Fig 3.

Hypotension and hypertension zones divided by the iso-pressure curve. The abscissa indicates CO, and the ordinate indicates SVR. The following equation governs the relationship among different systemic haemodynamic variables: $\text{CO}\times \text{SVR}=(\text{MAP}-\text{CVP})\times 80$. The black curve is the iso-pressure curve; for a given pair of CO and SVR values, as long as the CO–SVR product equals the difference of MAP and CVP times 80, the point determined by the CO–SVR pair falls on the iso-pressure curve. In this case, we assume MAP is 85 mm Hg and the CVP is 5 mm Hg to exemplify the concept; the point representing a CO of 5 L min⁻¹ and an SVR of 1280 mm Hg min L⁻¹ falls on the black iso-pressure curve because the CO-SVR product equals the MAP–CVP difference times 80. The left lower purple area is called the hypotension zone because any point in this area, no matter what CO and SVR values it presents, leads to a smaller MAP–CVP difference than the MAP-CVP difference dictating the iso-pressure curve. For example, point A, determined by a CO of 4 L min⁻¹ and an SVR of 1100 mm Hg min L⁻¹, corresponds to a MAP–CVP difference of approximately 55 mm Hg; if assuming CVP equals 5 mm Hg, MAP is 60 mm Hg in this example. The combinations of different CO and SVR changes represent different underlying pathophysiologies of hypotension. The right upper blue area is called the hypertension zone because any point in this area, no matter what CO and SVR values it represents, leads to a larger MAP–CVP difference than the MAP–CVP difference dictating the iso-pressure curve. For example, point B, determined by a CO of 6 L min⁻¹ and an SVR of 1400 mm Hg min L⁻¹, corresponds to a MAP–CVP difference of approximately 105 mm Hg; if assuming CVP equals 5 mm Hg, the MAP is 110 mm Hg in this example. The combinations of different CO and SVR changes represent different underlying pathophysiologies of hypertension. CO, cardiac output; SVR, systemic vascular resistance; CVP, central venous pressure.