The Cardiac Cycle Flashcards
What are the 2 main phases of eah heartbeat in the cardiac cycle?
How many phases are systole and diastole split into?
Systole = contraction (lasts approx. 1/3 of a beat)
Diastole = relaxation, heart fills with blood (lasts approx. 2/3 of a beat)
Systole = composed of 3 distinct phases
Diastole = composed of 4 distinct phases
What is the end-diastolic volume?
What is the end-systolic volume?
What is the stroke volume?
Measured when the heart is filled completely (usually around 108 ml at rest)
Measured when the heart has contracted fully (usually around 36 ml at rest)
Stroke volume = end-disatolic volume - end-sysotlic volume (usually around 72 ml at rest)
What is the ejection fraction?
Why is it used clinically?
What is the average stroke volume and average ejection fraction?
Ejection fraction (%) = [100 x stroke volume] / [end-disatolic volume]
Because ejection fraction describes the contractility of the heart resonably well
72 ml, 67%
Name the cardiac valves in the diagram below:
What is the sequence of events for the cardiac cycle?
- SAN activation excites the atria = atrial systole
- Isovolumetric contraction
- Rapid ejection
- Reduced ejection
- Isovolumetric relaxation
- Rapid passive filling
- Reduced passive filling
What is atrial systole?
Which part of the ECG corresponds to this?
Starts from SAN activation, wave of depolarisation acorss the atria causes contraction of the atria, fills the ventricles fully
P-wave = atrial excitation from the SAN
Cerulean blue line represents heart sounds - S4 = abnormal heart sounds perhaps indicating congestive heart failure, pulmonary embolism or tricuspid incompetence
What is isovolumetric contraction?
What does it show up as on the ECG?
Which part of the lub-dub does this form?
Tricuspid and bicuspid valves close just before this - generation of increasing pressure inside the left and right ventricles as all valves in the ventricles are closed so the volume remains the same yet the ventricles are contracting
Ventricles are contracting but the blood is not going anywhere yet (cells contracting are not yet shortening)
Q, R and S waves - indicative of ventricular excitation / depolarisation as the wave of depolarisation corsses the atria, travels down the bundle of his and travels up the purkinje fibres
‘lub’ from closing of the valves just before
What is rapid ejection?
Opening of pulmonary and aortic valves begins the phase - blood is expelled so ventricular volume decreases
Contraction of the ventricles eventually increases the pressure inside the ventricles to be greater than the diastolic (back) pressure holding the pulmonary and aortic valves closed, therefore overloads the afterload so blood is expelled as these valves are opened
Cells are contracting isotonicly as blood is being expelled (muscle fibres shorten)
No heart sounds for this phase
What is reduced ejection?
What wave on the ECG indicates repolarisation?
Blood ejected less rapidly, slower decrease in ventricular volume - aortic and pulmonary valves begin to close
This phase marks the end of systole
Pressure in the ventricles begins to fall, and the aortic pressure mimics this soon after
Fall in ventricular pressure below arterial pressure causes semilunar (aortic and pulmonary) valves to close and that marks the beginning of the next phase
T wave - indicates process of repolarisation
What is isovolumetric relaxation?
Which part of the ‘lub-dub’ sound does this pahse produce?
The closing of the aortic and pulmonary valves begins this phase - no change in volume in the ventricles as both sets of valves are closed
Ventricular muscles relax whilst the volume is maintained, so the ventricular pressure falls whilst the atrial pressure rises due to the filling of the atria
This phase is marked by the closing of the aortic and pulmonary valves at the start - producing the ‘dub’ sound
What is rapid passive filling?
What could be some issues in this phase?
The tricuspid and mitral (bicuspid) valves open as the ventricular pressure falls below the atrial pressure, which results in rapid, passive ventricular filling
Flat ECG (isoelectric) between the cardiac cycles
Issues with the mitral valve i.e. mitral valve incompetence due to not openning properly, etc. or severe hypertension = extra sound - S3 (turbulent ventricular filling)
What is the reduce passive filling phase?
Pahse may be called diastasis - ventricular volume increases more slowly until atrial systole is reached (when ventricular volume is topped up by atrial contraction)
Longest phase of the cardiac cycle
How do the pressures between the pulmonary and systemic circuits differ?
Do the right and left ventricles eject the same volume of blood?
What is the systemic circuit pressure Vs the pulmonary circuit pressure?
Essentially same patterns but the pulmonary circuit has a lower pressure overall than the systemic
Yes - the right ventricle is still pumping the same volume of blood as the left ventricle, just into a lower pressure circuit
Systemic = 120/80, Pulmonary = 25/5
How can pulmonary circuit pressure be measured?
Placing a catheter in the pulmonary artery, pulling it back through the pulmonary artery to measure pressures in the right ventricle, right atrium and pulmonary artery. Also blowing up a balloon on the catheter measures back pressures - the pulmonary capillary wedge pressure, which changes a lot during heart pressure
How does ventricular volume and ventricular pressure correlate with each other? Fill in the graph:
Why are these pressure volume loops used?
It is a pressure volume loop:
A- ventricular volume high but no pressure applied yet
A-B - ventricular volume remains high and pressure applied from isovolumetric contraction
B-C - ventricular contraction decreases ventricular volume
C-D - pressure decreases as ventricles enter relaxation
D-A - ventricular filling
Can give an idea of the contractility of the heart
How does the preload and afterload affect the pressure volume loop?
What way does the preload or afterload need to be changed to increase the stroke volume (SV)?
What occurs with an increased afterload?
The filling of the ventricles to the end-diastolic volume governs and so the stretching of the muscle
At point B, the ventricles encounter either the aortic or pulmonary arterial pressure (i.e. the afterload)
Increasing the preload stretches the muscles further so increases the SV
Decrease in shortening of the muscle fibres = decreased SV
How does the PV loop change with:
a) hardening of the aortic valve
b) acute blood loss
c) exercise
? Draw them into the diagram:
Hint: think of it in terms of preload and afterload
a) Constriction of the aortic valve from the hardening and narrowing increases the back pressure (afterload), so reduced shortening of the muscles = smaller SV
b) Loss of blood reduces venous blood return, which decreases the preload
c) Venous return increases due to venoconstriction and skeletal muscle pump, and SNS increases contractility
