Cardiac Cycle

Question 1

define cardiac cycle

Answer 1

all events that occur from the beginning of one heart beat to the beginning of the next

the orderly depolarisation/repolarisation sequence triggers a recurring cardiac cycle of atrial and ventricular contractions and relaxations

diastole - heart ventricle are relaxed and fill with blood
systole - heart ventricles contract and pump blood into the aorta (left ventricle) and pulmonary artery (right ventricle)
at heart rate of 75 BPM - ventricular diastole ~0.5 sec, ventricular systole ~0.3 sec

Question 2

describe different phases of cardiac cycle

Answer 2

1. passive filling
2. atrial contraction
3. isovolumetric ventricular contraction
4. ventricular ejection
5. isovolumetric ventricular relaxation

Question 3

describe normal cardiac pressure curves

Answer 3

refer PP
left atrium

left ventricle

aorta

Question 4

explain the origin of normal heart sounds

Answer 4

heart valves produce a sound when they shut, they do not normally produce a sound when they are open
first heart sound (s1) is caused by closure of mitral and tricuspid valves - lub
S1 heralds the beginning of systole
the second heart sound (S2) is caused by closure of aortic and pulmonary valves - dub
S2 heralds the end of systole and the beginning of diastole
additional heart sounds (s3 or s4) may be heard

aortic valve area - right 2nd intercostal space
pulmonary - left 2nd intercostal space
tricuspid - left 4th intercostal space
mitral - left 5th intercostal space (apex beat)

Question 5

explain how the arterial pressure is maintained during ventricular diastole

Answer 5

when blood get ejected during systole it stretches arteries and capillaries
it then recoils back and this keeps the blood moving forward and prevents the pressure from falling (despite ventricular pressure being close to 0)

Question 6

outline jugular venous pressure (JVP) waveforms

Answer 6

refer pp;
JVP occurs after right atrial pressure waves (indirect measurement of central venous pressure)
a - atrial contraction
c - bulging of tricuspid valve into atrium during ventricular contraction
v - rise of atrial pressure during atrial filling, release as mitral/tricuspid valves open

Question 7

describe passive filling

Answer 7

first phase cardiac cycle;
pressure in atria and ventricles close to 0
mitral valves open so venous return flows into ventricles
aortic pressure ~80mmHg and aortic valve is closed
similar events happen in the right side of the heart, but the pressures (right ventricular and pulmonary artery) are much lower
ventricles become ~80% full by passive filling

Question 8

describe atrial contraction

Answer 8

second phase cardiac cycle;
P wave in ECG signals atrial depolarisation
atria contracts between P wave and QRS
atrial contraction complete the end diastolic volume (~130ml in resting normal adult) - the end diastolic pressure is few mmHg

Question 9

describe isovolumetric ventricular contraction

Answer 9

third phase cardiac cycle;
ventricular contraciton starts after the QRS (signals ventricular depolarisation) in the ECG
ventricular pressure rises
when the ventricular pressure exceeds atrial pressure the mitral valve shut - producing first heart sound (lub)
the aortic valve is still shut, so no blood can enter or leave the ventricle
tension rises around a closed volume isovolumetric contraction
the ventricular pressure rises very steeply

Question 10

describe ventricular ejection

Answer 10

fourth phase cardiac cycle;
ventricular pressure exceeds aorta/pulmonary artery pressure
aortic/pulmonary valve open (silent on stethoscope)
stroke volume is ejected by each ventricle, leaving behind the end systolic volume
stroke volume = end diastolic volume (135) - end systolic volume (65)
aortic pressure rises
the T wave in ECG signals ventricular repolarisation
ventricles relax and ventricular pressure starts to fall
when ventricular pressure falls below aortic/pulmonary pressure; aortic pulmonary valves shut
this produces second heart sound (dub)
the valve vibration produces the dicrotic notch in aortic pressure curve

Question 11

describe isovolumetric relaxation

Answer 11

fifth phase cardiac cycle;
closure of aortic and pulmonary valves signals that start of isovolumetric relaxation
ventricle s again a closed box, as the mitral valve is shut
tension falls around a closed volume isovolumetric relaxation
when ventricular pressure falls below atrial pressure, mitral valves open (silent) and heart starts new cycle