A method for calculation of arterial acid–base and blood gas status from measurements in the peripheral venous blood
Introduction
Measurement of the acid–base and blood gas status of arterial blood is a helpful and often necessary tool in assessing the status of the acutely ill patient. In wards other than the intensive care unit, other emergency medicine departments and departments of lung medicine, the sampling of arterial blood is not usually a routine part of clinical care. In patients residing in medical departments such as internal medicine, indwelling arterial catheters are uncommon, meaning that arterial blood samples are usually only taken when the patient's state has deteriorated and the clinician requires further information. At this point, arterial blood is only usually taken by specially trained staff [1] via an arterial puncture with a needle, with the associated elevated risk of side effects such as haematoma, and the pain associated with radial artery puncture [2], [3], [4]. It is possible that delayed or infrequent assessment of a patient's arterial acid–base status could result in insufficient diagnosis and delayed referral to the ICU.
In contrast, large numbers of peripheral venous blood samples are taken easily and safely in medical departments. Indeed, for most patients, a peripheral venous blood sample is one of the first diagnostic tests performed on hospital admission. It has been argued, and to a large extent clinically accepted, that venous blood in particular peripheral venous blood, is not adequate for assessing the acid–base and respiratory state of patients [5], [6], [7], [8]. As a consequence only a small amount of information is obtained about the acid–base status from these samples, typically the standard bicarbonate (SBC), and the haemoglobin concentration (Hb). Other acid–base values, including pH, carbon dioxide pressure (PCO2), oxygen saturation (sO2), and oxygen pressure (PO2), are not measured. Indeed, since these venous samples are usually taken aerobically with no attempt to ensure a constant level of oxygen and carbon dioxide in the sample, these values would probably not reflect the true levels in the blood.
Previously, methods have been developed to approximate arterial blood from peripheral venous blood by warming the sampling site, causing blood flow to increase sufficiently such that venous blood samples have similar characteristics to arterial [9], [10], [11], [12]. Whilst this method may be used in clinical practice, automation of the warming process requires extra equipment, and takes 15 min for the hand to reach the appropriate temperature [13].
This paper presents a new method for calculating values of the acid–base status of arterial blood, from anaerobically taken venous blood samples, supplemented with arterial oxygen saturation measured by a pulse oximeter. Such a method may allow the large number of routinely and acutely taken peripheral venous blood samples in non-emergency wards to be used to obtain an acid–base status as part of routine clinical care. The purpose of this paper is: to present the method for calculating ‘arterialised values’; to illustrate its use by a few clinical examples with very different acid base status; and to analyse the sensitivity of the method to measurement errors and to the physiological assumptions included in the method.
Section snippets
Method for calculating ‘arterialised’ acid–base status
Fig. 1 illustrates a method for calculating values of the acid–base status of arterial blood from values in the peripheral venous blood, plus arterial oxygen saturation measured with a pulse oximeter. The principle of the method is that peripheral venous values of blood gas and oxygenation status can be mathematically transformed into arterial values by simulating the transport of blood back through the tissue. To perform this simulation two assumptions are required. First, it is assumed that
Use of the method for calculating ‘arterialised’ acid–base status
Three patients were studied to illustrate the method. In these patients, simultaneous samples of arterial and peripheral venous blood were taken (ABL 625, Radiometer, Copenhagen) anaerobically (PICO syringe, Radiometer, Copenhagen) with a measurement of the arterial oxygen saturation obtained from a pulse oximeter (COSMO plus, Novametrix medical systems, Wallingford, Connecticut). Blood samples were taken at what was considered to be a well-perfused arm, i.e. warm and of normal colour, while
Sensitivity of calculated values of ‘arterialised’ acid–base status to measurement error, physiological assumptions, and handling of the blood
The previous section described the results of calculating values of arterial acid–base status in three intensive care patients. In this section, the method is tested for its sensitivity to measurement errors, the physiological assumptions included in the method, and poor handling of the blood.
The sensitivity analysis is based on a hypothetical patient with normal arterial and venous values (pHv 7.370, PCO2,v = 6.1 kPa, PO2,v = 5.5 kPa, SO2,v = 75%, tNBBpv = 23.5 meq/l, Hbv = 9.3 mmol/l, SO2,a = SpO2 = 97%, RQ =
Discussion and conclusions
Measurement of the acid–base status of arterial blood is an important part of the evaluation of the acutely ill patient. Outside the high dependency department routine sampling of arterial blood is uncommon, usually requiring trained staff to perform arterial blood sampling via arterial puncture with a needle. Far more routine in these departments is the sampling and analysis of venous blood. This paper has proposed a method for calculating the values of acid base chemistry in the arterial
Conflict of interest
All authors are board members and shareholders of OBI APS. OBI APS is currently applying for a patent on the method presented in this manuscript.
Acknowledgement
This work was partially supported by a grant awarded by the IT-committee under the Danish Technical Research Council.
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2011, Computer Methods and Programs in BiomedicineCitation Excerpt :The principle of the method is that venous values can be mathematically transformed into arterial values by simulating the transport of blood back through the tissue. The steps included in the method are presented in detail in Rees et al. [72] with a brief summary given here. Step A: An anaerobic venous blood sample is drawn and measurements of pHv, PCO2,v, SO2,v, PO2,v, Hbv, Methaemoglobin (MetHbv), and carboxyhaemoglobin (COHbv) are taken.