Blood Pressure

It is a common mistake to equate a normal blood pressure with the absence of shock. The body’s ability to compensate means blood pressure is maintained until a late stage in the progression from physiological insult to an irreversible shock state. This is demonstrated well by acute blood loss [20].

Class of shock Class I Class II Class III Class IV
Volume of blood loss (ml) Up to 750 750-1500 1500-2000 >2000
Volume of blood loss (%) 0-15% 15-30% 30-40% >40%
Heart rate <100 >100 >120 >140
Blood pressure Normal Normal Decreased Decreased
Pulse pressure Normal or increased Decreased Decreased Decreased
Respiratory rate 14-20 20-30 30-40 >35
Mental state Slightly anxious Mildly anxious Anxious, confused Confused, lethargic

Not until 30-40% of the circulating blood volume is lost does the blood pressure begin to fall. Note also a reduction in pulse pressure occurs before a reduction in systolic BP as the diastolic increases in response to vasoconstriction.

The mean arterial pressure (MAP = (systolic + 2 x diastolic)/3) is a better representation of organ perfusion than the systolic. A MAP of 65mmHg is considered to be sufficient for organ perfusion in a healthy adult.