Fig 2.

The pressure–output–resistance triangle. Organ perfusion is in the centre of the triangle. This diagram is based on the following premises: (1) changes in systemic vascular resistance (determinant of blood pressure) and regional vascular resistance (determinant of organ perfusion) are concordant, and (2) the share of cardiac output among various organs remains stable. Blood pressure is proportional to cardiac output and systemic vascular resistance. Organ perfusion is proportional to perfusion pressure and inversely proportional to regional vascular resistance; it is also a percentage share of cardiac output (a). Hypotension can be caused by a decrease in systemic vascular resistance (close red circle and red arrow), and in this case, organ perfusion remains stable because of the proportional decreases in blood pressure and regional vascular resistance (b). Hypotension can be caused by a significant decrease in systemic vascular resistance (closed red circle and red arrow), although there is a lesser degree of increase in cardiac output (closed blue circle). In this case, organ perfusion is increased because of the lesser decrease in blood pressure than the decrease in regional vascular resistance (c). Hypotension can be caused by a decrease in cardiac output (closed red circle and red arrow), and in this case, organ perfusion is decreased because of decreased perfusion pressure in the face of an unchanged regional vascular resistance (d). Hypotension can be caused by a significant decrease in cardiac output (closed red circle and red arrow), although there is a lesser degree of increase in systemic vascular resistance (closed blue circle). In this case, organ perfusion has a significant decrease because of decreased perfusion pressure in the face of an increased regional vascular resistance (e). Hypotension can be caused by simultaneous decreases in cardiac output and systemic vascular resistance (closed red circles and red arrows). In this case, organ perfusion is decreased because of the more significant decrease in perfusion pressure than the decrease in regional vascular resistance (f). CO, cardiac output; VR, vascular resistance which can be either SVR (systemic vascular resistance) or RVR (regional vascular resistance) depending on the context; ΔP, perfusion pressure.