Calculation of Tissue Oxygen Debt
The Voa need was estimated from the patients own baseline preoperative Vo2 corrected for the effects of temperature and anesthesia. The temperature correction assumed metabolic activity increased or decreased 7 percent per degree Fahrenheit; ie, the temperature corrected Vo2 (Vo2c) was calculated as follows: Vo2c = Vo2x ; where T is the patients core temper ature in degrees Fahrenheit. Lowe and Ernst reported the following for the Vo2 of patients under anesthesia: Vo2 (anesth) = 10 x kg”. This value, also corrected for temperature, was used for calculation of the Vo2 need during the time the patient underwent general anesthesia. After the effects of the anesthetic were reversed after surgery, the temperature-corrected Vo2 was used.
The estimated Vo2 deficit or excess was calculated from the difference between the actual measured rate of oxygen consumed (Vo2 measured) and the preoperative baseline rate of Vo2 corrected for anesthesia and temperature (ie, the Vo2 baseline or Vo2 need). The rate of Vo2 deficit or excess at any given time is as follows: Vo2 deficit (or excess) = (Vo2 measured) — (Vo2 need).
The net cumulative oxygen balance at any given time was calculated from the integrated area describd by the time-Vo2 deficit curve (Fig 1).
Figure 1. Upper section, serial measurements of Vo2 (solid line) and estimated Vo2 requirements (dashed lines) in illustrative case. Middle section, Vo^ deficit measured as difference between actual measured Vo2 and Vo2 need. Lower section, cumulative Vo2 deficit below or excess (above) zero line.