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THE CARDIAC CYCLE
By definition the sequence of events in a single pump stroke of the heart.
Movement of the heart
Palpation and percussion
Auscultation - listening with the stethoscope
auscultation tells us most about the functioning of the valves which we hear opening and closing
first heart sound is - closing of the a-v valves (the mitral and the tricuspid)
second heart sound - the closing of the aortic & pulmonary
second heart sound - often split - aortic closing just a "split" second before the pulmonary.
third sound mid way between the second and first caused by very rapid filling of the ventricles
Abnormal sounds and murmurs
- due to abnormal directions of flow or restrictions of flow
- narrowing of valves - mitral stensosis
- leaky valves, tricuspid incompetence
Investigations - ECG
- we can use it to check on the sequence of cardiac events.
- P wave, corresponding to atrial depolarisation
- QRS complex corresponding to ventricular depolarisation
- S-T segment corresponding to the plateau phase of the ventricular muscle cardiac action potential
- T wave corresponding to the period of ventricular repolarisation
- atria complete their contraction before the ventricles
- that the apex of the heart starts to contract before the base
- nearly synchronous contraction of all the muscle
Investigations - echocardiography
- uses ultrasound - echoes from the walls of the heart chambers and from the valve leaflets
- useful information about 2 - dimensional diameters and distances - less useful when it comes to measuring volumes.
These URLs deal with echocardiography and also have a few links to sites showing echocardiograms. The second one was down at the time of preparation of this handout but the first one was not.
Investigations - cardiac catheterisation
can get information about pressure changes using this - often a prelude to cardiac surgery and not a routine investigation.
Investigations - Doppler ultrasound
orientation is critical
flow velocity varies across the diameter of a vessel
diameter of the vessel may change during observation
Flow is velocity times cross sectional area.
|Five phases of the cardiac cycle
| i) Isovolumetric ventricular relaxation.
|ii) Rapid ventricular filling.
|iii) Atrial contraction.
|iv) Isovolumetric contraction.
|v) Ventricular ejection.
|Phases (i), (ii) and (iii) together -ventricular diastole.
|Phases (iv), (v) - ventricular systole
- 1) 50% of ventricular emptying in first 25% of the ejection phase
- 2) emptying complete by 75% of the way through ventricular systole
- 3) young hearts relax rapidly produces suction effect (N.B. all hearts fill fast at first).
- 4) very slow entry of blood into the ventricles in period known as diastasis as ventricles almost fill up
- this period can be reduced without any significant effect on ventricular filling
- heart can speed up to about 180 beats per minute without compromising ventricular filling, above this rate the cardiac output may fall.
- 5) atrial contraction can add about 20-30% to ventricular volume, but is not essential for maintenance of cardiac output at rest
The effect of changing heart rate on the duration of the phases of the cardiac cycle
|Duration of cardiac cycle
|Duration of ventricular systole
|Duration of ventricular diastole
Some facts you need to LEARN
- Human end-diastolic volume (EDV) is 110-120ml;
- Human end systolic volume is 40-50ml,
- Human stroke volume of 70ml
- Human heart rate 72 beats per minute gives a cardiac output of 5l/min.
- horses resting heart rate 40 beats per minute
- horses cardiac output 15l/min
- horses stroke volume of 375ml.
- Blood volume roughly 70ml/kg, cardiac output roughly 70 ml/kg (but perhaps 90ml/kg in smaller mammals)
pressures on the right side of the heart lower than on the left.
right ventricular ejection begins before and goes on longer than left ventricular ejection
ventricles are the regions with the biggest pressure fluctuations in the whole of the circulation.
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