ANAPHY

Cardiac output ($\text{CO}$) represents the volume of blood pumped by each ventricle of the heart per unit of time, typically measured in $dm^3\,\text{min}^{-1}$. It reflects the total capacity of the cardiovascular system to deliver oxygen and essential nutrients to peripheral tissues.

1. Fundamental Equations

Cardiac output is determined by the product of heart rate and stroke volume:

$\text{CO} = \text{HR} \times \text{SV}$

1. Heart Rate ($\text{HR}$)

   • Defined as the frequency of ventricular contractions per minute.

   • Typical adult resting heart rate ranges from $60$ to $100\,\text{beats}\,\text{min}^{-1}$, averaging approximately $70\,\text{beats}\,\text{min}^{-1}$.

2. Stroke Volume ($\text{SV}$)

   • Defined as the volume of blood ejected by a ventricle during a single contraction.

   • Calculated as the difference between end-diastolic volume and end-systolic volume: $\text{SV} = \text{EDV} - \text{ESV}$.

   • Standard adult resting stroke volume is approximately $70\,cm^3$.

3. Resting Cardiac Output Calculation

   • Baseline resting output is calculated as: $\text{CO} = 70\,\text{beats}\,\text{min}^{-1} \times 70\,cm^3 = 4900\,cm^3\,\text{min}^{-1} \approx 5.0\,dm^3\,\text{min}^{-1}$.

2. Determinants of Stroke Volume

Stroke volume is modulated by three primary physiological factors:

1. Preload

   • Definition: The degree of myocardial sarcomere stretch at the end of diastole, determined largely by end-diastolic volume ($\text{EDV}$) and venous return.

   • Frank-Starling Law: Increased end-diastolic stretch optimizes actin-myosin overlap, producing a more forceful ventricular contraction and greater stroke volume.

   • Regulating Factors: Total blood volume, venous vasomotor tone, body posture, skeletal muscle pump activity, and intrathoracic respiratory variations.

2. Myocardial Contractility (Inotropy)

   • Definition: The intrinsic contractile strength of the ventricular myocardium at any given preload, independent of muscle stretch.

   • Cellular Basis: Regulated by the magnitude of intracellular calcium ion ($\text{Ca}^{2+}$) concentration available to troponin C during systole.

   • Modulating Factors:

   • Positive inotropic agents: Sympathetic stimulation via $\beta_1$-adrenergic receptors, circulating epinephrine, and cardiac glycosides.

   • Negative inotropic agents: Parasympathetic stimulation, severe acidosis, hyperkalemia, hypoxia, and beta-blockers.

3. Afterload

   • Definition: The resistance or tension against which the ventricular myocardium must contract to open the semilunar valves and eject blood.

   • Determinants: Left ventricular afterload is governed primarily by systemic vascular resistance ($\text{SVR}$) and aortic pressure; right ventricular afterload depends on pulmonary vascular resistance ($\text{PVR}$).

   • Hemodynamic Effect: An isolated increase in afterload elevates end-systolic volume ($\text{ESV}$) and leads to a reduction in stroke volume.

3. Regulation of Heart Rate (Chronotropy)

Heart rate is controlled by intrinsic pacemaker tissues alongside neural and hormonal influences:

1. Autonomic Nervous System

   • Sympathetic System: Cardiac accelerator nerves release norepinephrine, activating $\beta_1$-adrenergic receptors at the sinoatrial (SA) node to accelerate phase $4$ depolarization, increasing heart rate (positive chronotropy).

   • Parasympathetic System: The vagus nerve releases acetylcholine, binding to $M_2$ muscarinic receptors to open potassium channels, hyperpolarizing SA node pacemaker cells and slowing heart rate (negative chronotropy).

2. Baroreceptor Reflex

   • High-pressure stretch mechanoreceptors located in the carotid sinus and aortic arch monitor systemic arterial blood pressure.

   • Elevated arterial pressure increases afferent nerve firing to the medullary cardiovascular center, evoking enhanced parasympathetic outflow and diminished sympathetic outflow to lower heart rate and reduce cardiac output.

3. Humoral and Electrolyte Modulators

   • Catecholamines: Adrenal release of epinephrine and norepinephrine stimulates chronotropy and inotropy.

   • Thyroid Hormones: Free triiodothyronine ($T_3$) enhances transcription of cardiac $\beta_1$-adrenergic receptors and contractile proteins.

   • Electrolytes: Hyperkalemia depolarizes baseline resting membrane potential and reduces conduction velocity, while hypokalemia and calcium alterations disrupt rhythmic pacing.

4. Physiological Variations and Clinical Metrics

1. Cardiac Index ($\text{CI}$)

   • Normalizes cardiac output to an individual's body surface area ($\text{BSA}$): $\text{CI} = \frac{\text{CO}}{\text{BSA}}$.

   • Reference resting values range from $2.5$ to $4.0\,dm^3\,\text{min}^{-1}\,\text{m}^{-2}$.

2. Response to Exercise

   • During intense dynamic exercise, cardiac output can elevate from $5.0\,dm^3\,\text{min}^{-1}$ up to $20$ to $25\,dm^3\,\text{min}^{-1}$ in healthy adults, and can exceed $35\,dm^3\,\text{min}^{-1}$ in trained endurance athletes.

   • Increases are achieved through profound tachycardia coupled with heightened contractility and enhanced venous return via the muscle and respiratory pumps.

3. Clinical Measurement Methods

   • Fick Principle: Determines flow by dividing total oxygen consumption ($V\text{O}_2$) by the arteriovenous oxygen content difference: $\text{CO} = \frac{V\text{O}_2}{C_{\text{a}}\text{O}_2 - C_{\text{v}}\text{O}_2}$.

   • Thermodilution: Measures thermal dissipation along a pulmonary artery catheter following a rapid saline bolus injection.

   • Echocardiography: Provides non-invasive assessment by computing velocity time integrals and cross-sectional areas of the left ventricular outflow tract using Doppler ultrasound.