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Detailed descriptions of the various waveforms are

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Detailed descriptions of the various waveforms are often not a feature of MRCS A (although they areon the syllabus). However, they are a very popular topic for surgical physiology in the MRCS Bexam.Electrical propertiesIntrinsic myogenic rhythm within cardiac myocytes means that even the denervated heart iscapable of contraction.In the normal situation the cardiac impulse is generated in the sino atrial node in the rightatrium and conveyed to the ventricles via the atrioventricular node.
The sino atrial node is also capable of spontaneous discharge and in the absence ofbackground vagal tone will typically discharge around 100x per minute. Hence the higherresting heart rate found in cardiac transplant cases. In the SA and AV nodes the restingmembrane 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 theSA node will depolarise first. The cells have a refractory period during which they cannot bere-stimulated and this period allows for adequate ventricular filling. In pathologicaltachycardic states this time period is overridden and inadequate ventricular filling may thenoccur, cardiac output falls and syncope may ensue.Parasympathetic fibres project to the heart via the vagus and will release acetylcholine. Sympatheticfibres release nor adrenaline and circulating adrenaline comes from the adrenal medulla.Noradrenaline binds to β 1 receptors in the SA node and increases the rate of pacemaker potentialdepolarisation.Cardiac cycleImage sourced fromWikipediaMid 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 diastolicvolume 130-160ml.
Early systole: AV valves shut. Ventricular pressure rises. Isovolumetric ventricularcontraction. AV Valves bulge into atria (c-wave). Aortic and pulmonary pressure exceeded-blood is ejected. Shortening of ventricles pulls atria downwards and drops intra atrialpressure (x-descent).Late systole: Ventricular muscles relax and ventricular pressures drop. Although ventricularpressure drops the aortic pressure remains constant owing to peripheral vascular resistanceand elastic property of the aorta. Brief period of retrograde flow that occurs in aortic recoilshuts the aortic valve. Ventricles will contain 60ml end systolic volume. The average strokevolume is 70ml (i.e. Volume ejected).Early diastole: All valves are closed. Isovolumetric ventricular relaxation occurs. Pressurewave associated with closure of the aortic valve increases aortic pressure. The pressure dipbefore this rise can be seen on arterial waveforms and is called the incisura. During systolethe atrial pressure increases such that it is now above zero (v- wave). Eventually atrialpressure exceed ventricular pressure and AV valves open - atria empty passively intoventricles and atrial pressure falls (y -descent )

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