Cardiovascular system revision note - Cardiac output
Cardiac output = stoke volume(SV) × heart rate(HR)
Cardiac output
- The volume of blood pumped by each ventricle within a unite time ( minute )
- Average = 5l / min
- Normal range = 4-8 l/min
Stroke volume
- The volume of blood pumped by each ventricle per heart beat
- Average = 70ml/ min
- Normal range = 55-100ml/min
Heart rate
- Changes in heart rate are more important quantitatively in producing changes in cardiac output than in stroke volume
- Average = 72 beats per min
- = 70ml/min × 72 beats / min
Stroke volume
- Stroke volume (SV) = End diastolic volume ( EDV) – End systolic volume ( ESV)
End diastolic volume ( EDV)
- The volume of blood remaing I the ventricle at the end of ventricular diastole
- 120ml
End systolic volume ( ESV)
- The volume of blood remaining in the ventricle at the end of ventricular systole
- 50ml
Ejection fraction (EF)
- The fraction of blood pumped out of a ventricle in a heart beat
- Normal range 55-70 %
- EF= SV/ EDV
- EF= EDV- ESV / EDV
Preload
- The degree to which the myocardium is stretched before it contracts
- The more the ventricle is stretched the more force it generates to push blood out
- Increased EDV cause increase stroke volume
Frank – starling law of the heart
- The energy of contraction is proportionate to the initial length of cardiac muscle fiber
Factors affecting EDV / preload
- Strength of atrial contraction
- Total blood volume
- Venous tone
- Skeletal muscle pump
- Respiratory pump
- Posture
- Intra – pericardial pressure
- Ventricular compliance
Factors affecting ESV
- After load
- Moyocardial contractility
After load
- The resistance against which the blood is expelled
- Is increased by –
- increased aortic pressure
- Increased systemic vascular resistance
Myocardial contractility ( inotropy )
- Sympathetic stimulation increases the strength of contraction of myocardium ( positive inotrophic effect)
- Parasympathetic stimulation has the opposite effect ( negative inotrophic effect )
- Hypoxia, hypercapnia, acidosis , quinidine , procainamide, barbiturates , heart failure depresses myocardial contractility .
- Metabolic end products increase force of contraction
- Xanthine produces a negative inotrophic effect
- Epinephrine and thyroxine increase contractility
- Digitalis increases contractility
Cardiac index
- Cardiac output per unit body surface area ( depend on the size of the person )
Measuring of cardiac output
- Direct method
- Indicator dilution method
- Doppler combined with echo cardiography
Labels: CVS, CVS Revision, Revision