varies with age and heart rate (HR) – the slower the HR, the longer the P-R interval.
  
• QRS complex: This represents ventricular depolarization or the passage of electricity from the AV node, down the branch bundles in the septum and across both ventricles. The usual length of time for the QRS complex is 0.06–0.08 seconds in infants, 0.1 seconds in children and 0.12 seconds in adolescents.
  
• Q-T interval: This represents the time taken from the beginning of the QRS complex to the end of the T wave. It represents ventricular depolarization and repolarization. The rate is dependent on the age and HR of the patient, the higher the HR, the shorter the Q-T interval.
  
• S-T interval: This represents the interval between the S and T waves and is normally flat, as there is little or no electrical activity at this point.
  
• T wave: This represents ventricular repolarization.

Normal sinus rhythm indicates the beat has arisen from the SA node, represented as a P wave. This P wave is generally of normal shape, length and direction (pointing upwards). This P wave is then followed by a normal P-R interval, then the QRS complex. Every P wave is followed by a QRS and every QRS must be preceded by a P wave for the rhythm to be ‘sinus’.

While the heart rate is supplied in a variety of forms (manual pulse, on the ECG or Sa02 monitor), calculating the rate is a valuable skill. Using standard ECG recording paper, a useful rough calculation is to divide 300 by the number of large squares between one R-R interval. This is a rough guide but will give an indication of tachycardia or bradycardia. Another method is presuming the standard 12-lead ECG machine is set to record for 10 seconds, multiply the number of QRS complexes in the strip by 6.

These are the so-called ‘reversible causes’ and sit within Resuscitation UK’s Paediatric Advanced Life Support algorithm (2015). It is worth having an appreciation of the main causes of cardiovascular collapse in children and infants to supplement any attempts to interpret an ECG recording.

• Hypoxia: oxygen deprivation causing acidosis.
  
• Hypovolaemia: significant fluid loss, regardless of cause.
  
• Hypo-/hyperkalaemia/metabolic: several electrolytes are crucial for myocardial function, in particular, potassium, calcium and magnesium.
  
• Hypothermia: profound cooling.
  
• Tension pneumothorax: rapid collection of air in the pleural space.
  
• Toxins: these could be ingested medicines or other products,
  
• Tamponade: cardiac tamponade is a collection of fluid (blood, serous fluid or clots) in the pericardial sac, leading to compression of the heart.
  
• Thromboembolism: a blood clot which can migrate.