2.3 Critical Care Medicine, Shock Syndromes & Mechanical Ventilation

1. Introduction to Shock Syndromes

Shock is a life-threatening state of cellular and tissue hypoxia resulting from reduced oxygen delivery, increased oxygen consumption, or inadequate oxygen utilization. It clinically manifests as profound hypotension, altered mental status, oliguria, and elevated serum lactate levels. Understanding the hemodynamic profiles is essential for diagnosis and management.

Hemodynamic Profiles of Shock

• Hypovolemic Shock: Caused by severe fluid loss (e.g., hemorrhage, severe dehydration).
  • Hemodynamics: Decreased Central Venous Pressure (CVP/Preload), decreased Cardiac Output (CO), and increased Systemic Vascular Resistance (SVR) as a compensatory mechanism.
  • Treatment: Rapid volume resuscitation with crystalloids or packed red blood cells (in hemorrhagic shock), and definitive control of the bleeding source.
• Cardiogenic Shock: Caused by primary pump failure (e.g., massive acute myocardial infarction, advanced heart failure, fatal arrhythmias).
  • Hemodynamics: Increased CVP and Pulmonary Capillary Wedge Pressure (PCWP) indicating volume backup, severely decreased CO, and increased SVR.
  • Treatment: Inotropes (e.g., Dobutamine, Milrinone), careful diuresis, mechanical circulatory support (intra-aortic balloon pump or ECMO), and urgent revascularization for ischemia.
• Distributive Shock (e.g., Septic, Anaphylactic, Neurogenic): Caused by severe peripheral vasodilation.
  • Hemodynamics: Decreased SVR (the hallmark), normal or decreased CVP, and generally increased CO (hyperdynamic state) initially to compensate.
  • Treatment: Vasopressors to restore vascular tone, IV fluids, and treatment of the underlying cause (e.g., antibiotics for sepsis, epinephrine for anaphylaxis).
• Obstructive Shock: Caused by extracardiac obstruction to blood flow (e.g., Cardiac Tamponade, Tension Pneumothorax, Massive PE).
  • Hemodynamics: Increased CVP/PCWP (due to backup), decreased CO, and increased SVR.
  • Treatment: Immediate relief of the obstruction (pericardiocentesis, needle decompression, or thrombolytics).

2. Sepsis and Septic Shock

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Septic shock is a subset of sepsis characterized by profound circulatory, cellular, and metabolic abnormalities, defined clinically by the need for vasopressors to maintain MAP ≥ 65 mmHg and a serum lactate > 2 mmol/L despite adequate fluid resuscitation.

The Surviving Sepsis Campaign Bundle

Early recognition and intervention within the "golden hours" are critical to mortality reduction. The core immediate actions (ideally within 1 to 3 hours of recognition) include:

1. Measure Lactate: Re-measure if initial lactate is elevated.
2. Obtain Blood Cultures: Must be drawn prior to administration of antibiotics.
3. Administer Broad-Spectrum Antibiotics: Cover likely pathogens based on the suspected source (e.g., Piperacillin-Tazobactam and Vancomycin).
4. Administer Fluids: Rapid administration of 30 mL/kg of IV crystalloid (e.g., Lactated Ringer's or Normal Saline) for hypotension or lactate ≥ 4 mmol/L.
5. Apply Vasopressors: If the patient remains hypotensive after the fluid challenge, initiate vasopressors. Norepinephrine is universally the first-line agent. Vasopressin can be added as a second-line agent to reduce the norepinephrine dosage requirement.

3. Acute Respiratory Distress Syndrome (ARDS)

ARDS is a catastrophic form of acute inflammatory lung injury leading to increased pulmonary vascular permeability, increased lung weight, and loss of aerated lung tissue. It is clinically recognized by the sudden onset of severe hypoxemia and bilateral opacities on chest imaging not entirely explained by cardiac failure.

The Berlin Definition Criteria:

1. Timing: Within 1 week of a known clinical insult or new/worsening respiratory symptoms.
2. Imaging: Bilateral opacities on chest X-ray or CT.
3. Origin of Edema: Respiratory failure not fully explained by heart failure or fluid overload.
4. Oxygenation: Categorized by the PaO2/FiO2 (P/F) ratio (Mild: 200-300, Moderate: 100-200, Severe: < 100 mmHg) with a minimum PEEP of 5 cmH2O.

4. Mechanical Ventilation Principles

Invasive mechanical ventilation involves delivering positive pressure to the lungs via an endotracheal or tracheostomy tube.

Basic Modes of Ventilation

• Assist-Control (AC) or Continuous Mandatory Ventilation (CMV): The ventilator delivers a preset tidal volume or pressure for every breath, regardless of whether the breath is machine-initiated or patient-initiated. It guarantees minute ventilation but can lead to hyperventilation and respiratory alkalosis if the patient breathes too fast.
• Synchronized Intermittent Mandatory Ventilation (SIMV): The machine delivers a set number of mandatory breaths synchronized with patient effort. Any additional breaths taken by the patient are entirely their own effort (supported only by pressure support).
• Pressure Support Ventilation (PSV): A spontaneous breathing mode where the patient initiates every breath, and the machine provides a set amount of positive pressure to overcome airway resistance. Used primarily for weaning.

ARDS Ventilation Strategy (Lung-Protective Ventilation)

Traditional large tidal volumes damage ARDS-afflicted lungs due to barotrauma (high pressures) and volutrauma (overdistension). The landmark ARDSNet trial established the necessity of low tidal volume ventilation:

• Target Tidal Volume: 6 mL/kg of predicted body weight (not actual weight).
• Plateau Pressure Goal: Maintain < 30 cmH2O.
• Permissive Hypercapnia: Allow the PaCO2 to rise (accepting a lower pH, usually down to 7.20-7.25) to avoid hazardous plateau pressures.
• Optimizing PEEP: High PEEP is applied to keep alveoli open at the end of expiration, preventing atelectrauma (shearing from repeated opening and closing of alveoli) and improving oxygenation.