Week 2: Shock & Fluids

Shock: Present when metabolic demands of the body are insufficiently met by supply, OR when tissues are unable to use supplied oxygen adequately.

The Key Equations:

• MAP (Mean Arterial Pressure) = CO (Cardiac Output) × TPR
• Cardiac Output (Cardiac Output) = Heart Rate (Heart Rate) × Stroke Volume (Stroke Volume)
• Perfusion Pressure = MAP (Mean Arterial Pressure) − CVP (Central Venous Pressure)

Distributive shock causes include sepsis, anaphylaxis, neurogenic (spinal), and adrenal crisis.

Absolute hypovolaemia = actual fluid loss (hole in the bucket; vessels normal/constricted).

Relative hypovolaemia = vasodilation increases vascular space (bucket got bigger; fluid volume unchanged).

Lactate rises when oxygen delivery is inadequate and tissues switch to anaerobic metabolism.

Holliday-Segar electrolyte rules: sodium 3 mmol per 100 mL water and potassium 2 mmol per 100 mL water.

Why 4% and 1/5th is the ideal maintenance fluid:

2.5 L provides: ~2400 mL water, ~74 mmol sodium = Perfect match for Holliday-Segar!

Fluid intent matters:

• Resuscitation fluids restore intravascular volume rapidly
• Replacement fluids match ongoing losses (e.g., GI, drains)
• Maintenance fluids cover daily water/electrolyte needs

Excess IV fluid can cause tissue oedema, impaired oxygenation/ventilation, and venous congestion that may worsen renal function.

In septic shock, the first-line vasopressor is noradrenaline; add vasopressin if inadequate response.

In cardiogenic shock, the preferred inotrope is dobutamine.

Q: Noradrenaline receptor profile: dominant [] effect with some [] inotropy. A: alpha-1; beta-1

Dobutamine is beta1-dominant and improves contractility.

Vasopressin acts on V1 receptors and is a non-catecholamine vasoconstrictor.

Refractory shock is often considered when noradrenaline requirements are above 0.2-0.3 mcg/kg/min despite adequate fluids; add adjuncts (vasopressin, hydrocortisone) and reassess cause.

Vasopressin in septic shock is a fixed-dose infusion of 0.03 units/min and is not titrated.

If MAP (Mean Arterial Pressure) is adequate but cardiac output remains low, add an inotrope (e.g., dobutamine) rather than escalating pure vasoconstriction.

Shock management includes specific treatment of the underlying cause (e.g., antibiotics for sepsis, PCI for MI).

Shock management also requires supportive care to maintain physiology while definitive treatment works.

In sepsis, give broad-spectrum antibiotics within 1 hour of recognition, then de-escalate based on source and cultures.

Obtain blood cultures (x2) before antibiotics when feasible - but do not delay antibiotics in septic shock.

In sepsis with unknown source, start empiric broad-spectrum coverage with piperacillin-tazobactam or meropenem.

Respiratory sepsis: ceftriaxone + azithromycin for CAP; pip-tazo for HAP.

Urinary sepsis: gentamicin + ampicillin or ceftriaxone.

Intra-abdominal sepsis: pip-tazo or metronidazole + gentamicin.

Skin/soft tissue sepsis: flucloxacillin (or vancomycin if MRSA risk).

Suspected meningitis: ceftriaxone + vancomycin +/- dexamethasone.

Q: Where should empiric sepsis antibiotic regimens come from? A: The local antimicrobial guideline (e.g., Westmead Sepsis Kills); always adapt to local policy and renal function.

Gentamicin sepsis dosing (Westmead): 7 mg/kg IV single dose (max 640 mg).

MRSA risk: add vancomycin 25-30 mg/kg IV loading dose.

In acute haemorrhage, treat based on physiology (shock, lactate, ongoing bleeding) — Hb can be normal early.

In most haemodynamically stable upper GI bleeding, use a restrictive strategy: transfuse PRBCs when Hb (Haemoglobin) < 70 g/L (target 70–90 g/L).

5 Criteria for Delirium (DSM-5):

1. Disturbance of attention and awareness
2. Acute onset (hours–days), change from baseline
3. Additional cognitive disturbance
4. Not explained by pre-existing dementia, not in coma
5. Evidence of organic cause

Delirium is acute, fluctuating, and features inattention - unlike dementia (chronic progressive) or primary psychosis (attention usually preserved).

Sepsis (Sepsis-3, 2016): Life-threatening organ dysfunction caused by a dysregulated host response to infection. Operationally: suspected infection + acute SOFA score increase ≥ 2.

Septic shock (Sepsis-3): Sepsis requiring vasopressors to maintain MAP ≥ 65 mmHg AND lactate > 2 mmol/L despite adequate fluid resuscitation.

qSOFA (bedside screening): RR ≥ 22, altered mentation (GCS < 15), SBP ≤ 100. Score ≥ 2 prompts further assessment. Not diagnostic — just a screening tool.

SOFA score assesses 6 organ systems: respiratory (PaO2/FiO2), coagulation (platelets), liver (bilirubin), cardiovascular (MAP/vasopressors), CNS (GCS), renal (creatinine/urine output).

Lactate clearance — failure to clear lactate by 10–20% in 2–4 hours predicts worse outcomes in sepsis.

ISBAR — structured clinical handover framework: Identify, Situation, Background, Assessment, Recommendation.

On an AP film (portable), the heart appears magnified — do not over-call cardiomegaly on AP radiographs.

ASA Physical Status Classification:

• ASA 1 — healthy
• ASA 2 — mild systemic disease
• ASA 3 — severe systemic disease
• ASA 4 — severe disease that is a constant threat to life
• ASA 5 — moribund, not expected to survive without surgery
• ASA 6 — brain-dead organ donor
• Add E suffix for emergency surgery

A septic shock patient on vasopressors requiring emergency surgery is ASA 4E — severe systemic disease (constant threat to life) plus emergency.

Before emergency surgery in a shocked patient, optimise: haemodynamics (MAP target, vasopressors), bloods (FBC, coag, group & crossmatch), coagulopathy correction (FFP, cryo, platelets), glucose (target 6–10), antibiotics (redose if needed), and consent (patient or next of kin).

Q: Normal arterial pH range? A: 7.36-7.44 (pH >7.45 = alkalaemia, <7.35 = acidaemia)

Q: Normal pCO2 (Partial Pressure of Arterial Carbon Dioxide) range? A: 35-45 mmHg (~4.7-6.0 kPa)

Q: Normal HCO3- range? A: 24 ± 2 mmol/L (22-26); reflects the metabolic component of acid-base balance

Source: CC Bible (ABG reference ranges).

Q: Normal anion gap range? A: 8-12 (older assays: 12 ± 4). Calculated as Na - (Cl + HCO3)

Q: Normal lactate? A: <2.0 mmol/L; >2.0 suggests tissue hypoperfusion

Source: CC Bible; local pathology reference ranges vary.

Metabolic acidosis: ↓ pH with ↓ pCO2 (respiratory compensation = hyperventilation)

Metabolic alkalosis: ↑ pH with ↑ pCO2 (respiratory compensation = hypoventilation)

Respiratory acidosis: ↓ pH with ↑ pCO2 (CO2 retention)

Respiratory alkalosis: ↑ pH with ↓ pCO2 (hyperventilation blowing off CO2)

To identify the PRIMARY disturbance: pH direction must match either HCO3 or pCO2 direction. The component that matches the pH is primary; the opposite one is compensation.

Example: pH 7.48 (high), pCO2 48 (high), HCO3 34 (high) → HCO3 matches pH direction = primary metabolic alkalosis.

In metabolic alkalosis (HCO3 high, pH high), expected compensatory pCO2 = 0.7 × HCO3 + 20 (±5 mmHg).

If actual pCO2 is within this range → appropriate compensation. If outside → suspect mixed disorder.

In metabolic acidosis, expected pCO2 = 1.5 × HCO3 + 8 (± 2) - Winter's formula

In metabolic alkalosis, expected pCO2 = 0.7 × HCO3 + 20 (± 5)

In acute respiratory acidosis, expected HCO3 rises by 1 for every 10 mmHg rise in pCO2

Source: CC Bible (acid–base compensation rules).

In chronic respiratory acidosis, expected HCO3 rises by 4 for every 10 mmHg rise in pCO2

Source: CC Bible (acid–base compensation rules).

The delta ratio = ΔAG / ΔHCO3 where ΔAG = AG - 12 and ΔHCO3 = 24 - measured HCO3

Delta ratio (ΔAG/ΔHCO3) less than 0.4 indicates normal anion gap metabolic acidosis (hyperchloraemic) — the HCO3 is dropping more than the AG is rising.

Delta ratio (change in AG ÷ change in HCO3) of 0.4-0.8 indicates mixed high AG + normal AG metabolic acidosis — both anion gap and hyperchloremic acidosis are present.

Delta ratio (ΔAG/ΔHCO3) of 0.8-2.0 indicates a pure high anion gap metabolic acidosis — HCO3 drops proportionally to AG rise.

Delta ratio (change in AG ÷ change in HCO3) greater than 2.0 indicates high anion gap metabolic acidosis PLUS an overlapping metabolic alkalosis — the HCO3 hasn't dropped as much as expected given the AG rise.

Examples: DKA (Diabetic Ketoacidosis) + vomiting, lactic acidosis + diuretics, renal failure + NG suction.

A-a gradient = PAO2 - PaO2 (Partial Pressure of Arterial Oxygen). Normal A-a gradient (room air) ≈ (Age/4) + 4 mmHg (~Age/30 + 0.5 kPa)

Source: CC Bible (A–a gradient rule of thumb).

PAO2 (alveolar O2) = 713 × FiO2 (Fraction of Inspired Oxygen) - (pCO2 (Partial Pressure of Arterial Carbon Dioxide) × 1.25) at sea level (mmHg). If using kPa: PAO2 = 95 × FiO2 - (pCO2 × 1.25)

Elevated A-a gradient suggests: V/Q mismatch, shunt, or diffusion impairment

Surviving Sepsis Campaign Hour-1 Bundle — all elements should be initiated within 1 hour of sepsis recognition:

1. Measure lactate (repeat if initial lactate >2 mmol/L)
2. Obtain blood cultures before antibiotics (do not delay antibiotics to obtain cultures)
3. Administer broad-spectrum antibiotics
4. Begin rapid IV crystalloid (30 mL/kg for hypotension or lactate ≥4 mmol/L)
5. Apply vasopressors if hypotensive during or after fluid resuscitation (target MAP (Mean Arterial Pressure) ≥65 mmHg)