0630- Cardiac cycle- CG Flashcards
Aims
- understand what events are used to time the CC;
- realize that heart sounds (S1 and S2) delimit
systole and diastole;
- be able to draw the important features of pressure, volume and flow changes during cardiac cycle in ventricles, atria and arteries;
- know when and how ventricles are filled;
- appreciate how stroke volume is determined by
both, systolic and diastolic pressures; and
• appreciate the relative timing between left and right atria and ventricles.
What events are used to time the cardiac cycle?
Electrical (Ionic current flow and APs) or Mechanical events
Precisely defined by events seen on ECG
P wave= atrial deparisation,
QRS= ventricular depolarisation (start of systole)
T wave= repolarisation (systole end)
Mechanical events Muscle contraction- auscultation - movement of valves (murmurs, clicks) - Blood flow out of ventricle - volume changes
Explain the mechanical properties of heart
heart= 2 serially connected pumps Left = high pressure, Right = low Valve enforce directionality…AV = inflow, semilunar = outflow Failure = back pressure building in preceding parts
Describe S1, S2, S3, S4. How is diastole and systole defined?
S1 = lub = closing of atrioventricular valves (start of systole)-
S2 = dub = closing of semilunar valves (start of diastole). Typically split during inspiration, (negative pressure due to thorax expansion-increases venous return, hence delayed pulmonary valve sound)
Loudness depends on which area auscultation
May be pathological (gallop heart sounds)
S3- turbulent flow associated with rapid ventricular filling/LV function
S4- caused by forceful atrial contraction (ie to overcome resistance in ventricle)- a-stiff-wall
Systole- S1 to S2 (Q to T)
Diastole- S2- S1 (T to Q)
What are the phases of the cardiac cycle? Describe what is happening in the heart at each stage, and match to ECG morphology.
7 phases- 3 in systole, 4 in diastole
SYSTOLE
- Isovolumetric contraction phase = QRS complex (from mitral valve closure to opening of aortic valve). Rapid pressure increase, first exceeding atrial pressure, then exceeding aortic pressure
- Fast ejection phase= ST interval
- Slow ejection phase=** T wave** (ventricular repolarisation reduces tension)
DIASTOLE
- Isovolumetric relaxation phase = (from aortic valve closure to mitral valve opening)
- Passive ventricular filling (rapid)= atrial pressure>ventricular
- Passive ventricular filling (slow) = reduced pressure gradient as ventricles fill
- Atrial contraction = P wave (‘atrial kick’ to fill up ~15%- becomes more significant as HR increases, less time for passive filling)
Role of valves
Valves open when pressure difference between proximal and distal side
Enforce directionality of flow (closure prevents backflow)
Stenosis- narrowing of open valve diameter (requires increased ventricular/atrial pressure to maintain flow)
Regurgitation- failure of valve to seal properly- no pressure separation (flow in both directions), may involve large volumes
Describe pressure changes in the left atrium during the cardiac cycle
During systole
- Mitral valve closes and bulges back= small increase in pressure (c wave).
Subsequent pressure drop (x descent) as ventricular contraction pulls atrial base down, expanding chamber
Atrium fills throughout systole
During diastole
Mitral valve opens, pressure drops as blood goes into ventricles. Atrium contracts, causes small hump (a wave)
Pressure drops further
Mitral valve closes..
Valvular plane displacement aids atrial filling during systole and ventricular filling during diastole
Describe pressure changes in the aorta during the cardiac cycle
During systole (aortic valve open), pressure dominated by left ventricular pressure. As aortic valve shuts, small drop in pressure = backflow of blood back into the ventricles is the trigger for aortic valve to shut
During diastole (ie when aortic valve is closed), pressure drop from ~80 to 60mmHg. Another notch when aortic valve opens
Notches at aortic valve opening and closing.
What is valvular plane displacement and what purpose does it serve?
Displacement of heart due to systolic ejection movement causes valvular plane to displace towards the apex
This movement causes mechanical suction on central veins during last systole, which increases atrial filling speed.
Movement of heart back during diastole (early filling phase) - increases atrial pressure –> aids in ventricular filling
Describe pressure changes in the right atrium and ventricle during the cardiac cycle
Right ventricle + pulmonary artery
- analogous to left, except lower RV pressure, and pressure difference (~20 torr)
Ohm’s law- since pressure difference is of a smaller magnitude compared to Left side, pulmonary resistance must also be smaller than systemic resistance to maintain equal flow (matching)..
Right atrium and central veins (thoracic vena cava..)
a- atrial contraction c- bulging of valve out into atria x descent- valvular plane movement (build up of suction) v- tricuspid valve closes
can be measured on jugular vein pressure with a pulse transducer
Increased pressure in central vein if pump failure in heart backflows onto preceding elements ie LHF –> RHF, RHF –> high JVP
What is stroke volume determined by
Require 80mL for homeostasis
SV= end diastolic volume (systolic) - end systolic volume (diastolic)
Increase SV by increasing difference between systolic and diastolic pressure (lowered diastolic = higher SV)
Explain relative timing between left and right atria/ventricles. Why can S2 be split during inspiration?
Cycle starts in RIGHT atrium (location of SAN) and ends in RIGHT ventricle (LV contracting delays its contraction).
(also, the first to depolarise is the first to repolarise)
What is the split S2 sound caused by?
During inspiration, the pulmonary valve closes later, and aortic valve closes earlier. Expanded thorax –> negative pressure, causing:
- suction in central veins/RA, increasing venous return. Hence pulmonary valve stays longer during systole
- tendency for blood to stay in lungs, lowered return to LA and LV. Hence aortic valve closes sooner during systole
Timing between systole and diastole
resting: Systole- about 1/3rd, diastolic ~2/3 (can change)
Systole changes very little
Diastolic timing variable (atrial contraction necessary if diastole filling time is short ie increased HR)
Stricker Summary
- Systole: S1 → S2
- Diastole: S2 → S1
- The 7 phases of the cardiac cycle are: isovolumetric contraction, fast and slow ejection, isovolumetric relax- ation, fast and slow ventricular filling, atrial contraction.
- Whilst diastole is variable, systole is ± constant.
- SV is dependent on difference between systolic and
diastolic pressure.
- Most of atrial filling occurs during systole.
- “Insignificance” of atrial contraction at rest.
- “Pump failure” causes load on preceding element(s).
