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.