Hypotension classification
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Hypotension: Classification
Definition: Hypotension is a systemic blood pressure below 90/60 mmHg. It becomes clinically significant when perfusion pressure is insufficient to meet the metabolic demands of vital organs.
I. Broad Clinical Classification
Hypotension can be approached from two complementary angles: by clinical type/mechanism (non-shock states) and by shock classification (severe/life-threatening hypotension).
II. Types of Non-Shock Hypotension
1. Orthostatic (Postural) Hypotension
A drop in systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes of standing from a supine or sitting position.
Sub-types:
- Neurogenic OH - Caused by autonomic nervous system failure impairing vasoconstrictor reflexes. Associated with:
- Primary: Multiple system atrophy (Shy-Drager syndrome), pure autonomic failure, Parkinson's disease
- Secondary: Diabetes mellitus neuropathy, amyloidosis, Guillain-Barré syndrome, HIV, alcoholic neuropathy
- Non-neurogenic OH - No direct autonomic dysfunction; caused by volume depletion, medications, or venous pooling. More common than neurogenic.
- Drugs: Diuretics, antihypertensives, nitrates, alpha-blockers, sildenafil, TCAs, opiates, L-dopa, phenothiazines, MAOIs, calcium channel blockers
- Bradley and Daroff's Neurology in Clinical Practice, p. 433-436
2. Postprandial Hypotension
BP drop occurring 15-90 minutes after eating, due to splanchnic pooling of blood during digestion. More common in the elderly, and in patients with Parkinson's disease or autonomic neuropathy.
3. Neurally Mediated (Vasovagal / Vasodepressor) Hypotension
The "common faint." A reflex-mediated hypotension triggered by prolonged standing, emotional stimuli, pain, or venipuncture. It is often recurrent and affects 20-25% of young people. May or may not be accompanied by bradycardia. Signs include pallor, diaphoresis, nausea, and dilated pupils.
- Bradley and Daroff's Neurology in Clinical Practice, p. 429
4. POTS (Postural Orthostatic Tachycardia Syndrome)
Sympathetically mediated orthostatic symptoms (tachycardia ≥30 bpm on standing) often without frank hypotension, but syncope can occur. Considered a related autonomic disorder.
5. Drug-Induced Hypotension
Persistent or episodic low BP as an adverse effect of antihypertensives, diuretics, vasodilators, or other medications.
6. Chronic / Constitutional Hypotension
Persistently low BP without identifiable pathology; often benign in young, fit individuals.
III. Classification of Shock (Severe Hypotension with Organ Hypoperfusion)
When hypotension is severe enough to impair oxygen delivery, it is classified as shock. The four-category physiologic classification is standard across Harrison's, Miller's Anesthesia, Rosen's, and Schwartz's Surgery:
| Shock Type | Primary Derangement | CO | SVR | Examples |
|---|---|---|---|---|
| Distributive | Reduced SVR (vasodilation) | ↑ (compensatory) | ↓↓ | Septic shock, anaphylaxis, neurogenic shock, adrenal crisis, SIRS, toxic shock syndrome |
| Cardiogenic | Pump failure | ↓↓ | ↑ (compensatory) | MI, myocarditis, severe arrhythmia, valvular disease |
| Hypovolemic | Reduced preload / volume loss | ↓ | ↑ | Hemorrhage, GI losses, burns, DKA, diabetes insipidus |
| Obstructive | Mechanical obstruction to flow | ↓ | ↑ | Tension pneumothorax, cardiac tamponade, pulmonary embolism, aortic dissection, constrictive pericarditis |
- Harrison's Principles of Internal Medicine 22E, Table 314-1, p. 3053-3056
- Miller's Anesthesia 10e, p. 1237-1247
IV. Hemorrhagic Shock Sub-classification (ATLS Classes I-IV)
A specific sub-type of hypovolemic shock classified by volume lost:
| Class | Blood Loss (mL) | Blood Loss (%) | Heart Rate | Blood Pressure | CNS/Mental Status |
|---|---|---|---|---|---|
| I | <750 | <15% | <100 | Normal | Normal/anxious |
| II | 750-1500 | 15-30% | >100 | Orthostatic changes | Anxious |
| III | 1500-2000 | 30-40% | >120 | Hypotension | Confused |
| IV | >2000 | >40% | >140 | Severe hypotension | Lethargic/obtunded |
Note: hypotension only becomes clinically apparent at Class III-IV (>30% blood volume loss). Young, healthy patients may compensate and maintain near-normal BP until sudden cardiovascular collapse.
- Schwartz's Principles of Surgery 11e, Table 5-5, p. 169
V. Hemodynamic Profiles at a Glance
| Parameter | Distributive | Cardiogenic | Hypovolemic | Obstructive |
|---|---|---|---|---|
| Cardiac Output | ↑ | ↓ | ↓ | ↓ |
| SVR | ↓ | ↑ | ↑ | ↑ |
| CVP/Preload | ↓ | ↑ | ↓ | ↑ (tamponade) / ↓ (PE, tension PTX) |
| Skin | Warm, flushed | Cool, mottled | Cool, clammy | Variable |
VI. Key Clinical Signs of Hypoperfusion
Regardless of classification, the following indicate significant hypoperfusion (Rosen's Emergency Medicine):
- Tachycardia
- Low cardiac output
- Dusky or mottled skin
- Delayed capillary refill
- Altered mental status
- Low urine output
- Low central venous O₂ saturation (ScvO₂)
- Elevated lactate
- Rosen's Emergency Medicine, Box 3.4
Summary: Hypotension is classified by type (orthostatic, postprandial, vasovagal, drug-induced) for outpatient/chronic presentations, and by shock category (distributive, cardiogenic, hypovolemic, obstructive) for acute life-threatening presentations. The ATLS hemorrhage classification adds granularity for trauma contexts. Identifying the mechanism early drives targeted resuscitation.
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