Definition and Classification of Hypotension
Hypotension is defined as a systolic blood pressure <90 mmHg or a mean arterial pressure <65 mmHg. 1
Blood Pressure Thresholds for Hypotension
Hypotension can be classified based on specific blood pressure measurements:
Absolute thresholds:
Relative thresholds:
Clinical Contexts of Hypotension
Hypotension presents differently depending on the clinical context:
Trauma Setting
- Hemodynamic instability is defined as admission systolic blood pressure <90 mmHg, or requiring fluid boluses/transfusions/vasopressors, or base deficit >6 mmol/L, or shock index >1 1
- The Advanced Trauma Life Support (ATLS) definition considers a patient "unstable" with blood pressure <90 mmHg and heart rate >120 bpm, with evidence of skin vasoconstriction, altered consciousness, or shortness of breath 1
Surgical Setting
- Postoperative hypotension (systolic BP <90 mmHg) is associated with increased risk of myocardial injury, acute kidney injury, and death 1
- The risk increases with longer durations of hypotension 1
- For patients with preoperative hypertension, the threshold at which harm occurs may be higher than 90 mmHg systolic 1
Heart Failure Setting
- In heart failure with reduced ejection fraction (HFrEF), low blood pressure is defined as systolic BP <90-100 mmHg 1
- Approximately 3-4% of HFrEF outpatients have low BP (SBP <90-95 mmHg) 1
- The prevalence increases with HF severity, reaching 9-25% in hospitalized patients 1
Septic Shock
- Defined as hypotension requiring vasopressors to maintain MAP ≥65 mmHg despite adequate fluid resuscitation 2
- Target systolic BP is usually 80-100 mmHg 4
Special Populations and Considerations
Different patient populations may require different blood pressure targets:
- Chronic hypertension patients: May require higher MAP targets (75-85 mmHg) 2
- Elderly patients (>75 years): May benefit from lower MAP targets (60-65 mmHg) 2
- Trauma patients without brain injury: May benefit from permissive hypotension (SBP 80-90 mmHg) until bleeding is controlled 2
- Trauma patients with brain injury: Should maintain normal blood pressure 2
Symptoms and Manifestations
Hypotension may present with various symptoms depending on severity:
- Dizziness, syncope, headache
- Visual disturbances
- Nausea/vomiting
- Fatigue
- Altered mental status
- Cool, clammy skin
- Decreased capillary refill
- Reduced urine output (<0.5 mL/kg/hr)
- Tachycardia (compensatory mechanism)
Monitoring Parameters
When assessing hypotension, several parameters should be monitored:
- Blood pressure (continuous if possible)
- Heart rate and rhythm
- Urine output (target >0.5 mL/kg/hr)
- Mental status
- Skin color and capillary refill
- Serial lactate measurements
- Central venous oxygen saturation (if available, target >70%)
Common Pitfalls and Caveats
Relative vs. absolute hypotension: A patient with chronic hypertension may experience symptoms at "normal" blood pressures that represent a significant drop from their baseline.
Orthostatic measurements: Always check for orthostatic hypotension by measuring BP after the patient has been lying or sitting for 5 minutes, then again at 1 and 3 minutes after standing 1.
Medication effects: Many medications can cause or worsen hypotension, including antihypertensives, alpha-blockers (tamsulosin), sildenafil, trazodone, and carvedilol 5.
Circadian variation: Blood pressure normally decreases by 10-20% during sleep, but this pattern varies between individuals 1.
Between-arm differences: A between-arm systolic BP difference of >10 mmHg is associated with increased cardiovascular risk and may indicate arterial stenosis. Always measure BP in both arms initially and use the arm with the higher reading for subsequent measurements 1.