BACKGROUND: Analysis of venous blood gas (VBG) can represent arterial blood gas (ABG) analysis in patients with various diseases. The effects of hypotension on differences between the results of simultaneous venous and arterial blood gas analyses were reviewed.METHODS: This observational, cross-sectional study was conducted from March to October 2010 in emergency departments of two university hospitals in Tehran (Iran) on consecutive adult patients for whom ABG had been indicated for diagnosis/treatment. Arterial and peripheral venous bloods were simultaneously sampled with blood pressure measurement.The VBG-ABG amount of difference regarding pH, HCO3, PCO2, PO2, SO2, and Base Excess (BE) was compared between those with and without hypotension.RESULTS: During the study, 192 patients (51.6 ± 23.6 years, 67.7% males) were entered into the hypotension (n=78) and normotensive groups (n=114). The average VBG-ABG amount of difference (95% limits of agreement) in the hypotension versus normotensive group were -0.030 (-0.09 to 0.03) vs. -0.016 (-0.1 to 0.068) for pH (p=0.01), 1.79 (-1.91 to 5.49) vs.1.32 (-1.94 to 4.58) mEq/L for HCO3 (p=0.032), 2.69 (-20.43 to 25.81) vs.2.03 (-7.75 to 11.81) mmHg for PCO2 (p=0.295), -35.97 (-130.17 to 58.23) vs. -32.65 (-104.79 to 39.49) mmHg for PO2 (p=0.293), -18.58 (-14.66 to 51.82) vs. -9.06 (-31.28 to 13.16) percent (p < 0.001) for SO2, and 0.25 (-3.73 to 4.23) vs.0.79 (-2.51 to 4.09) for BE (p=0.036).CONCLUSIONS: Hypotensive status is associated with an increase in the amount of difference between VBG and ABG analysis regarding pH, HCO3, and BE, though the amount of increase does not seem to be clinically important. Studying the precise effects of replacing ABG with VBG on the clinical decision-making and the following outcomes is worthwhile.