EDIC Part I: Cardiovascular Dynamics

Cardiovascular dynamics tests shock classification, haemodynamic monitoring and fluid responsiveness.

Core rules

• MAP = CO × SVR; CO = HR × SV. SVR = 80 × (MAP − CVP)/CO.
• Hypovolaemic: low CO, high SVR. Cardiogenic: low CO, high SVR with elevated filling pressures. Distributive: high or normal CO, low SVR. Obstructive: low CO due to tamponade, tension pneumothorax or pulmonary embolism.
• Fluid responsiveness: passive leg raise or a 250-mL fluid challenge. PPV/SVV are valid only in controlled ventilation, sinus rhythm and tidal volume ≥8 mL/kg.
• Norepinephrine is first-line in septic shock; vasopressin is an adjunct; dobutamine supports low CO with adequate MAP.

Worked example

MAP 55 mmHg, CVP 8 mmHg, CO 8 L/min: SVR = 80 × (55 − 8)/8 = 470 dyn·s·cm⁻⁵, markedly low. With warm peripheries and lactate 4 mmol/L, this is distributive shock. Give norepinephrine and treat the source rather than more fluid alone.

Traps

• CVP does not reliably predict fluid responsiveness.
• PPV is invalid in spontaneous breathing, arrhythmia or open chest.
• A normal lactate does not exclude shock.
• Echocardiography distinguishes RV failure and tamponade before fluids are given.