Hemodynamic Monitoring and Shock
Shock questions on this Board are mechanism-first: name the type, prove it with numbers, then choose the drug. Expect cardiovascular physiology, bedside assessment of fluid responsiveness, and vasoactive drug selection.
The arithmetic
Mean arterial pressure = diastolic + one third of pulse pressure; a MAP of 65 mmHg is the usual resuscitation target. Cardiac output = heart rate x stroke volume; cardiac index divides by body surface area (normal 2.5-4.0 L/min/m2). Systemic vascular resistance = 80 x (MAP - CVP) / CO, normally 800-1200 dyn.s.cm-5. These lines let you classify any shocked patient at the bedside.
Four mechanisms
Distributive (septic, anaphylactic, neurogenic): low SVR, normal or high output, warm peripheries, low CVP. Hypovolaemic: low preload, low output, high SVR, collapsed IVC. Cardiogenic: high filling pressures, low output, cool peripheries, pulmonary oedema. Obstructive: tamponade, tension pneumothorax, massive pulmonary embolism - echo or clinical signs make the diagnosis, and treatment is mechanical, not a fluid bolus. Lactate, capillary refill above 3 seconds and mottling grade track perfusion better than blood pressure alone.
Fluid responsiveness and drugs
Only about half of unstable patients respond to fluid. Pulse pressure or stroke volume variation above 13%, a passive leg raise producing a 10% rise in cardiac output, or a 250-500 mL challenge with a measured response all predict it; none is valid in spontaneous breathing or arrhythmia. Noradrenaline is first-line for vasoplegic shock, with vasopressin 0.03 units/min added rather than escalating noradrenaline indefinitely. Dobutamine treats low output with high filling pressures; adrenaline is reserved for refractory shock.
Worked example
MAP 60, CVP 6, cardiac output 8 L/min, BSA 1.8. SVR = 80 x (60 - 6) / 8 = 540 dyn.s.cm-5; CI = 4.4 L/min/m2. Low resistance with high output is distributive shock - start noradrenaline and hunt the source rather than giving more fluid.
Traps
Over-damped arterial traces underestimate systolic and overestimate diastolic pressure; the mean stays most reliable. Trusting CVP as a volume gauge. Giving fluid boluses in cardiogenic or obstructive shock. Forgetting that a normal blood pressure with a lactate of 5 mmol/L is still shock. Accepting a MAP of 65 in a patient with known severe hypertension without checking perfusion.