Cardiac Output

Introduction

Cardiac output reflects the heart’s ability to deliver blood efficiently to meet the metabolic demands of the body. Its regulation depends on neural influences as well as intrinsic cardiac mechanisms that adjust pumping performance in response to changes in ventricular filling and arterial resistance.

• The heart maintains systemic and pulmonary circulation by continuously pumping blood.
• Adequate blood flow is essential for delivering oxygen and nutrients to tissues and removing metabolic waste products.
• Cardiac output is the volume of blood pumped by each ventricle per minute and is an important indicator of cardiac performance.
• Assessment of cardiac output provides valuable information about cardiovascular function in health and disease.
• Measurement of cardiac output is routinely performed in cardiology practice, critical care settings, and increasingly through noninvasive echocardiographic techniques.

Definitions and Factors Affecting

Definitions and Normal Values

Cardiac Output

• Cardiac output is the volume of blood pumped by each ventricle per minute.
• It is determined by the product of stroke volume and heart rate.
• Cardiac output = Stroke volume × Heart rate.
• In a healthy resting adult, normal cardiac output is approximately 5–6 liters per minute.

Stroke Volume

• Stroke volume is the amount of blood ejected by a ventricle during a single cardiac cycle.
• The average resting stroke volume is about 70 milliliters per beat.
• For example, a stroke volume of 70 milliliters and a heart rate of 70 beats per minute produce a cardiac output of approximately 4.9 liters per minute.

Cardiac Index (CI)

• The cardiac index is cardiac output adjusted for body surface area.
• It allows meaningful comparison of cardiac performance among individuals of different body sizes.
• Cardiac index is calculated by dividing cardiac output by body surface area.
• The normal value is approximately 2.5–4.0 liters per minute per square meter, with an average of about 3.0–3.5 liters per minute per square meter.

End-diastolic Volume

• End-diastolic volume is the volume of blood present in a ventricle at the end of ventricular filling.
• The normal value is approximately 120–130 milliliters.

Ejection Fraction

• Ejection fraction is the percentage of end-diastolic volume ejected during ventricular systole.
• It is calculated as stroke volume divided by end-diastolic volume, multiplied by 100.
• Normal ejection fraction is approximately 55–70%.
• It is an important indicator of ventricular contractile function.

End-systolic Volume

• End-systolic volume is the volume of blood remaining in a ventricle after systole.
• The normal value is approximately 50 milliliters.

Cardiac Reserve

• Cardiac reserve is the additional cardiac output that can be generated above the resting level.
• It reflects the heart’s capacity to meet increased metabolic demands.
• Cardiac reserve is greater in trained athletes than in untrained individuals.

Physiological Conditions that Alter Cardiac Output

A. Conditions that increase cardiac output

• Cardiac output increases during exercise, emotional stress, anxiety, excitement, pregnancy, after meals, and exposure to extreme heat.
• These conditions increase tissue metabolic demands or enhance cardiovascular activity.

B. Conditions that decrease cardiac output

• Cardiac output decreases when a person suddenly moves from a lying to a standing position.
• Excessive sweating may reduce cardiac output by decreasing plasma volume and venous return.

C. Conditions that do not change cardiac output

• Sleep generally causes little change in overall cardiac output because reductions in metabolic demand are balanced by cardiovascular adjustments.
• Mild to moderate changes in environmental temperature usually have minimal influence on cardiac output.

Age

• Cardiac output tends to decline with advancing age because of reductions in maximal heart rate and cardiac performance.
• In children, cardiac output is lower than in adults mainly because of a smaller stroke volume, despite a higher heart rate.

Gender

Average cardiac output is slightly lower in females than in males, largely because of differences in body size and body surface area.

Exercise

• Exercise markedly increases cardiac output.
• Sympathetic stimulation increases heart rate and myocardial contractility.
• Venoconstriction enhances venous return, increasing end-diastolic volume and stroke volume.
• During intense exercise, cardiac output may increase five- to sevenfold above resting levels.

Excitement

Emotional arousal increases sympathetic activity, leading to an increase in cardiac output.

Pregnancy

Pregnancy increases cardiac output because of expansion of blood volume and increased metabolic demands.

Posture

• Standing reduces venous return because blood temporarily pools in the veins of the lower limbs under the influence of gravity.

Environmental Temperature

• Exposure to high environmental temperatures increases cardiac output through sympathetic activation and increased blood flow to the skin.
• Exposure to severe cold has the opposite effect.

Measurement of Cardiac Output

• Cardiac output can be measured using direct or indirect methods.
• Direct measurement involves determination of blood flow in the ascending aorta using specialized flow-measuring devices and is mainly restricted to experimental studies or selected surgical settings.
• In clinical practice, Doppler echocardiography is a widely used noninvasive method for estimating cardiac output.
• Common indirect methods include the Fick principle, indicator dilution technique, thermodilution method, echocardiography, pulse-pressure analysis, and selected imaging-based techniques.
• Each method estimates cardiac output by assessing blood flow, oxygen utilization, or indicator distribution within the circulation.

Fick Method

Definition

• The Fick principle states that the uptake or release of a substance by an organ equals blood flow through the organ multiplied by the arteriovenous concentration difference of that substance.
• For cardiac output measurement, oxygen is commonly used as the indicator substance.

Procedure

• Cardiac output is calculated by dividing whole-body oxygen consumption by the difference between arterial and mixed venous oxygen content.
• Oxygen consumption is measured over a defined period using respiratory gas analysis.
• Arterial blood is obtained from a peripheral artery to determine arterial oxygen content.
• Mixed venous blood is collected from the pulmonary artery using a catheter to determine venous oxygen content.
• The formula is:

Cardiac Output = Oxygen Consumption ÷ (Arterial Oxygen Content − Mixed Venous Oxygen Content)

$$\mathrm{Output\ of\ left\ ventricle} = \frac{\mathrm{O_2\ consumption\ (mL/min)}}{\mathrm{(AO_2) – (VO_2)}}$$

$$\mathrm= \frac{\mathrm{250mL/min}}{\mathrm{200mL – 150mL}}$$

$$\mathrm= \frac{\mathrm{250mL/min}}{\mathrm{50mL}}$$

$$\mathrm= \mathrm{5L/min}$$

Advantages

• The method provides relatively accurate measurements of cardiac output.
• No indicator dye or tracer substance is required.
• It is considered a reference method for validating other techniques.

Disadvantages

• Pulmonary artery catheterization is an invasive procedure requiring trained personnel.
• The procedure is generally performed in specialized clinical settings.
• Anxiety and procedural stress may alter cardiovascular parameters and influence results.
• Simultaneous measurement of oxygen consumption increases technical complexity.
• The method is less practical for routine use during exercise or in ambulatory individuals.
• Advances in Doppler echocardiography and thermodilution techniques have reduced its routine clinical application.

Indicator Dilution Method

Principle

• The indicator dilution method estimates cardiac output by analyzing the dilution of a known quantity of indicator injected into the bloodstream.
• The indicator may be a dye, such as Evans blue or indocyanine green, or a suitable tracer substance.
• After injection, the indicator mixes with circulating blood and passes through the heart and systemic circulation.
• Cardiac output is calculated from the amount of indicator administered and its concentration in arterial blood during the first circulation.

Procedure

• A known quantity of indicator is injected rapidly into a peripheral vein, usually in the upper limb.
• Arterial blood samples are collected at short, regular intervals after injection.
• The concentration of the indicator in each sample is measured using appropriate analytical techniques, such as spectrophotometry for dyes.
• A concentration–time curve is constructed by plotting indicator concentration against time.
• Initially, indicator concentration rises as the leading portion of the indicator reaches the arterial sampling site.
• The concentration then reaches a peak when the largest amount of indicator arrives.
• Subsequently, the concentration declines as the indicator becomes progressively diluted within the circulation.
• The resulting graph contains an ascending limb, a peak, and a descending limb.
• After the first circulation, recirculation of the indicator may cause a secondary rise in concentration.
• To avoid errors from recirculation, the descending limb of the first-pass curve is extrapolated mathematically.
• Cardiac output is inversely related to the average concentration of the indicator during its first passage through the circulation.
• A larger cardiac output causes greater dilution of the indicator and therefore a lower measured concentration.
• Conversely, a lower cardiac output results in less dilution and a higher concentration.
• Mathematical analysis of the area under the first-pass dilution curve is used to calculate cardiac output accurately.

Advantages

• The method provides reliable quantitative assessment of cardiac output.
• It has contributed significantly to the development of modern hemodynamic monitoring techniques.
• The underlying principle is also applied in several contemporary indicator-based and thermodilution methods used in cardiovascular research and clinical practice.

Disadvantages

Repeated measurements within a short interval may produce errors due to residual indicator remaining in circulation.

Thermodilution Method

Principle

• The thermodilution method is based on the indicator dilution principle, using cold saline as the indicator.
• A specialized catheter with injection and temperature-sensing ports is used.
• Cold saline is injected into the right atrium through the catheter.
• A thermistor located in the pulmonary artery records the resulting change in blood temperature.
• The magnitude and duration of the temperature change are used to calculate cardiac output.
• Greater blood flow produces a smaller and shorter temperature change.

Advantages

• Saline is safe, inexpensive, and non-toxic.
• Temperature changes dissipate rapidly, minimizing errors from recirculation.
• Measurements can be repeated multiple times when necessary.
• The method is useful in critically ill patients requiring hemodynamic monitoring.
• It is widely used in intensive care settings.

Disadvantages

The procedure requires cardiac catheterization, making it an invasive method.

Other Methods

Ballistocardiography

• Ballistocardiography records body movements generated by the ejection of blood during each heartbeat.
• These mechanical vibrations are converted into waveforms and analyzed to estimate cardiac output.
• The method has limited accuracy and is rarely used in modern clinical practice.

Echocardiography

• Echocardiography is a noninvasive technique that uses ultrasound waves to visualize cardiac structures.
• When combined with Doppler imaging, it measures blood flow velocity and estimates flow volume across cardiac valves.
• It provides a reliable assessment of cardiac output.
• The technique is also useful for evaluating chamber size, ventricular function, and valvular abnormalities.

X-ray Method

• This method involves administration of a contrast agent followed by serial imaging of the heart during systole and diastole.
• Computer-based analysis of ventricular volume changes can be used to estimate cardiac output.
• Because of radiation exposure and availability of superior techniques, it is used infrequently.

Pulse-Pressure Method

• Pulse pressure, the difference between systolic and diastolic arterial pressures, provides only a rough estimate of cardiac output.
• The method is influenced by arterial compliance and should not be used for precise measurement.

Factors Affecting Cardiac Output

• Cardiac output is determined by stroke volume and heart rate.
• Any factor that alters either of these variables influences cardiac output.

Factors Affecting Stroke Volume

• Stroke volume depends on three principal determinants.
• Preload reflects ventricular filling and end-diastolic volume.
• Myocardial contractility represents the intrinsic force of ventricular contraction.
• Afterload is the resistance against which the ventricle ejects blood.
• Changes in preload, contractility, or afterload directly affect stroke volume and cardiac output.

Preload

• Preload refers to the degree of ventricular filling at the end of diastole and is commonly represented by the end-diastolic volume.
• Within physiological limits, an increase in end-diastolic volume increases stroke volume, whereas a decrease in end-diastolic volume reduces stroke volume.
• This relationship is explained by the Frank–Starling mechanism, which states that greater myocardial fiber stretch before contraction produces a stronger contraction.
• The initial length of cardiac muscle fibers depends on the extent of ventricular filling during diastole.
• Increased ventricular filling stretches myocardial fibers and enhances the force of contraction.
• This intrinsic adjustment of stroke volume in response to changes in ventricular filling is called heterometric autoregulation.

End-diastolic volume depends primarily on venous return, atrial contraction, and ventricular compliance.

Venous Return

• Venous return is the volume of blood returning to the right atrium from the systemic circulation.
• It is a major determinant of ventricular filling and cardiac output.
• Factors influencing venous return include skeletal muscle activity, respiratory movements, blood volume, and sympathetic stimulation.

Skeletal Muscle Pump

• Veins of the limbs are surrounded by skeletal muscles and contain one-way valves.
• Contraction of skeletal muscles compresses veins and propels blood toward the heart.
• Venous valves prevent backward flow of blood.
• Increased muscle activity, such as walking or exercise, enhances venous return and cardiac output.
• Prolonged standing without movement promotes venous pooling in the lower limbs.
• Reduced venous return during prolonged standing may lower cardiac output and occasionally cause syncope.

Thoracic Pump

• Inspiration enhances venous return through the thoracic pump mechanism.
• Expansion of the thoracic cavity lowers intrathoracic pressure.
• The resulting decrease in central venous pressure facilitates blood flow toward the heart.

Abdominal Pump

• During inspiration, descent of the diaphragm increases intra-abdominal pressure.
• Compression of abdominal veins directs blood toward the thoracic cavity and right atrium.
• Venous valves prevent backward flow into the lower limbs.

ECF Volume

• Venous return is strongly influenced by plasma volume and total extracellular fluid volume.
• Fluid loss from conditions such as diarrhea, vomiting, or hemorrhage reduces venous return and cardiac output.
• Expansion of plasma volume, as occurs during pregnancy, increases venous return and cardiac output.

Sympathetic Activity

• Veins contain abundant sympathetic innervation.
• Sympathetic stimulation causes venoconstriction, which mobilizes blood from the venous reservoir and increases venous return.
• Reduced sympathetic activity causes venodilation and promotes venous pooling, thereby decreasing cardiac output.

Atrial Pump Activity

• Most ventricular filling occurs passively during ventricular diastole.
• At rest, atrial contraction contributes approximately 15–20% of ventricular filling.
• Under normal conditions, this contribution has a modest effect on stroke volume.
• During exercise or increased metabolic demand, atrial contraction becomes more important.
• Sympathetic stimulation strengthens atrial contraction and augments ventricular filling.

Ventricular Compliance

• Ventricular compliance refers to the ability of the ventricular wall to expand during filling.
• Normal ventricular compliance allows adequate filling at relatively low pressures.
• Reduced compliance impairs ventricular filling and lowers end-diastolic volume.
• Conditions such as restrictive cardiomyopathy, hypertrophic cardiomyopathy, and infiltrative myocardial diseases decrease ventricular compliance.
• External restriction of ventricular expansion can also reduce filling.
• In pericardial effusion, excess fluid within the pericardial cavity increases intrapericardial pressure and limits ventricular expansion.
• Severe pericardial effusion causing cardiac tamponade markedly reduces ventricular filling, stroke volume, and cardiac output.

Myocardial Contractility

• Myocardial contractility is the intrinsic ability of cardiac muscle to generate force during contraction.
• It is a major determinant of stroke volume and cardiac output.
• Factors that increase contractility are termed positive inotropic factors, whereas those that decrease contractility are termed negative inotropic factors.
• Contractility is influenced by myocardial mass, autonomic nervous activity, circulating hormones, drugs, electrolyte balance, and heart rate.
• Increased contractility enhances ventricular emptying and stroke volume, whereas reduced contractility lowers cardiac output.
• On ventricular function curves, enhanced contractility shifts the curve upward and to the left, while reduced contractility shifts it downward and to the right.

Ventricular Muscle Mass

• Adequate ventricular muscle mass is essential for normal cardiac pumping function.
• Loss of myocardium following myocardial infarction reduces contractile capacity and decreases cardiac output.
• Certain cardiomyopathies may also reduce effective muscle mass and impair ventricular performance.
• Regular physical training can increase ventricular muscle mass through physiological cardiac adaptation.
• In trained athletes, mild physiological hypertrophy improves cardiac performance.
• During exercise, these individuals often achieve higher cardiac outputs primarily through increased stroke volume rather than marked increases in heart rate.

Autonomic Activity

• Sympathetic stimulation increases myocardial contractility and enhances stroke volume by activating β₁-adrenergic receptors in ventricular muscle.
• Reduced sympathetic activity decreases contractile force and cardiac output.
• Ventricular innervation by parasympathetic fibers is relatively sparse.
• Therefore, parasympathetic stimulation has only a minor direct effect on ventricular contractility.
• However, parasympathetic activity decreases cardiac output primarily by reducing heart rate.

Hormonal Factors

Catecholamines

• Catecholamines are powerful positive inotropic agents.
• They stimulate β₁-adrenergic receptors and increase intracellular cyclic adenosine monophosphate.
• This enhances calcium entry into cardiac muscle cells and strengthens contraction.

Acetylcholine

• Acetylcholine activates muscarinic receptors and reduces cyclic adenosine monophosphate formation.
• Consequently, myocardial contractility decreases slightly.

Glucagon

• Glucagon increases intracellular cyclic adenosine monophosphate and enhances myocardial contractility.
• It may be used clinically in selected situations to improve cardiac performance.

Insulin

Insulin exerts a mild positive inotropic effect and supports normal myocardial metabolism.

Thyroxine

• Thyroxine increases cardiac output by enhancing heart rate and myocardial contractility.
• It increases β₁-adrenergic receptor expression and sensitivity to catecholamines.
• It also increases myosin adenosine triphosphatase activity, thereby improving the force of ventricular contraction.

Chemical Factors

Xanthines

Xanthines, such as caffeine and theophylline, increase myocardial contractility by inhibiting cyclic adenosine monophosphate degradation.

Inhibiting Factors

• Hypercapnia, hypoxia, and acidosis depress myocardial contractile function.
• General anesthetic agents and various toxins can also reduce myocardial contractility, leading to decreased cardiac output.

Drugs

Digitalis

• Digitalis increases myocardial contractility by inhibiting sodium–potassium adenosine triphosphatase activity in cardiac muscle cells.
• This action increases intracellular calcium availability, resulting in stronger myocardial contraction.
• Digitalis is therefore classified as a positive inotropic agent.
• Some drugs depress myocardial contractility and may reduce cardiac output.

Other Drugs

Examples include quinidine, procainamide, and barbiturates, which exert negative inotropic effects on the myocardium.

Afterload

• Afterload is the resistance that the ventricles must overcome to eject blood into the arterial circulation.
• It is closely related to systemic vascular resistance and arterial pressure.
• An increase in afterload makes ventricular ejection more difficult and tends to reduce stroke volume.
• A decrease in afterload facilitates ventricular emptying and increases stroke volume.
• Regulation of cardiac output through changes in afterload occurs without altering initial myocardial fiber length.
• This mechanism is known as homometric autoregulation.

Vessel Diameter

• Arteriolar diameter is a major determinant of peripheral resistance.
• Vasoconstriction increases peripheral resistance and elevates afterload.
• Increased afterload may reduce stroke volume and cardiac output.
• Vasodilation decreases peripheral resistance, lowers afterload, and facilitates ventricular ejection.

Viscosity of Blood

• Blood viscosity influences resistance to blood flow within the circulation.
• Increased viscosity, such as in polycythemia, raises peripheral resistance and may reduce cardiac output.
• Reduced viscosity, as seen in anemia, lowers vascular resistance and can contribute to an increase in cardiac output.

Factors Affecting Heart Rate

• Heart rate is regulated primarily by the autonomic nervous system.
• Sympathetic stimulation increases heart rate by enhancing the rate of impulse generation in the sinoatrial node.
• Parasympathetic stimulation, mainly through the vagus nerve, decreases heart rate.
• Under normal conditions, heart rate reflects the balance between sympathetic and parasympathetic influences.
• Since cardiac output equals stroke volume multiplied by heart rate, changes in heart rate can influence cardiac output.
• An increase in heart rate generally tends to increase cardiac output.
• However, the increase in cardiac output is not always proportional to the rise in heart rate.
• During marked tachycardia, ventricular diastole shortens more than systole.
• Reduced diastolic duration limits ventricular filling and decreases end-diastolic volume.
• The resulting reduction in stroke volume may offset the effect of the increased heart rate.
• Consequently, cardiac output may rise only slightly or may even decrease in severe tachycardia.
• In bradycardia, ventricular filling time increases because diastole is prolonged.
• Greater ventricular filling increases end-diastolic volume and stroke volume.
• This compensatory increase in stroke volume may help maintain cardiac output despite a slower heart rate.

Heart-Lung Preparation

• The heart–lung preparation is an experimental model used to study the effects of preload and afterload on cardiac performance.
• In this preparation, the circulation is arranged so that blood returning from the systemic circuit re-enters the right atrium and passes repeatedly through the heart and lungs.
• Because autonomic influences are excluded, heart rate remains relatively constant.

Demonstration of Afterload

• Increasing resistance to ventricular outflow increases afterload and initially reduces stroke volume.
• The resulting increase in ventricular end-diastolic volume subsequently enhances stroke volume through the Frank–Starling mechanism.

Demonstration of Preload

• Raising venous pressure increases preload and ventricular filling.
• Increased end-diastolic volume augments stroke volume.
• Lowering venous pressure has the opposite effect and reduces stroke volume.

Clinical Relevance

• The principles demonstrated in heart–lung preparations form the physiological basis for understanding preload and afterload regulation.
• Similar concepts are applied in the use of heart–lung machines during open-heart surgery.

Regulation of Cardiac Output

• Regulation of cardiac output occurs through intrinsic and extrinsic mechanisms.
• Intrinsic mechanisms operate within the heart itself and are collectively known as autoregulatory mechanisms.
• These mechanisms allow the heart to adjust its pumping performance according to changing physiological demands.

Intrinsic Regulation

The two principal intrinsic mechanisms are the Frank–Starling mechanism and rate-induced regulation.

Frank-Starling’s Mechanism

• The Frank–Starling mechanism relates ventricular performance to the degree of ventricular filling.
• An increase in end-diastolic volume stretches myocardial fibers and enhances the force of contraction.
• The stronger contraction increases stroke volume and cardiac output.
• A decrease in ventricular filling produces the opposite effect.
• This response occurs even in a denervated heart and therefore represents a true intrinsic mechanism.
• Regulation of cardiac output through changes in myocardial fiber length is called heterometric autoregulation.

Rate-induced Regulation

• Cardiac performance is also influenced by the frequency of cardiac contractions.
• Within physiological limits, an increase in heart rate enhances the force of myocardial contraction.
• This phenomenon is known as the force–frequency relationship.
• Increased contraction frequency allows calcium ions to enter myocardial cells more often because each action potential includes a calcium-dependent plateau phase.
• Repeated depolarizations increase intracellular calcium availability.
• Higher intracellular calcium concentrations strengthen myocardial contraction and increase stroke volume.
• At moderately increased heart rates, this mechanism helps maintain or enhance cardiac output.
• However, excessively high heart rates may shorten ventricular filling time and limit the beneficial effect.
• Rate-induced enhancement of contractility enables the heart to adapt rapidly to increased circulatory demands, particularly during exercise and sympathetic stimulation.

Extrinsic Regulation

Cardiac output is influenced by afterload, neural factors, and humoral factors.

Afterload

• Changes in afterload alter ventricular ejection and stroke volume.
• Increased peripheral resistance raises afterload and tends to reduce cardiac output.
• Decreased peripheral resistance lowers afterload and facilitates ventricular ejection.
• Regulation through changes in afterload occurs without alteration of myocardial fiber length and is termed homometric regulation.

Neural Control

Autonomic regulation plays a major role in controlling cardiac output.

Sympathetic Influence

Sympathetic stimulation increases heart rate, myocardial contractility, and venous return, thereby increasing cardiac output.

Parasympathetic Influence

• Parasympathetic stimulation primarily reduces heart rate and consequently decreases cardiac output.
• Because ventricular parasympathetic innervation is limited, its direct effect on ventricular contractility is relatively small.

Humoral Control

Humoral control includes regulation by hormones and chemicals.

Hormonal Regulation

• Humoral regulation of cardiac output is mediated by circulating hormones and chemical messengers.
• These substances influence heart rate, myocardial contractility, and vascular resistance.

Adrenomedullary Hormones

• Epinephrine, norepinephrine, and dopamine increase heart rate and myocardial contractility.
• These effects enhance stroke volume and increase cardiac output.

Thyroid Hormones

• Thyroxine increases myocardial contractility by enhancing excitation–contraction coupling within cardiac muscle.
• It increases the number and sensitivity of β₁-adrenergic receptors in the heart.
• As a result, heart rate and contractile force increase.
• Thyroxine also promotes myocardial protein synthesis, contributing to physiological cardiac growth.
• Increased tissue metabolism produces peripheral vasodilation, which lowers afterload and facilitates cardiac output.

Glucagon

• Glucagon has positive inotropic and chronotropic actions.
• It increases intracellular cyclic adenosine monophosphate concentration.
• This enhances calcium entry into myocardial cells and promotes calcium release from the sarcoplasmic reticulum.
• Increased intracellular calcium strengthens cardiac contraction and supports a higher cardiac output.

Growth Hormone

Growth hormone increases cardiac output primarily by enhancing myocardial contractility and supporting normal cardiac growth and function.

Chemical Regulation

Chemical regulation of cardiac output is influenced primarily by blood oxygen, carbon dioxide, and hydrogen ion concentrations.

Oxygen

• Mild hypoxia may increase heart rate through chemoreceptor activation.
• Severe hypoxia depresses myocardial contractility and reduces cardiac output.

Carbon Dioxide

Carbon dioxide exerts a direct depressant effect on cardiac muscle and also influences cardiovascular function through chemoreceptor-mediated reflexes.

Acidosis

• Acidosis decreases myocardial contractility by reducing calcium availability and lowering myofilament sensitivity to calcium.
• Alkalosis generally produces the opposite effect and may enhance myocardial performance.