Cardiac Output and Stroke Volume

• Intended learning outcomes include defining cardiac output, explaining intrinsic regulation of stroke volume and listing factors that affect venous return

Cardiac Output (COP)

• COP is the amount of blood pumped by each ventricle per minute
• Normal value is 5.5 L/min and is equal in both ventricles
• Equals minute volume
• Calculated as Heart Rate (HR) x Stroke Volume (SV)
• Not an actual volume, but a calculated volume
• COP can be expressed as L/min, with a normal value of ~5 L/min
• Cardiac Index = COP / m^2 of body surface area = 3.2 L/m2/min

Stroke Volume (SV)

• SV is the amount of blood pumped around the body by the left ventricle in one contraction
• SV = End Diastolic Volume (EDV) - End Systolic Volume (ESV)
• SV is a real volume
• Normal SV is 60-90 ml with a mean of 80 ml
• Calculated as 130 ml (EDV) - 50 ml (ESV) = 80 ml

End Diastolic Volume (EDV)

• EDV is the volume of blood in the ventricles at the end of diastole
• Normal EDV ranges from 110-140 ml with a mean of 130 ml
• Increased venous return increases EDV which increases preload, that consequently increases SV according to Frank Starling law

End Systolic Volume (ESV)

• ESV is the volume remaining in the ventricle after the end of ejection
• Normal ESV ranges from 50-70 ml, with an average of 60 ml

Heart Rate (HR)

• Heart Rate (HR) is the number of beats per minute
• Normal resting HR is 60-90 bpm, with a mean of 70 bpm
• Increased HR (up to 150 bpm) occurs by sympathetic stimulation which increases COP, provided SV is not decreased

Ejection Fraction

• Ejection Fraction is the fraction of EDV that is ejected during each contraction
• Normal is around 50-65%
• Ejection fraction is calculated as SV/EDV x 100, normally equals 65% which is 2/3 EDV

Cardiac Vocabulary

• Preload is the muscle length prior to contractility that is dependent on ventricular filling or end diastolic volume (EDV)
• Preload is influenced by venous return and is the fiber length or tension from which the heart contracts
• Venous return is the most important determining factor for preload
• The value of preload is related to right atrial pressure
• Afterload is the tension/arterial pressure against which the ventricle must contract
• If arterial pressure increases, afterload also increases
• Afterload for the left ventricle is determined by aortic pressure
• Afterload for the right ventricle is determined by pulmonary artery pressure
• An Increase in afterload results in decreased strength, velocity and duration of LV ejection, decreasing SV and increasing ESV
• The LV ms will become thicker (LV hypertrophy) to maintain normal SV against high resistance at low costs of oxygen consumption
• Decrease in afterload results in increased SV and COP at a low cost of energy and oxygen consumption.

Variations in Cardiac Output (COP)

• Varies in different individuals and in the same individual under different conditions
• Posture affects venous return and COP
  • COP decreases 25% from its level during standing after recumbency
• Physical and mental activity
  • Exercise increases COP up to 25L/min in sedentary and 35 L/min in athletes
  • Increases in anxiety and excitation
  • It does not change in quiet sleep only in dreams of exercise or stressful conditions.
• Temperature
  • Hot humid climate increases COP 40%
  • Low body temperature as in hypothermic cardioplegia & open heart surgery decreases COP
• Eating increases COP, especially with a heavy meal
• During Pregnancy, COP increases 10% in late pregnancy due to increased metabolism of fetus and mother
• Hormones and drugs increase COP (E, NE, thyroxine, histamine)
• Pathological conditions
  • Anemia, fevers, thyrotoxicosis increase COP
  • Heart failure, rapid arrhythmias & valve disease decrease COP

Regulation of Cardiac Output

• The COP is regulated extrinsically and intrinsically
• Extrinsic factors affecting SV and HR, include
  • ANS [symp* →↑ COP (↑HR & ↑ SV) and parasympath →↓ COP (↓HR & ↓ SV).
  • Hormones such has CA & thyroxine
  • Glucagon (positive inotropic)
• Intrinsic factors affect the SV only and include
  • Heterometric regulation
• Factors affecting SV include Preload
  • Increased initial length of myocardium (↑ EDV) by increasing in VR, increasing contraction & SV
  • According to Starling law: within limit, Preload, cardiac suction, and nervous and hormonal compensation all help the heart function.

Factors Affecting Heart Rate

• Parasympathetic stimulation inhibits the heart rate; negative chronotropic effect
• Sympathetic stimulation increases the heart rate; positive chronotropic effect
• Epinephrine hormone (released from the adrenal medulla) increases the heart rate; positive chronotropic effect
• The nervous system controls heart rate
  • Two nerves link the cardiovascular center in the medulla oblongata of the brain with the SA node of the heart
  • Accelerator nerve: sympathetic NS, when stimulated releases neurotransmitter at the SA node to INCREASE heart rate
  • Vagus nerve: parasympathetic NS, when stimulated releases neurotransmitter at the SA node to DECREASE heart rate

Venous Return

• Represents volume of blood flow returning to heart from veins
• The skeletal muscle pump involves muscle contraction & valves
• The respiratory pump moves blood into the right atrium
• Lower thoracic pressure increases abdominal pressure during inhalation
• VR = (MSFP - RAP)/RVR, where
  • MSFP = Mean systemic filling pressure
  • RAP= Right atrial pressure
  • RVR = Resistance to venous return
• MSFP (mean systemic venous pressure) is the pressure measured everywhere in the systemic circulation after blood flow has been stopped by clamping large BVs at heart
• MCP (mean circulatory pressure) is the mean pressure in both systemic & pulmonary circulation; MSFP MCP (pulmonary circulation with little contribution)
• CVP equals central venous pressure
• MCP= 7 mmHg is the systemic filling pressure in the living, peripheral venous pressure (6-8 mmHg)
• Central venous = 0-5 mmHg is the pressure in great veins at their entry to the heart = RAP

Physiological Factors Helping Venous Return Against Gravity

• Respiratory pump
  • During inspiration, the intra-thoracic pressure becomes more negative
  • The intermittent inspiration functions as an intermittent suction pump
  • During inspiration, the intra-abdominal pressure becomes more positive i.e. intermittent pressure pump.
• Skeletal muscle pump
  • Skeletal muscles surround the veins in the lower limbs
  • During muscle contraction, the veins are squeezed & the blood is shifted toward the heart
  • During muscle relaxation, the valves prevent the back flow of blood
• Cardiac suction
  • Refers to the rapid filling phase, during which the rapid expansion of the ventricles decreases Ventricular pressure & blood is sucked into the ventricles
• The sympathetic vasoconstrictor tone
  • It prevents the shift of blood to the veins of lower limbs (being constricted)