Term 2 Lecture 2: Cardiac Cycle Flashcards
Phase 1: diastole
-Blood flows through atria to ventricles (AV valves open)
- pressure in veins sufficient to drive blood into atria of the heart (venous return)
- semilunar valves are closed
- ventricular pressure lower than in aorta and pulmonary arteries
- end of phase 1 atria contract driving more blood into the ventricles
- atria relax and ventricular systole begins
Phase 2: systole
- ventricles contract
- ventricular pressure exceeds atrial pressure (early in systole)
- AV valves close
- semilunar valves closed as ventricular pressure is not high enough to force them open
- no blood flows in/out of ventricles - constant vol.
- iso-volumetric contraction
- by end of phase 2 ventricular pressure is enough to force open semilunar valves
Phase 3: systole
-blood ejected into aorta and pulmonary arteries through semilunar valves and ventricular volume falls
- ventricular ejection
- ventricular pressure rises then declines
- falls below aortic pressure and semilunar valves close ending ejection (and systole)
- distole begins
Phase 4: distole
- ventricular myocardium relaxes
- ventricular pressure too low to keep semilunar valves open
- all valves close, blood vol. constant, ventricles relax - iso-volumetric relaxation
- ventricular pressure low, AV valves open
- blood enters ventricles from atria
- ventricular filling
Duration of systole/diastole
Not equal
Heart beat ~72/min (1 beat per 0.8sec)
Diastole ~0.5 sec - adequate filling time for efficient pumping
Systole ~0.3 sec
Aortic pressure
1) Diastole: no blood in aorta, aortic valves closed, blood leaves aorta downstream systemic circulation
- loss of vol/pressure
= Min. Diastolic pressure
2) Systole: aortic pressure continues to fall, aortic valve only opens when ventricular pressure is high enough to force it open
3) systole: aortic valves open, ejection begins, aortic pressure rises quickly and blood flows into the aorta
Pulse pressure and mean arterial pressure
Pulse pressure (PP) diff between systolic pressure (SP) and diastolic pressure (DP)
PP= SP-DP = ~120-80= ~40mmHg
MAP is the average pressure occuring in aorta during one cardiac cycle
MAP=DP+1/3(SP-DP)= ~ 80+1/3(120-80)
= ~93.3mmHg
BP = systole/diastole = ~120/80
Ventricular volume terminology
EDV- volume of blood in ventricles at end of diastole referred to as end diastolic value (EDV) represents max. ventricular volume attained during cardiac cycle - reached just before start of ejection
ESV- vol of blood in ventricles at the end of systole called end systolic volume (ESV) represents min ventricular vol attained just after ejection.
SV - stroke volume = vol of blood in ventricles just before ejection minus vol of blood in ventricles just after ejection
SV=EDV-ESV
E.g. norm at rest EDV=~135ml
ESV= ~65ml
So SV= ~135-65ml = ~70ml
EF - ejection fraction= ratio of vol ejected in one beat (SV) compared to vol contained in ventricle just prior to ejection (EDV) so EF=SV/EDV
E.g. 70ml/135ml=0.52=52%
This tells us if heart is pumping efficiently a low % equates to muscle weakness
Heart sounds
Stethoscope detects lub-dup sound
Lub: first sound, soft, low pitched occurs at the start of systole (phase 2) as AV valves close
Dup: louder, sharper, higher pitched second sound occurs at the start of diastole (phase 4) as semilunar valves close
The sound is caused by turbulent blood flow as valves narrow and not by the valves snapping shut
Recording heart activity by electrocardiogram (ECG)
-Non-invasive way of monitoring heart electrical activity
- records overall spread of electrical current through the heart as function of time during the cardiac cycle
-electrodes used
-mechanical abnormalities not detected
ECG lead positions
Limb leads
Lead 1 Left arm to right arm
Lead 2 left leg to right arm
Lead 3 Left leg to left arm
Augmented unipolar leads
-AVR right arm
-AVL left arm
-AVF left leg
Chest leads
V1 right chest
V2-6 left chest
Reading an ECG
P wave = atrial contraction (e.g. 0.2 mV in 0.1 sec)
QRS complex= ventricular contraction (e.g. 1.0mV in 0.08-0.12 sec)
T wave = ventricular repolarisation (e.g. 0.2-0.3mV in 0.16-0.27 sec)
PR segment = AV nodal delay
ST segment = ventricle completely depolarised (cardiac cells in plateau phase - ventricular activation completed - contraction/emptying done)
T-P segment = heart muscle completely repolarised and at rest - ventricles filling
Duration of ECG periods in seconds
0.12-0.21
P-Q (P-R) interval onset P wave and onset
Q-S complex = conduction time through AV node
0.30-0.43
Q-T interval onset QRS complex to end of T wave- ventricle contraction (systole)
0.85-1.00
R-R interval timing between QRS peaks = time between heart beats
^ to determine HR divide 60secs by RR interval.
Abnormalities in heart rhythm - cardiac myopathies
Tachycardia - loss of T to P segment
Extrasystole - premature ventricular contraction
Ventricular fibrillation - uncoordinated/chaotic contraction
Heart block - defect in cardiac system
Myocardial infarction (heart attack) - death aka necrosis of heart muscle
Summary
Mechanical events of cardiac cycle (contract/relax) and resultant changes in blood flow in heart are brought about by rhythmic changes in cardiac electrical activity
Cardiac cycle consists of alt. Periods of systolic (contract/empty) and diastole (relax/full)
Wigger’s diagram correlates various events that occur concurrently during the cardiac cycle including ECG, pressure/vol changes, valve activity and heart sounds.
ECG/EKG is a record of overall spread of electrical activity throughout the heart.
Diff parts of ECG record can be correlated to specific cardiac events
ECG can be used to diagnose abnormal heart rates, arrhythmias and heart muscle damage, enlarged chambers, heart disease and abnormal heart rhythm
