Novel method for estimating the total blood volume: the importance of adjustment using the ideal body weight and age for the accurate prediction of haemodilution during cardiopulmonary bypass.

OBJECTIVES: Although total blood volume (TBV) is central to the estimation of the haemodilution rate during cardiopulmonary bypass (CPB), conventional formulas lack sufficient accuracy. The aim of this study was to establish a new formula using ideal body weight (BW) with adjustment for gender or age to estimate TBV for a more accurate prediction of the haemodilution rate during CPB.

METHODS: A total of 214 consecutive patients who underwent cardiac surgery with CPB were included in this study. TBV was retrospectively estimated using the following formulae: (1) Conventional TBV = actual BW × fixed 70 ml/kg, (2) gender-based modified TBV = ideal BW × 75 ml/kg (male) or 65 ml/kg (female) and (3) age-based modified TBV = ideal BW × 70 ml/kg (<65 years old) or 60 ml/kg (≥65 years old). The relationship between actual and predicted haemodilution rates calculated by these formulas was examined.

RESULTS: The actual haemodilution rate based on the haematocrit value was 24.4 ± 4.4%. There was no significant correlation between the actual and predicted haemodilution rates obtained by the conventional formula, whereas both modified formulae with the ideal BW showed a significant correlation. Furthermore, the age-based modified formula showed the highest correlation level (r = 0.45, P < 0.001) as well as a strong correlation between the actual and predicted postdilution haematocrit values (y = 0.903x + 3.385, R2 = 0.892).

CONCLUSIONS: The conventional formula is unable to predict the actual haemodilution rate accurately. Our new formula with a combination of the ideal BW and adjustment for age was shown to be useful for the accurate prediction of the haemodilution rate during CPB.