Assessing Your Risk of Shock and Calculating MAP
How to Determine If You're in Shock
You are likely in shock if you have hypotension (systolic blood pressure <90 mmHg or MAP <65 mmHg for >30 minutes) PLUS signs of tissue hypoperfusion such as decreased mentation, cold extremities, urine output <30 mL/hour, or lactate >2 mmol/L. 1
Clinical Criteria for Shock Diagnosis
To meet shock criteria, you must have BOTH:
Blood pressure criteria: Systolic BP <90 mmHg OR mean BP <60 mmHg for >30 minutes, OR requirement of vasopressors to maintain these thresholds 1
At least one sign of hypoperfusion 1:
- Altered mental status or confusion
- Cold, clammy extremities or mottled skin (livedo reticularis)
- Urine output <30 mL/hour
- Elevated lactate >2 mmol/L
Hemodynamic Criteria (If Monitoring Available)
If you have invasive monitoring, shock is confirmed by cardiac index <2.2 L/min/m² combined with systolic BP <90 mmHg and pulmonary capillary wedge pressure >15 mmHg. 1
Additional hemodynamic markers include 1:
- Cardiac power output <0.6 W
- Shock index (heart rate/systolic BP) >1.0
Low Stroke Volume Index as a Shock Indicator
A stroke volume index (SVI) <30 mL/m² in patients with preserved ejection fraction, or <35 mL/m² in those with reduced ejection fraction, indicates significantly increased mortality risk and suggests inadequate cardiac output. 2
- In severe low-gradient aortic stenosis with preserved EF, SVI <30 mL/m² was associated with 1.8-fold higher 1-year mortality 2
- In reduced EF patients, the mortality threshold is higher at SVI <35 mL/m² 2
- During hemorrhagic shock, SVI can decrease by 70% from baseline 3
How to Calculate Mean Arterial Pressure (MAP)
MAP = Diastolic BP + [1/3 × (Systolic BP - Diastolic BP)]
Or simplified: MAP = [(2 × Diastolic BP) + Systolic BP] / 3 4
Example Calculation
If your blood pressure is 120/80 mmHg:
- MAP = 80 + [1/3 × (120 - 80)]
- MAP = 80 + 13.3
- MAP = 93 mmHg
Critical MAP Thresholds
A MAP of 65 mmHg is the minimum target for maintaining adequate organ perfusion in shock states. 1, 4
- Below 65 mmHg, organ autoregulation fails and blood flow becomes linearly dependent on pressure 1, 4
- Patients with chronic hypertension may require MAP ≥70 mmHg to maintain renal perfusion 4
- Elderly patients (>75 years) may tolerate lower MAP targets of 60-65 mmHg 4
Perfusion Pressure Calculation (More Accurate)
The true perfusion pressure = MAP - Central Venous Pressure (CVP). 4
- This trans-organ perfusion pressure should ideally be >60 mmHg for adequate kidney perfusion 4
- Elevated CVP from venous congestion reduces net perfusion pressure independent of cardiac output 4
Common Pitfalls to Avoid
MAP or blood pressure alone does NOT reliably indicate adequate tissue perfusion or cardiac output. 1, 4
- You can have "normal" MAP (>65 mmHg) but still be in hemorrhagic shock with severely reduced stroke volume 5
- In one animal study, MAP remained >65 mmHg despite 70% reduction in stroke volume during bleeding 5
- Always assess additional perfusion markers: lactate clearance, urine output, mental status, skin perfusion, and capillary refill 4
Key Warning Signs Despite "Adequate" MAP
Even with MAP >65 mmHg, you may still be in shock if you have 4, 5:
- Persistent lactate elevation
- Oliguria (<0.5 mL/kg/hour)
- Altered mental status
- Cold, mottled extremities
- Decreasing stroke volume index
The pulse pressure/heart rate ratio (PP/HR) correlates better with stroke volume changes than MAP alone during hemorrhage and resuscitation. 3 A 3.3-fold decrease in PP/HR ratio corresponded to 70% stroke volume reduction in hemorrhagic shock 3.