8 - CARDIAC CYCLE AND OUTPUT Flashcards
CARDIAC CYCLE
- Cardiac cycle is one heart beat
- Systole is contraction. Generally taken to mean ventricular contraction and ejection
- Diastole is relaxation. Or the rest of the cycle: ventricular relaxation and filling
- ECG traces the phases by measuring current flow (lecture 3)
CARDIAC CYCLE BASICS
- Atrial contraction
- isovolumetric contraction
- ventricular ejection
- isovolumetric relaxation
- ventricular filling
CYCLE SUMMARY: TIMINGS
• At a heart rate of 75bpm, each whole cardiac cycle lasts 0.8 seconds
• The whole heart is in diastole for 0.4 seconds, atrial systole lasts 0.1 seconds and ventricular systole lasts 0.3 seconds
The heart is physically capable of beating at more than 200bpm, however the volume of blood pumped by the heart starts to decrease above this rate –why?The cardiac cycle at 200bpm would last 0.3 seconds, with the time available for atrial filling, about 0.2 seconds – not enough!
HEART SOUNDS
There are 4 heart sounds, but only 2 are loud enough to be heard (auscultation)
First heart sound (LUBB)– turbulence caused by closure of the AV valves (happens when the ventricles contract)
Second heart sound (DUPP) – turbulence caused by semilunar valves closing (when the ventricles stop contracting)
3rd and 4th sounds from ventricular filling and atrial systole. 4th Sound audible when ventricles are stif
- Atrial Systole
• Atria contract, squeezing blood into the ventricles, through the valves.
• AV valves open. Pulmonic and Aortic closed
• Slight increase in atrial pressures
End Diastolic Volume = ventricular vol + atrial contribution (10% at rest)
EDV= 105ml +25ml = 130ml
- Isovolumetric Contraction:
- All valves closed.
- Beginning of systole- ventricles depolarise.
- Increase in intraventricular pressure from contraction.
- Heart shape change but no blood is ejected.
- Pushes AV valves closed. First sound
- Rapid ejection.
• AV valves closed , others open.
• When intraventricular P is higher than the aortic and Pulmonary P, the valves open and blood is ejected.
• Atria continue to fill.
- L: LVP exceeds aortic P of 80mm/Hg for SL valves to open and ejection. P increase to 120mm/Hg
- R: RVP exceeds pulmonary P of 20mm/Hg for SL valves to open and ejection. P increases to 25-30mm/Hg
No heart sounds in healthy patient. Ejection sounds indicate a shunt or valve disease.
- Reduced ejection.
- Aortic and pulmonary valves stay open and AV valves stay closed. No movement of blood
- Ventricular repolarisation and muscle relaxation. Ventricular P decreases slightly but blood still leaves the heart (kinetic energy). Atrial P increasing as atria continue to fill
- Isovolumetric relaxation.
• Valves close ( Heart sound 2). Aortic first, then pulmonic valve.
Ventricle volume remains the same as valves are closed (dicrotic wave). Atrial pressure and volume increase from venous return
End Systolic Volume = volume remaining in the ventricles after ejection
Ie ESV = (EDV)130ml – 70ml = 60ml
- Rapid Filling.
- AV valves open. Aortic and pulmonary valves close.
- Ventricular filling. Relaxation phase still.
- Amount of filling decreases when HR increases.
- Atrial P falls
- Third sound (not usually audible without specialist equipment)
- Reduced Filling.
- Difficult to distinguish these phases.
- When filling is nearly finished , ventricles at full stretch so P rises. P in large vessels drops as blood flows into circulation
CARDIAC OUTPUT
To work out a cardiac output you first need to know the Stroke Volume (SV):
SV = End-diastolic vol (EDV) – (ESV) End-systolic vol
EDV: Amount of blood collecting in ventricle (~130ml) Related to filling time and rate of venous return
ESV: amount remaining after contraction (~60ml) Preload, Contractility, Afterload
CALCULATING CARDIAC OUTPUT
Cardiac output = stroke volume X Heart rate
CO = (mL/min) SV = (mL/beat) HR = (beat/min)
REGULATION OF CARDIAC OUTPUT: HEART RATE
Cardiac Output is affected by the control of heart rate
• Neural control: physical or emotional stress
– Sympathetic nervous system stimulates heart rate (SA node). Up to 100-
200%
– Parasympathetic nervous system steadies HR
• Ion levels:
– Calcium: too little: weak. Too much: long contractions
– Potassium: involved in muscle contraction and nerve conduction.
Heart rate increase can increase CO to a point. CO increase not proportional to HR increase.
STARLING’S LAW AND STROKE VOLUME
Starlings law: bigger SV ejected if there is a larger degree of filling at the end of diastole.
- When the rate at which blood flows into the heart from the veins changes, the heart automatically adjusts its output to match the inflow
- If an increase in end-diastolic volume occurs, the force of ventricular contraction rises, producing an increase in stroke volume and cardiac output
- Stretching of muscle fibres increases contraction force
