## Algor Mortis: Rate of Body Cooling **Key Point:** Algor mortis (postmortem body cooling) occurs at an average rate of **1.5–2.0°F (0.8–1.1°C) per hour** under standard ambient conditions (~20–25°C / 68–77°F). ### Henssge Nomogram and Cooling Rate The classic formula for estimating postmortem interval (PMI) from body temperature is: $$\text{PMI (hours)} = \frac{37°C - \text{Rectal Temperature}}{1.0°C/\text{hour}}$$ This assumes: - Ambient temperature ~20°C - Body weight ~70 kg - No external heat source or insulation **High-Yield:** The **Henssge nomogram** is the gold standard forensic tool for PMI estimation from algor mortis. It accounts for ambient temperature, body weight, and surface area. ### Factors Affecting Cooling Rate | Factor | Effect on Cooling | | --- | --- | | **Cold ambient temperature** | Accelerates cooling | | **Hot ambient temperature** | Slows or reverses cooling | | **Thin body/low weight** | Faster cooling | | **Obese body/high weight** | Slower cooling | | **Wet clothing/immersion** | Accelerates cooling | | **Fever/sepsis at death** | Initial temperature higher → longer cooling time | | **Extreme exertion before death** | Initial temperature elevated | **Clinical Pearl:** Algor mortis is **least reliable** for PMI estimation in the first 12 hours and after 24–36 hours (when body temperature plateaus with environment). It is most useful between 12–24 hours postmortem. **Mnemonic:** **1.5–2.0°F PER HOUR** — Think "roughly 1 degree per hour" as a quick mental estimate; the precise range is 1.5–2.0°F. ### Limitations of Algor Mortis 1. **High variability** — Can range from 0.5–3.0°F/hour depending on conditions 2. **Ambient temperature dependency** — Must be known accurately 3. **Plateau effect** — Body temperature equalizes with environment; no further cooling occurs 4. **Early unreliability** — First few hours show variable cooling due to metabolic heat
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