BACKGROUND: We investigated changes in heart rate variability (HRV) across different degrees of acute dilutional anemia (hemoglobin [Hb]=9, 7, 5, 4, and 3 g/dL) in a pig model. METHODS: Twelve anesthetized mechanically ventilated pigs of either gender (mean body weight 27.5+/-5.5 kg) were hemodiluted by exchange of blood for hydroxyethyl starch (6%; 200000/0.5) from baseline values to each animal\'s individual critical hemoglobin concentration (Hbcrit 3.3 [2.3/3.6] g/dL). Differences in time- and frequency-domain calculations of HRV were analyzed throughout the hemodilution procedure by using short-term electrocardiogram recordings (analysis of variance+Dunn\'s post-hoc test). RESULTS: During the hemodilution procedure, the standard deviation of normal R-R intervals and the coefficient of variation changed at Hb 5.3 (4.2/5.7) g/dL. Thereafter, the high-frequency power (HF), total power of the variance, and root mean square of successive N-N interval differences changed at Hb 3.9 (3.1/4.3) g/dL. The low-frequency power (LF) and the LF/HF ratio remained unaffected by hemodilution to Hbcrit. CONCLUSION: Acute dilutional anemia resulted in significant changes in different time- and frequency-domain variables in HRV analysis. These changes occurred considerably earlier than did commonly recognized transfusion triggers or signs of general tissue hypoxia. Further investigation is warranted to elucidate whether these changes can be considered as indicators of imminent tissue hypoxia.