Cardiac Cycle And Output Flashcards
Automaticity
Heart muscle is stimulated by nerves and is self excitable
How long is the Cardiac cycle?
0.8 seconds (75bpm)
What is the cardiac cycle?
Period between the start of one heart beat to the beginning of the next. Systole and diastole
Systole
Contraction of heart muscle, blood is pumped into the arteries, approx. 0.1 seconds
Diastole
Relaxation of heart muscle, heart filling with blood, approx. 0.7 seconds
What are the phases of the cardiac cycle?
Atrial systole > isovolumetric ventricular contraction > ejection > isovolumetric ventricular relaxation > passive ventricular filling
Atrial systole
Heart blood pressure is low as blood enters the atria from the pulmonary and systemic circulations and then flows into the ventricles. Approx. 80%, atrial kick forces the remaining 20% (AV valves open, semilunar valves closed)
Ventricular systole
Atria relaxes, ventricular pressures rise resulting in AV valves closing
Isovolumetric ventricular contraction
The ventricles are completely closed chambers, and pressure in the ventricles increase until the pressure is more than in the aorta/pulmonary trunks
Ventricular ejection
Opens semilunar valves
Isovolumetric ventricular relaxation
Ventricles relax, ventricular pressure drops. Backflow of blood in aorta and pulmonary trunk closes semilunar valves (dicrotic notch)
Dicrotic notch
Brief rise in aortic pressure caused by backflow of blood rebounding off semilunar valves
Passive ventricular filling
Blood has accumulated in the atria behind the closed atrioventricular valves passes rapidly into the ventricles
What are the cardiac cycle pressures?
Right atrium: 0-4 mmHg, right ventricle: 25 systolic mmHg, pulmonary arteries: 25 systolic mmHg, left atrium: 8-10mmHg, left ventricle: 120 systolic mmHg, aorta: 120 systolic mmHg
What is cardiac output (CO)?
The amount of blood pumped by each ventricle in one minute, it is the best indicator of adequate blood flow to the peripheral tissues. Product of heart rate (HR) and stroke volume (SV)
Heart rate
Number of beats per minute
Stroke volume
Amount of blood pumped out by a ventricle with each beat
Cardiac reserve
Difference between resting and maximal cardiac output
The pump at work cardiac output conversion
CO=SVxHR
Ejection fraction
The percentage of blood pumped out of a ventricle with each contraction. Used as a measure of ventricular function.
What is the normal ejection fraction?
Between 50% and 65%
Regulation of stroke volume
SV- end diastolic volume (EDV) minus end systolic volume (ESV)
End diastolic volume
Amount of blood collected in a ventricle during diastole
End systolic volume
Amount of blood remaining in a ventricle after contraction
What are the 4 factors affecting stroke volume?
Venous return, preload, contractility, afterload
Venous return
Amount of venous blood returned to the heart
Preload
Amount ventricles are stretched by contained blood
Contractility
Cardiac cell contractile force due to factors other than EDV
Afterload
Back pressure exerted by blood in the large arteries leaving the heart
Critical factor controlling stroke volume?
Preload or degree of stretch of the cardiac muscle cells before they contract
What increases stroke volume?
Slow heart beat and exercise increase venous return to the heart
What decreases stroke volume?
Blood loss and extremely rapid heart beat
Frank-Starling Law
More blood in more blood out. The greater the volume of blood in the ventricle, the stronger the contraction. Both increased filling time and increase blood volume = increased SV
What are the Extrinsic factors influencing stroke volume that increase contractility?
Increased sympathetic stimuli, certain hormones, Ca2+ and some drugs
What are the extrinsic factors influencing stroke volume that decrease contractility?
Acidosis, increased extra cellular K+, beta blockers and calcium channel blockers
Signs and symptoms of decreased cardiac output?
Acute changes in BP, acute changes in mental status, cold/clammy skin, colour changes in the skin and mucous membranes, crackles (rales), dyspnea, dysrhythmias, fatigue, orthopnea, restlessness
