Systemic vascular resistance

SVR · Derived: SVR = 80 × (MAP − RAP) / CO, with CO from LVOT (card 42) and RAP from the IVC (card 24)

Normal range & thresholds

Normal 800–1200 dyn·s·cm⁻⁵ (SVR index ≈1970–2390 dyn·s·cm⁻⁵·m²).

• <800 — vasodilated circulation (sepsis, anaphylaxis, liver failure, vasoplegia after bypass).
• >1600 — vasoconstriction (cardiogenic or hypovolaemic shock, severe hypertension) ^[33].

Pathophysiology

Ohm’s law for the circulation: the pressure drop across the systemic bed divided by the flow through it. It describes only the steady, resistive part of afterload — the arterioles. It ignores the pulsatile part (compliance, wave reflection), which is why Ea (card 48) is the better description of total arterial load on the ventricle ^[109].

Raised by

• Cardiogenic and hypovolaemic shock (compensatory vasoconstriction)
• Vasopressor therapy; hypothermia; severe hypertension; phaeochromocytoma
• Aortic coarctation and peripheral vascular obstruction (apparent)

Lowered by

• Sepsis and septic shock, anaphylaxis, adrenal crisis, spinal/neurogenic shock
• Liver failure, arteriovenous fistula/shunts, severe anaemia, thyrotoxicosis
• Vasodilators, general anaesthesia, post-cardiopulmonary-bypass vasoplegia

Technique & pitfalls

• MAP from an arterial line if present; cuff MAP otherwise (state which).
• RAP from IVC size and collapse — or measured CVP; the default 5 mmHg is a guess — say so ^[3,50].
• CO from LVOT SV × HR averaged over several beats.

Pseudo-change & artefact

• Error in CO propagates inversely and is dominated by the LVOT diameter.
• RAP misestimation matters little when MAP is high, a lot when MAP is low.
• Positive-pressure ventilation invalidates the IVC-derived RAP.

Treatment thresholds

• Distributive shock: low SVR with preserved CI supports vasopressor therapy to a MAP ≥65 mmHg (Surviving Sepsis 2021) ^[155].
• Cardiogenic shock: high SVR with low CI supports inotropy and, if pressure allows, afterload reduction ^[151,154].
• No isolated SVR threshold triggers therapy; it is interpreted with CI and MAP.

Next step

• Next: cardiac power output (card 44) and arterial elastance (card 48).
• Then: LV and RV function, dynamic LVOT obstruction (vasodilated hyperdynamic hearts can obstruct), fluid responsiveness.

Drugs

• Raise: noradrenaline, vasopressin, phenylephrine, methylene blue (vasoplegia).
• Lower: nitrates, nitroprusside, hydralazine, dihydropyridine calcium blockers, milrinone, general anaesthetics.

Reversibility

Changes within minutes with vasoactive drugs and resolves with the underlying cause (e.g. infection control in sepsis).