Cardiac Output Flashcards
What is cardiac output and how is it calculated?
Cardiac output is the volume of blood pumped by the ventricle per minute
Cardiac output = Stroke volume x Heart rate
What is the average value of cardiac output?
5-5.5l
How does exercise affect cardiac output?
Increases the cardiac output up to 25L/min
How are minute volume and cardiac output related?
Minute volume = cardiac output at rest
What is cardiac index?
Volume of blood pumped by each ventricle per meter square body surface area per minute (3.2 L/m2/min)
What is venous return?
Venous return is the volume of blood returning to the right side of the heart in a minute
Equal to CO
What is ejection fraction?
Percentage ratio of stroke volume to the end diastolic volume
How is ejection fraction calculated?
Ejection Fraction (%) = (Stroke Volume/End DIastolic Volume) x 100
Range of regular EF% values?
>60%
What is the significance of EF%?
Indicator of myocardial contractility and it decreases in heart failure
How can cardiac output be measured?
Echocardiography
Fick’s principle
Dye dilution method
What is Fick’s principle?
Cardiac Output = (O2 consumption)/(Arterial PO2-Venous PO2)
What are the types of variations in cardiac output?
Physiological variations
Pathological variations
What physiological factors may influence cardiac output?
Posture: Cardiac output decreases in standing position
Digestion: Cardiac output decreases
Temperature: Increases in high temperature and very low temperature (shivering)
Excitation: Increases
Exercise: Increases
Pregnancy: Increases
What pathological factors may increase cardiac output?
Anemias
Hyperthyroidism and fever
Aortic regurge
What physiological factors decrease cardiac output?
Paroxysmal tachycardia
Complete heart block
Myocardial ischemia and infarction
Shock
Mitral stenosis or regurge & aortic stenosis
Heart failure
Why is aortic regurge the only valve disease which increases cardiac output?
Aortic regurge increases stroke volume and causes blood to flow back to the ventricles from the aorta
What are the factors affecting cardiac output
Preload
Afterload
Heart rate
Ventricular contraction
How does venous return/preload affect cardiac output?
Preload determines the degree of stretch of the myocardium
Increased venous return increases the stroke volume (by Starling Law) and Heart Rate (by Bainbridge reflex)
What are the factors affecting venous return?
Pumping action of the heart: Increase VR
Right atrial pressure (RAP): Decrease VR
Respiratory movement: Increase VR
Gravity: Decreases VR
Diameter of arterioles and veins and opening of capillary: Increases venous return
Skeletal muscle contraction: Increases venous return
Blood volume: Increases venous return
How does arterial blood pressure/afterload affect the cardiac output?
Afterload affects the aortic pressure, arterial wall rigidity and blood viscosity (i.e. Arterial blood pressure/ABP )
Sudden rise in ABP
First systolic beat: Decreased stroke volume
Second systolic beat: EDV is increased → increase contraction (Starling law)
How does heart rate between 50 and 200 bpm with constant venous return affect cardiac output?
No cardiac output effect
How does heart rate below 50bpm with constant venous return affect cardiac output?
Complete heart block → marked decrease in heart rate
How does heart rate above 200bpm with constant venous return affect cardiac output?
Similar to situation in paroxysmal tachycardia: shortening of the diastole → decrease filling → marked decrease in systolic volume
