Cardiac Output: Factors and Influences

• Cardiac output is the volume of blood pumped from each ventricle per minute.
• Cardiac output (ml/minute) is calculated by multiplying stroke volume (ml/beat) by heart rate (beats/min).
• An average heart rate is 70 bpm.
• An average stroke volume is 70-80 ml/beat.
• An average cardiac output is 5,500 ml/minute.

Factors Influencing Cardiac Output

• Cardiac output increases in younger people.
• Cardiac output is generally higher in males.
• Eating a meal can increase cardiac output by 30%.
• Exercise can increase cardiac output by up to 700%.
• Anxiety and fear can increase cardiac output from 50-100%.
• Body size influences cardiac output.
• Cardiac index measures cardiac output per minute per meter squared of the body surface area, averaging 3.2L/MIN/M^2.
• Changing from a recumbent position to an upright position will decrease cardiac output by 20-30% due to gravity.
• Venous return is the volume of blood returning to the right atrium per minute; an increase in cardiac output will increase venous return.
• Cardiac output can be affected by changes in cardiac rate, stroke volume, or both.
• The autonomic nerves primarily control cardiac rate; sympathetic stimulation increases the rate, while parasympathetic stimulation decreases it.
• Stroke volume is determined by neural input, where sympathetic stimuli strengthen myocardial muscle fiber contraction and parasympathetic stimuli have the opposite effect.
• Cardiac output distributes according to metabolic activity and function.
• Some vascular beds receive a larger percentage while others receive smaller amounts.
• Lungs receive 100% of Cardiac output.
• Brain receives 14% of Cardiac Output.
• Skeletal Muscle receives 15% of Cardiac Output.
• Bone receives 5% of Cardiac Output.
• Gastrointestinal system and Spleen receive 21% of Cardiac Output.
• Liver receives6% of Cardiac Output.
• Kidneys receive 22% of Cardiac Output.
• Skin receives 6% of Cardiac Output.
• Other areas receive 8% of Cardiac Output.

Stroke Volume

• Stroke volume is the amount of blood pumped by each ventricle per beat.
• The average stroke volume of a resting young man is about 70ml per beat.
• SV=EDV-ESV
• 70ml= 120ml-50ml
• Increased force of contraction decreases End Systolic Volume (ESV) by 10-20ml.
• Increased venous return leads to an increase in the End Diastolic Volume (EDV).
• An increase in End Diastolic Volume (EDV) or a decrease in End Systolic Volume (ESV) will increase Stroke Volume (SV).

Heart Rate

• Heart rate: the number of heart beats per minute, with a normal rate being 72 beats per minute.
• Average heart rate ranges between 60-100 beats per minute.
• The radial pulse can be felt on the wrist, just below the thumb.
• Heart rate can be calculated by counting arterial pulsation, counting the R-R interval, or counting heart beats by stethoscope.

Factors influencing heart rate:

• Age: Highest in newly born infants (130 beats/min), decreasing to a steady value at maturity.
• Sex: Higher in females at 85 beats/min.
• Posture: Increases up to 25% during standing, then returns to its normal basal value in recumbent position.
• Emotion: Excitement, fear, and anger increase heart rate.
• Food intake: Caffeine increases heart rate.
• Temperature: A 1°C increase in body temperature leads to an increase of 20 beats/min in heart rate.
• Exercise: Increases heart rate.
• Time of day: Highest in the evening (85 beats/min) and lowest in the early morning (65 beats/min) due to variation in body temperature.
• Body temperature is lowest (36.5°C) in the early morning and highest (37.5°C) in the evening.
• Sleep: Slows down to 60 beats/min during deep sleep.
• Pain: Pain sensation increases heart rate.
• Effect of Potassium Ions: Excess potassium causes the heart to become dilated and flaccid, slowing heart rate and potentially blocking cardiac impulse conduction; High potassium concentration weakens the heart, causes abnormal rhythm, and can lead to death.
• Effect of Calcium Ions: Excess calcium causes effects opposite to those of potassium, causing heart to go toward spastic contraction.
• Conversely, deficiency of calcium ions causes cardiac flaccidity, similar to the effect of high potassium.

Heart Rate on Systole and Diastole

• With a normal heart rate of 72 beats/min, systole lasts 40% of the cardiac cycle, and diastole lasts 60% of the cardiac cycle.
• An increased heart rate decreases the duration of both systole and diastole.
• The decrease in the duration of the relaxation phase (filling phase) is greater than the decrease in the contraction phase.
• At very high heart rates (more than 200 beats/min), systole lasts about 60% of the cardiac cycle, while diastole lasts about 40%. The ventricle may not relax enough to obtain a maximum filling at very high rates.
• Decreased filling leads to decreased End Diastolic Volume (EDV), reduced Stroke Volume (SV), and decreased Cardiac Output (CO) as SV=EDV-ESV.

Arterial and Venous Pressure

• Pressure pulse (pressure wave) refers to the waves of pressure generated by changes in pulse.
• Examples include ventricular pulse, atrial pulse, aortic pulse, radial pulse, and jugular pulse.
• Blood flowing in vessels has a pressure wave that travels faster than the blood's velocity, expanding vessel walls, and being palpable as the pulse.
• With each contraction, the left ventricle ejects blood into the aorta and the arterial tree.
• A pressure wave moves rapidly through the arterial system, creating the arterial pulse.
• Pressure changes from the left ventricle's ejection into the aorta travel through the arterial tree faster than the forward movement of blood.
• The pressure change can be felt as pulsation (pulse) in large vessels and recorded by a transducer as a pressure pulse curve.
• The pulse shape changes peripherally from the aorta and through the distributing arteries.
• The pressure in the capillary is not pulsatile.

Height of Jugular Venous Pressure (JVP)

• The waveform of the JVP results from changes in pressure in the right atrium.
• The mnemonic ASK ME can be used to remember the JVP waveform.
• The 'a wave' is due to atrial contraction; JVP is highest here.
• The 'c wave' is due to the beginning of systole, which may be due to pulse from the internal carotid artery or the closure of the tricuspid valve pushing into the right atrium.
• The 'x descent' occurs when the atrium is relaxed and the tricuspid valve is closed.
• The 'v wave' occurs at the end of systole when the atrium is at its highest volume
• The 'y descent' occurs at the beginning of diastole when blood moves from the right atrium to the right ventricle.
• Tricuspid stenosis has abnormal ‘a’ wave.
• Tricuspid regurgitation shows a giant ‘c’ wave.

Preload and Afterload

• Preloadis the degree to which the myocardium is stretched before it contracts.
• Afterload is the resistance against which blood is expelled.