Detailed descriptions of the various waveforms are

Detailed descriptions of the various waveforms are often not a feature of MRCS A (although they are
on the syllabus). However, they are a very popular topic for surgical physiology in the MRCS B
exam.
Electrical properties
 Intrinsic myogenic rhythm within cardiac myocytes means that even the denervated heart is
capable of contraction.
 In the normal situation the cardiac impulse is generated in the sino atrial node in the right
atrium and conveyed to the ventricles via the atrioventricular node.

 The sino atrial node is also capable of spontaneous discharge and in the absence of
background vagal tone will typically discharge around 100x per minute. Hence the higher
resting heart rate found in cardiac transplant cases. In the SA and AV nodes the resting
membrane potential is lower than in surrounding cardiac cells and will slowly depolarise from
-70mV to around -50mV at which point an action potential is generated.

Differences in the depolarisation slopes between SA and AV nodes help to explain why the
SA node will depolarise first. The cells have a refractory period during which they cannot be
re-stimulated and this period allows for adequate ventricular filling. In pathological
tachycardic states this time period is overridden and inadequate ventricular filling may then
occur, cardiac output falls and syncope may ensue.
Parasympathetic fibres project to the heart via the vagus and will release acetylcholine. Sympathetic
fibres release nor adrenaline and circulating adrenaline comes from the adrenal medulla.
N oradrenaline binds to β 1 receptors in the SA node and increases the rate of pacemaker potential
depolarisation.
Cardiac cycle
Image sourced from Wikipedia

Mid diastole : AV valves open. Ventricles hold 80% of final volume. Outflow valves shut.
Aortic pressure is high.

Late diastole : Atria contract. Ventricles receive 20% to complete filling. Typical end diastolic
volume 130-160ml.


Early systole : AV valves shut. Ventricular pressure rises. Isovolumetric ventricular
contraction. AV Valves bulge into atria (c-wave). Aortic and pulmonary pressure exceeded-
blood is ejected. Shortening of ventricles pulls atria downwards and drops intra atrial
pressure (x-descent).
 Late systole : Ventricular muscles relax and ventricular pressures drop. Although ventricular
pressure drops the aortic pressure remains constant owing to peripheral vascular resistance
and elastic property of the aorta. Brief period of retrograde flow that occurs in aortic recoil
shuts the aortic valve. Ventricles will contain 60ml end systolic volume. The average stroke
volume is 70ml (i.e. Volume ejected).
 Early diastole : All valves are closed. Isovolumetric ventricular relaxation occurs. Pressure
wave associated with closure of the aortic valve increases aortic pressure. The pressure dip
before this rise can be seen on arterial waveforms and is called the incisura. During systole
the atrial pressure increases such that it is now above zero (v- wave). Eventually atrial
pressure exceed ventricular pressure and AV valves open - atria empty passively into
ventricles and atrial pressure falls (y -descent )