Lecture 20: Excitation-Contraction Coupling of the Heart and Cardiac Cycle Flashcards
Two parts of the cardiac cycle
systole and diastole
Systole
Contraction and emptying
Diastole
Relaxation and filling
Contraction occurs as a result of
excitation across the heart
Relaxation follows
the subsequent repolarization of the cardiac muscle
Cardiac cycle is divided into how many stages
7
Phase A of cardiac cycle
- Atrial contract
- Final phase of ventricular filling
- Increased atrial pressure is reflected back to veins causing venous pressure “a” wave
What is associated with the fourth heart sound
Phase A: Atrial systole
Fourth heart sound
- Not heard under normal conditions
- May be heard when ventricle compliance is decreased and forceful ventricular filling produces a sound
- Ex. Ventricle hypertrophy
Phase B: Isovolumertric Ventricular Contraction
- Ventricles contract and ventricular pressure increases
- All valves are closed, and closure of AV valves produces the first heart sound (S1)
- C wave occurs here
S1 signals the onset of
Ventricular systole
“C” wave
Increase of ventricular pressure leads to bulging of AV valve into atria, causing a small atrial pressure wave
Occurs during phase B
Two atrioventricular valves
Tricuspid valve
Mitral valve
First heart sound
Two nearly superimposed components:
-Mitral component slightly precedes triscupid component because the earlier electrical stimulation of left ventricular contraction
Phase C: Rapid Ventricular Ejection
- Ventricles contract
- Ventricular pressure increases and reaches the maximum
- Aortic valve opens
- Ventricles eject blood into arteries
- Ventricular volume decreases
- Aortic pressure increases and reaches maximum
Two semilunar valves
- Pulmonary valve
2. Aortic valve
Afterload
The pressure against which the heart pumps blood into the circulation
Phase D: Reduced Ventricular Ejection
- Ventricles eject blood into the aorta at a slower rate
- Ventricles do not empty completely
- Aortic pressure begins to fall
End-systolic volume
The amount of blood remaining in the ventricle at the end of systole
Stroke volume
The amount of blood ejected by the ventricle in one beat
The proportion of EDV that is ejected
(SV/ESV) = Ejection fraction
Phase E: Isovolumetric Ventricular Relaxation
- Ventricles relaxed
- Ventricular pressure decreases
- Aortic valve closes
- Second heart sound is heard
- Ventricular pressure is constant (end-systolic volume)
- V wave
V wave
Atrial pressure increases as a result of filling from the veins
Dicrotic notch
Closure of the aortic valve produces a disturbance on the aortic pressure curve
Second heart sound
- Aortic valve closes slightly before the pulmonary valve, leading to this sound
- Inspiration delays the closure of the pulmonic valve, which causes the splitting of the S2
Why does splitting of S2 occur during inspiration?
- Associated decrease of intrathoracic pressure produces an increase in venous return to the right side of the heart
- Resulting increase in right ventricular EDV causes an increase in right ventricular SV and prolongs right ventricular ejection time
- Prolongation of ejection time delays the closure of the pulmonic valve relative to the aortic valve
Phase F: Rapid Ventricular Filling
- Ventricles relaxed
- Mitral valve opens
- Ventricles fill passively with blood from the atria
- Ventricular volume increases
- Ventricular pressure low
- Rapid ventricular filling may produce as a third heart sound
Third heart sound
Occurs early in diastole following the opening of the AV valves
Phase G: Rapid Ventricular Filling
- Ventricles relaxed
- Final phase of ventricular filling
Ventricular pressure-volume loop
Ventricular pressure plotted against volume
Heart murmurs
Sounds generated by turbulent flow in the surrounding
Five things heart murmurs may result from
- Flow across a partial obstruction (ex. aortic stenosis)
- Regurgitant flows across an incompetent valve (ex. mitral reguritation)
- Increase flow through normal structures (ex. Systolic murmurs associated with a high output state, during anemia)
- Ejection into dilated chamber (ex. dilated aorta)
- Abnormal shunting of blood from one vascular chamber to a lower pressure chamber (ex. ventricular septal defect)
Heart murmurs may be described by their (5)
- Timing
- Intensity
- Pitch
- Shape
- Location
Timing
Whether it occurs during systole, diastole, or is continuous (begins in systole and continues into diastole)
Intensity
- Ease in which it is identified/heard
- Murmurs are typically quantified by a grading system
Systolic murmur intensity grading
1/6 to 6/6 with 1 being barely audible and 6 being audible without stethoscope directly on chest wall
Diastolic murmur intensity grading
1/4 to 4/4 with 1 being barely audible and 4 being very loud
Pitch
- Frequency of the murmur ranging from high to low
- High frequency murmurs are caused by large pressure gradients between chambers (ex. aortic stenosis)
- Low frequency murmurs implies less of a pressure gradient between chambers (mitral stenosus)
Shape
How the murmur changes in intensity from onset to completion
Three examples of murmur shape
- Crescendo-decrescendo - first rises then fall off in intensity
- Decrescendo - murmur begins at maximum intensity, then falls
- Continuous - heard throughout the cardiac cycle
Location
- Region of maximal intensity
- Murmurs are often heard to radiate from their primary location to other areas of the chest. Such patterns of transmission relate to the direction of the turbulent flow
Three defects that cause heart murmurs
- Ventricular septal defect
- Changes in blood pressure in a particular region of the body
- Cardiac hypertrophy
Ventricular septal defect (murmur consequences)
- Initially, the increased blood return to the left ventricle augments stroke volume (via the Frank-Starling Mechanism)
- Over time the increased volume load can result in chamber dilation and heart failure
Changes in blood pressure in a particular region of the body - mitral stenosis example (murmur consequences)
- Blood dams up in the left atrium which means the left atrial pressure in higher than normal
- High left atrial pressure is passively transmitted to the pulmonary circulation which leads to…
- Increased pulmonary venous and capillary pressures leads to….
- The transduction of plasma into the lung interstitium which leads to…
- Congestive heart failure
Cardiac hypertrophy (murmur consequences)
Increase in workload of one or both ventricles
