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Let’s start our discussion of the cardiac rhythms at the very beginning by learning about normal sinus rhythm and its abnormalities. Sinus rhythms originate in the sinoatrial (SA) node, located within the wall of the high right atrium near the inlet of the superior vena cava (Figure 14-1). From this anatomical location, the SA node first depolarizes the right atrium in a leftward, inferior, and anterior direction, followed by the left atrium in a leftward, inferior, and posterior direction (Figure 14-2). The mean axis of the normal P wave in the frontal plane is between 0 and +75 degrees. Accordingly, we see normal sinus P waves that are positive in leads I, II, and aVF with a negative P wave in lead aVR. Remember that the SA nodal depolarization is too small to be seen on the surface ECG so our first evidence of this discharge is the appearance of a normal P wave, indicating successful depolarization of the atria. By convention, the normal sinus rate is defined as 60 to 100 bpm. The term normal sinus rhythm implies depolarization by the SA node at a standard rate with normal atrial depolarization, followed by transmission through the AV node and His-Purkinje system to the ventricles (Figure 14-3). This results in a normal P wave that is accompanied by a normal PR interval (0.12-0.20 sec). The P-P interval between complexes is regular, with any variation between the longest and shortest cycle lengths no more than 0.12 seconds or 10% (Figure 14-4).
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SA nodal discharge is sensitive to modulation by the autonomic nervous system. Sympathetic influences increase and parasympathetic (vagal) effects decrease the heart rate (HR). Temperature and metabolic status also influence HR. The rate of SA nodal discharge increases with elevated environmental and body temperature as well as with hyperthyroidism. Cold ambient and body temperature and hypothyroidism slow the HR.
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Sinus bradycardia describes an otherwise normal sinus rhythm with a HR <60 bpm (see Figure 14-5...