Objective-This study aims to determine the extent of correlation of arterial and venous pH with a view to identifying whether venous samples can be used as an alternative to arterial values in the clinical management of selected patients in the emergency department. Methods-This prospective study of patients who were deemed by their treating doctor to require an arterial blood gas analysis to determine their ventilatory or acid-base status, compared pH on an arterial and a venous sample taken as close to simultaneously as possible. Data were analysed using Pearson correlation and bias (Bland-Altman) methods. Results-Two hundred and forty six patients were entered into the study; 196 with acute respiratory disease and 50 with suspected metabolic derangement. The values of pH on arterial and venous samples were highly correlated (r=0.92) with an average diVerence between the samples of −0.4 units. There was also a high level of agreement between the methods with the 95% limits of agreement being −0.11 to +0.04 units. Conclusion-Venous pH estimation shows a high degree of correlation and agreement with the arterial value, with acceptably narrow 95% limits of agreement. Venous pH estimation is an acceptable substitute for arterial measurement and may reduce risks of complications both for patients and health care workers.

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The aim of this study was to determine the extent of correlation and agreement between arterial oxygen saturation and oxygen saturation as recorded by transcutaneous pulse oximetry, with a view to identifying whether pulse oximetry can be used as an alternative to arterial values in the clinical management of patients with acute exacerbations of chronic obstructive airways disease (COAD) in the emergency department. It also aims to determine whether there is a cut-off level of oxygen saturation by pulse oximetry that can screen for significant systemic hypoxia in this group. This prospective study of patients with acute exacerbations of COAD who were deemed by their treating doctor to require an arterial blood gas analysis to determine their ventilatory status, compared arterial oxygen saturation with simultaneously recorded oxygen saturation measured by transcutaneous pulse oximetry. Data were analysed using Pearson correlation, bias plot (Bland-Altman) methods for agreement and the receiver operator characteristic (ROC) curve method for determination of a screening cut-off. Sixty-four sample-pairs were analysed for this study. Nine (14%) had significant hypoxia (arterial PO2 less than 60 mmHg). The correlation coefficient was 0.91. The bias (Bland-Altman) plot shows a constant bias of -0.758% and only fair agreement, with 95% limits for agreement of -8.2 to + 6.7%. With respect to the ROC curve analysis, the 'best' cut-off for detection of hypoxia was at oxygen saturation by pulse oximetry of 92% (sensitivity 100%, specificity 86%). In conclusion, there is not sufficient agreement for oxygen saturation measured by pulse oximetry to replace analysis of an arterial blood gas sample in the clinical evaluation of oxygenation in emergency patients with COAD. However, oxygen saturation by pulse oximetry may be an effective screening test for systemic hypoxia, with the screening cut-off of 92% having sensitivity for the detection of systemic hypoxia of 100% with specificity of 86%.

Venous bicarbonate estimation shows a high level of agreement with the arterial value, with acceptably narrow 95% limits of agreement. These results suggest that venous bicarbonate estimation may be an acceptable substitute for arterial measurement.