AMS 2.10 Shock

Introduction to Critical Circulation

• Review of cardiopulmonary and systemic circuits.
• Importance of understanding blood flow direction in relation to common conditions: heart failure, pulmonary embolisms (PEs), trauma, myocardial infarctions (MIs).

Blood Flow Pathway

• Blood returns from systemic circuit to right side of the heart.
  • Movement from right atrium to right ventricle.
  • Blood enters pulmonary circuit for oxygenation.
• Oxygenated blood flows into:
  • Left atrium
  • Left ventricle
• Blood is pumped into the aorta and distributed to the systemic circuit (rest of body).

Key Definitions and Measurements

• Cardiac Output
  • Defined as the amount of blood pumped out of the left ventricle per minute.
  • Normal range: $3.5$ to $5$ liters per minute.
  • Variations in cardiac output based on body size (smaller stature = lower cardiac output, larger stature = higher output).
  • Influenced by:
    • Stroke Volume: Amount of blood pumped out of the left ventricle per beat.
    • Average: $70$ mL per beat.
    • Heart Rate: Normal range between $60$ to $100$ beats per minute.

Example Calculation of Cardiac Output

• Given patient's stroke volume = $70$ mL/beat and heart rate = $70$ beats/minute:
  • Cardiac Output = Stroke Volume $imes$ Heart Rate = $70$ mL/beat $imes$ $70$ beats/minute = 4900 mL/minute
  • Converted to liters: 4.9 liters/minute, within normal range.

Importance of Stroke Volume and Heart Rate

• Problems with stroke volume or heart rate indicate cardiac output issues:
  • Causes: dehydration, trauma, or heart rhythm abnormalities.
• Preload: Measures volume returning to the heart from systemic circulation.
  • Normal preload pressure: Approx. 8 mmHg (measured via central venous pressure).
• Afterload: The pressure the heart must work against to eject blood:
  • Pulmonary Circuit Pressure: Average Systole/Diastole: 25$/$10$mmHg, MAP:$15-$20 mmHg.
  • Systemic Circuit Pressure: Average Blood Pressure: $120$/$80$ mmHg, MAP: $70$ mmHg.
• Contractility: forcefulness of the heart's contraction.

Hemodynamic Stability

• Vital for proper organ perfusion and oxygenation.
• Needs volume in the vascular network, not 'trapped' fluid in interstitial spaces (third space).

Shock Overview

• Shock: Condition where perfusion is inadequate to maintain tissue oxygenation.
  • Leads to organ dysfunction: tissue dies without oxygenation.

Classifications of Shock

• Hypovolemic Shock:
  • Caused by decreased intravascular volume (fluid loss or shifts).
  • Loss of preload affects stroke volume and cardiac output directly.
• Cardiogenic Shock:
  • Result of impairment or failure of myocardium (common cause: myocardial infarction).
  • Affects contractility, leading to reduced stroke volume and cardiac output.
• Circulatory Shock:
  • Includes three subtypes: septic, neurogenic, and anaphylactic shock.
  • Problems with vasculature lead to compromised perfusion.
  • Vasodilation leads to pooling of blood in distal extremities.

Cellular Effects of Shock

• Intact vs. compromised cells:
  • Compromised: broken cell walls, influx of sodium and water leading to cell swelling and apoptosis.

Stages of Shock

• Initial Stage: Often referred to as compensatory phase. Actual shock event occurs here.
• Compensatory Stage: Body activates homeostatic responses.
  • Sympathetic nervous response (epinephrine, norepinephrine) increases heart rate, blood pressure, contractility, and cardiac output.

Management Strategies for Shock

• General management:
  • Fluid replacement (crystalloids, colloids, blood).
  • Vasoactive medication therapy (to support vascular tone and contractility).
  • Nutritional support.
  • Proactive positioning (modified Trendelenburg position) to enhance venous return to the heart.

Specific Management for Different Types of Shock

• Hypovolemic Shock:
  • Target underlying cause (e.g. bleeding, fluid shifts, burns).
  • Fluid and blood replacement, redistribution as needed.
• Cardiogenic Shock:
  • Restore myocardial perfusion (e.g. coronary intervention for MI).
  • Caution with fluid administration; use mechanical assistive devices if necessary.
• Circulatory Shock:
  • Focus on fluid resuscitation and vasoactive drugs.
  • Nutritional support critical, particularly for hypermetabolic state.
• Anaphylactic Shock:
  • Screen and prevent triggers (foods, insects, etc.).
  • Immediate administration of epinephrine.

Complications of Shock

• MODS (Multiple Organ Dysfunction Syndrome): Failure of two or more organ systems due to shock.
  • Can be primary (initially healthy organs failing) or secondary (pre-existing organ dysfunction worsened by shock).
• DIC (Disseminated Intravascular Coagulopathy): Not a distinct disease but a symptom of underlying conditions involving shock and sepsis.
  • Involves a cycle of clotting and bleeding due to injury from shock.
  • Early intervention required to prevent progression into multi-organ failure.

Conclusion

• Recognition and intervention are crucial in shock management.
• Understanding the complexities of each type of shock essential for effective treatment and patient outcomes.