Hemodynamics and Cardiac Output Overview

Hemodynamics

• The study of blood movement and the forces involved in blood circulation.
• Arterial blood pressure is the force exerted by blood on vessel walls.
• Systolic blood pressure: The peak pressure during ventricular contraction
• Diastolic blood pressure: The minimum pressure during ventricular relaxation
• Mean arterial pressure (MAP) is the average pressure during the cardiac cycle, crucial for organ perfusion
• MAP calculation: Typically approximated as 1/3 SBP + 2/3 DBP, but this formula may not be accurate in critically ill patients
• MAP = CO x SVR (Cardiac Output x Systemic Vascular Resistance)

Cardiac Output (CO)

• CO is the volume of blood pumped per minute.
• Derived from the product of stroke volume (SV) and heart rate (HR): CO = SV x HR
• Stroke volume is the volume of blood ejected from the left ventricle each heartbeat
• SV is influenced by preload (blood available for ejection), afterload (resistance to ejection), and contractility (heart's force generation)

Shock

• A syndrome of impaired tissue perfusion, often accompanied by hypotension
• Leads to cellular dysfunction, organ damage and ultimately death if not addressed
• Types of shock: Hypovolemic, obstructive, cardiogenic, distributive (sepsis)
• Treatment varies depending on the underlying cause

Causes of Shock

• Reduction of intravascular volume (Hypovolemic: blood loss, dehydration)
• Circulation obstruction (Obstructive: pulmonary embolism, pericardial tamponade)
• Myocardial pump failure (Cardiogenic: heart attack, heart failure)
• Increased vascular capacitance (Distributive: sepsis, anaphylaxis)

Advanced Shock

• Different subtypes of shock become difficult to distinguish in advanced stages, as compensatory mechanisms fail
• Clinical features of advanced shock become similar across different types
• Multiple types of shock can coexist in a single patient

Cellular Effects of Shock

• Ischemia, inflammation, and oxygen radical generation contribute to cellular damage
• During prolonged periods of ischemia, cells switch to anaerobic metabolism, depleting ATP stores
• Lactic acid and toxins accumulate, impairing mitochondrial function
• This leads to cell death if prolonged
• Irreversible cellular damage in advanced shock results in multiple organ dysfunction syndrome (MODS)

Systemic Inflammatory Response Syndrome (SIRS)

• A generalized inflammatory response to various insults, including shock, infection, trauma, and injury
• Hallmarks of SIRS include profound vasodilation (decreased perfusion) and increased capillary permeability (fluid leakage from vessels)
• SIRS is common in hypovolemic shock and septic shock, less frequent in cardiogenic shock
• Clinical features of SIRS involve impaired perfusion and reduced intravascular volume

Shock Diagnosis

• Confirmed through physical examination findings of impaired tissue perfusion
• Hemodynamic and laboratory changes consistent with impaired perfusion
• Hypotension is a common symptom, but not always present in all patients
• Hemodynamic monitoring is crucial for determining the type of shock and response to treatment

Systemic Vascular Resistance (SVR)

• The resistance the heart encounters while pumping blood
• Measured using a pulmonary artery catheter
• Influenced by vasoconstriction (increased resistance) and vasodilation (decreased resistance)
• Values are generally adjusted for body surface area (BSA)

Cardiac Index (CI)

• Cardiac output adjusted for body surface area
• Measures the effectiveness of the heart in terms of blood pumping relative to size

Stroke Index (SI)

• Stroke volume adjusted for body surface area
• Reflects the efficiency of ventricular contraction per unit of body size

Systemic Vascular Resistance Index (SVRI)

• Systemic vascular resistance adjusted for body surface area
• Represents the total resistance to blood flow throughout the circulatory system relative to body size

Pulmonary Artery Wedge Pressure (PAWP)

• Reflects left atrial pressure
• Measured by inflating a balloon catheter that wedges into a small pulmonary artery branch
• Provides information about left ventricular filling pressure and left ventricular function

Mixed Venous Oxygen Saturation (SVO2)

• Saturation of oxygen in mixed venous blood
• Measured using a pulmonary artery catheter
• Indicator of oxygen delivery and tissue oxygen consumption
• Lower values reflect reduced oxygen delivery or increased oxygen consumption, indicating potential tissue hypoxia

Explore the fundamental concepts of hemodynamics and cardiac output through this quiz. Understand key terms like arterial blood pressure, mean arterial pressure, and the factors influencing stroke volume. Test your knowledge on the dynamics of blood circulation and its importance to organ perfusion.