Critical Care Medicine Part 1&2

Critical Care Medicine – Study Notes (Part 1)

Oxygen Content & Delivery

Physiologic Goal

• Oxygen’s end-point: mitochondrial ATP production.

• Hypoxemia/desaturation → ↓ ATP, failure of membrane integrity → apoptosis/necrosis.

Oxygen Gradient

• Required across all steps: atmosphere → alveoli → arterial blood → tissues → mitochondria.

Oxygen Transport

• Carried mostly by hemoglobin, with a small dissolved fraction.

O₂ Content Formula:

$$CaO₂ = (1.34 \times Hgb \times SaO₂) + (0.003 \times PaO₂)$$

• Hemoglobin & saturation = main determinants.

• PaO₂ contributes little.

Hemoglobin–Oxygen Binding

• Influenced by p50 (~26 mmHg).

• Right shift (↑ p50, ↓ affinity): acidosis, ↑ temperature, ↑ CO₂, ↑ 2,3-DPG.

• Left shift (↓ p50, ↑ affinity): alkalosis, ↓ temperature, ↓ CO₂, ↓ 2,3-DPG.

Oxygen Delivery (DO₂)

$$DO₂ = CaO₂ \times CO \times 10$$

• Cardiac output × oxygen content (×10 for unit correction).

• DO₂ is what matters clinically.

Oxygen Consumption (VO₂)

• Normal ≈ 250 mL/min (varies).

• Measured by direct or indirect calorimetry (metabolic cart).

Fick Principle:

$$CO = \frac{VO₂}{CaO₂ - CvO₂}$$

• Must use oxygen content, not just saturation.

• Errors: using venous saturation instead of content.

VO₂ increased by: sepsis, hyperthyroidism, seizures, shivering, fever, malignant hyperthermia.
VO₂ decreased by: anesthesia, neuromuscular blockade, hypothyroidism, hypothermia.

Mechanical Ventilation

Non-Invasive Ventilation

• BiPAP: improves both ventilation (↑ IPAP) and oxygenation (↑ EPAP).

• CPAP: improves oxygenation only; classically for cardiogenic pulmonary edema.

• Contraindications: altered mental status, vomiting/aspiration risk, bowel obstruction, gastroparesis.

• High-flow nasal cannula (HFNC) increasingly replacing CPAP.

Ventilatory Cycle

• Trigger → Inspiratory flow → Cycle → Expiration.

• Triggers: machine, pressure, flow.

• Cycling: volume, time, flow, or pressure.

Ventilation Modes

• Volume Control: constant tidal volume; variable pressures.

• Pressure Control: constant pressure; variable tidal volume.

• Assist Control (AC): minimum set rate + patient-triggered breaths.

• SIMV: mandatory breaths + spontaneous breaths (may add pressure support).

• Pressure Support: fully patient-driven; no backup rate (except apnea safety mode).

Key Pressures

• Peak Inspiratory Pressure (PIP): resistance + compliance.

• Plateau Pressure: compliance only (measured during inspiratory pause).

• Interpretation:

  • High PIP, normal plateau → resistance problem.

  • High PIP & plateau → compliance problem.

PEEP (Positive End-Expiratory Pressure)

• Prevents alveolar derecruitment.

• Improves oxygenation but ↑ intrathoracic pressure → ↓ venous return.

• Overdistension risk if too high.

ARDS (Acute Respiratory Distress Syndrome)

• Non-cardiogenic pulmonary edema due to inflammatory injury of alveolo-capillary membrane.

• Heterogeneous distribution in lungs.

• Causes: pneumonia, aspiration, trauma, burns, pancreatitis, sepsis.

• CXR: bilateral infiltrates, diffuse.

No curative treatment.

• Focus = treat underlying cause + supportive care.

• Lung-protective ventilation:

  • Low tidal volume (6 mL/kg IBW).

  • Plateau pressure < 30 cmH₂O.

  • Avoid volutrauma/barotrauma.

• Inhaled nitric oxide: may transiently improve oxygenation, but no mortality benefit. Rescue use only.

Infection Control in Critical Care

Precautions

• Standard: baseline for all patients.

• Contact: gown/gloves.

• Droplet: surgical mask.

• Airborne: N95, negative pressure.

Ventilator-Associated Pneumonia (VAP)

• From microaspiration around ETT cuff.

• Organisms differ from community pneumonia.

• Specialized ETTs may help but limited mortality impact.

VAP Prevention Bundle:

• Daily sedation breaks / awakening trials.

• Early extubation.

• Stress ulcer prophylaxis.

• DVT prophylaxis.

• Chlorhexidine oral care.

Central Line–Associated Bloodstream Infection (CLABSI)

• Caused by poor insertion or maintenance.

• Prevention Bundle:

  • Hand hygiene.

  • Full barrier precautions (cap, gown, mask, sterile gloves, full drape).

  • Chlorhexidine skin prep.

  • Daily assessment & prompt removal when no longer needed.

  • Antimicrobial discs/occlusive dressings reduce risk.

Anesthesia Keyword Review – Critical Care (Part 2 of 2)

Topics Covered

• Shock states

• Sepsis

• Resuscitation & fluids

• Nutrition

• Burns

• Drowning

Shock States

Definition

• Circulatory shock = syndrome of reduced tissue perfusion.

• Not just hypotension → key problem = inadequate capillary blood flow to supply O₂ + glucose for ATP production.

• Result: electrolyte imbalance → cell membrane pump failure → ischemia → infarction → necrosis.

Mortality

• Still very high.

• Septic or cardiogenic shock: mortality may reach ~50%.

Phases of Shock

1. Reversible/Early Shock

   • Reflexes (macro/microvascular + cellular + immunologic) try to preserve cardiac output & arterial pressure.

   • Organ dysfunction absent or minimal.

2. Late Shock

   • Perfusion of heart/major organs begins to fail.

   • Organ dysfunction becomes obvious.

   • Not a sharp cutoff; continuum.

3. Irreversible/Terminal Shock

   • Vasomotor dysfunction, loss of arteriolar tone.

   • Paradoxical ↑ central venous tone (pathologic).

   • Ischemia → infarct → necrosis.

Types of Shock

• Distributive (hyperdynamic)

  • ↑ Cardiac output.

  • ↓ SVR.

  • ↑ Mixed venous saturation.

  • ↓ Wedge pressure.

  • Ex: Septic shock.

• Hypovolemic

  • ↓ CO, ↑ SVR.

  • ↓ Mixed venous saturation.

  • ↓ Wedge pressure.

• Cardiogenic

  • ↓ CO, ↑ SVR.

  • ↓ Mixed venous saturation.

  • ↑ Wedge pressure.

• Obstructive

  • ↓ CO, ↑ SVR.

  • ↓ Mixed venous saturation.

  • ↑ Wedge pressure.

Sepsis & Septic Shock

• Distributive shock subtype.

• Pathophysiology: dysregulated host immune response, pro-inflammatory imbalance.

• Genetics: influence severity/outcome.

• Early recognition = survival.

• Treat septic shock like a code (time-critical).

Definitions

• Sepsis definitions evolving (Sepsis-3 replaced SIRS-based).

• Still: infection → systemic inflammation → sepsis → septic shock (hypotension + tissue malperfusion).

Management Principles

• Initial resuscitation: aggressive fluids + vasopressors if needed.

• Antibiotics: broad spectrum within 1 hour of symptom onset.

  • Blood cultures first if possible (then urine → lung).

  • De-escalate once pathogen identified.

• Source control: drain abscess, remove infected hardware.

• Fluids:

  • Crystalloids preferred (no consistent advantage to colloids).

  • May require 10–14 L/24 hrs early.

  • Remove excess fluid aggressively by day 2–3.

• Hemodynamic goals: MAP ≥ 65 mmHg.

• Vasopressors:

  • 1st line = norepinephrine.

  • Add vasopressin to reduce norepi dose.

  • Rescue = epinephrine.

• Inotropes:

  • Dobutamine if myocardial dysfunction + low CO.

  • Avoid supraphysiologic oxygen delivery goals.

• Steroids:

  • Consider if refractory to catecholamines.

  • Watch for hyperglycemia.

Resuscitation Fluids

Crystalloids vs Colloids

• No mortality difference overall.

• Colloids (albumin) indicated in specific conditions:

  • Cirrhosis + SBP (spontaneous bacterial peritonitis).

  • Hepatorenal syndrome.

  • Large-volume paracentesis.

• Hydroxyethyl starches (HES): avoided (coagulopathy, AKI risk).

Crystalloids

• Options: D5W, ½NS, NS, LR, Plasma-Lyte.

• Electrolyte content varies.

• Balanced solutions (LR, Plasma-Lyte) preferred for large resuscitation.

Nutrition in Critical Care

• Enteral (preferred) over TPN when possible.

  • Improves mortality, ↓ infection, ↓ ventilator days.

• TPN (yellow bag): hypertonic glucose, amino acids, electrolytes, vitamins, lipids.

  • Indications: short gut, high-output fistula, prolonged obstruction or ileus.

  • Complications: infection (CLABSI), liver dysfunction, cholelithiasis, refeeding syndrome, hyperglycemia.

• Caloric needs: ~25 kcal/kg/day (↑ in burns, trauma, sepsis).

• Protein needs: 1.5–2 g/kg/day (↑ in burns, trauma, AKI).

Burns

• Cause massive inflammation and fluid loss.

• Airway management:

  • Intubate early if facial soot, singed nasal hairs, mouth burns → prevent airway edema obstruction.

• Fluid resuscitation: Parkland formula (starting point, not absolute).

• Neuromuscular blockade considerations:

  • Succinylcholine contraindicated after 24–48 hrs (risk hyperkalemia).

  • Resistance to non-depolarizers possible.

Drowning

• Primary cause of death: hypoxemia from aspiration.

• Even if rescued: risk of ARDS, inflammation.

• Hypothermia common (even in non-arctic water).

• Cardiogenic shock possible (↑ CVP/wedge with immersion).

• Always evaluate for secondary injuries (trauma → spinal cord injury, internal lacerations).