Cardiac Output Measurement by Fick Equation in Hypothermia at 32°C
The Fick equation will underestimate cardiac output in a hypothermic patient at 32°C (option b).
Physiological Basis for Underestimation
The Fick equation calculates cardiac output as: CO = VO₂ / (CaO₂ - CvO₂), where oxygen consumption (VO₂) is divided by the arteriovenous oxygen content difference. Hypothermia at 32°C causes a substantial reduction in metabolic rate and oxygen consumption, which directly affects the accuracy of this calculation 1, 2.
Metabolic Changes at 32°C
- Cerebral metabolism decreases by approximately 6-7% for each 1°C reduction in core temperature 2
- Whole body oxygen consumption (VO₂) decreases significantly with hypothermia, with studies showing VO₂ declining by approximately 19 mL O₂ per minute for each degree reduction in temperature 3
- At 32°C specifically, the medullary respiratory center becomes depressed, leading to decreased minute ventilation 1, 2
- In the absence of shivering (such as when patients are paralyzed with neuromuscular blocking agents), metabolic rate decreases by 8% for each degree of heat lost 1
Impact on Fick Equation Components
The arteriovenous oxygen content difference (CaO₂ - CvO₂) changes during hypothermia due to multiple factors 4:
- Oxygen extraction ratio (O₂ER) decreases significantly during hypothermia (from 0.29 ± 0.05 to 0.22 ± 0.03 at 32°C) 4
- The oxyhemoglobin dissociation curve shifts leftward, as evidenced by P50 decreasing from 23.8 ± 1.7 to 20 ± 0.9 mm Hg at 32°C 4
- This leftward shift means hemoglobin holds onto oxygen more tightly, reducing oxygen delivery to tissues and narrowing the arteriovenous oxygen content difference 4
Why Underestimation Occurs
When VO₂ is measured at the reduced hypothermic level but cardiac output is actually maintained or even increased (due to increased systemic vascular resistance), the Fick equation will calculate a falsely low cardiac output 1, 2. The equation assumes that the measured VO₂ accurately reflects tissue oxygen delivery requirements, but in hypothermia:
- Cardiac output may be relatively preserved or even elevated in mild hypothermia (<36°C) due to increased sympathetic tone 1, 2
- However, by 32°C, moderate hypothermia begins to depress cardiac activity 1, 2
- The dramatically reduced VO₂ (decreased by approximately 20-23% from baseline to 32°C) creates a mismatch in the Fick calculation 3, 5
Clinical Implications
Studies comparing Fick-derived cardiac output with other methods have shown inherent limitations even in normothermic conditions, with bias of 58 mL/min and precision of 35 mL/min 6. These measurement errors are likely magnified during hypothermia due to the altered metabolic state 6, 7.
Alternative cardiac output measurement methods (such as continuous thermodilution via pulmonary artery catheter) may be more reliable during hypothermia, though even pulse contour analysis shows increased percentage error during hypothermia (±34% vs ±24% during normothermia) 7.
Key Pitfall to Avoid
Do not assume that a low cardiac output measured by Fick equation in a hypothermic patient represents true cardiogenic shock 1, 2. The reduced metabolic demands at 32°C mean that what appears to be inadequate cardiac output by Fick calculation may actually be appropriate for the patient's reduced oxygen consumption needs 3, 4.