Venous blood gases and other alternatives to arterial blood gases

INTRODUCTION — An arterial blood gas (ABG) is the traditional method of estimating the systemic carbon dioxide tension and pH, usually for the purpose of assessing ventilation and/or acid-base status. However, the necessary sample of arterial blood can be difficult to obtain due to diminished pulses or patient movement. Diminished pulses may reflect poor peripheral circulation or low blood pressure, while patient movement is frequently caused by the pain associated with arterial puncture.

A venous blood gas (VBG) is an alternative method of estimating systemic carbon dioxide and pH that does not require arterial blood sampling. Performing a VBG rather than an ABG is particularly convenient in the intensive care unit and in the emergency department, either peripherally or from a central venous catheter from which venous blood can be quickly and easily obtained.

The sites from which venous blood can be sampled, measurements that can be performed on venous blood, and correlation of venous measurements with arterial measurements are reviewed here. Other alternatives to ABGs for estimating systemic carbon dioxide and pH are also described, including end-tidal carbon dioxide and transcutaneous carbon dioxide. ABGs, capnography, and mechanisms of oxygenation are reviewed separately. (See "Arterial blood gases" and "Carbon dioxide monitoring (capnography)" and "Measures of oxygenation and mechanisms of hypoxemia".)

VENOUS BLOOD GASES

Sampling sites — A VBG can be performed using:

●A peripheral venous sample (obtained by venipuncture)

●A central venous sample (obtained from a central venous catheter)

●Mixed venous sample (obtained from the distal port of a pulmonary artery catheter)

Central venous blood gases have been preferred because their correlation with arterial blood gases is the most well-established by research and clinical experience. Peripheral venous blood gases have been studied in critically ill patients as an alternative for patients who do not have central venous access [1-3]. If a tourniquet is used to facilitate venipuncture, it should be released about one minute before the sample is drawn to avoid changes induced by local ischemia [4]. Mixed venous blood gases are a reasonable alternative for patients whose venous access is a pulmonary artery catheter; however, a pulmonary artery catheter should not be inserted for the sole purpose of venous blood sampling. (See "Pulmonary artery catheterization: Indications, contraindications, and complications in adults".)

Measurements — A VBG measures the venous oxygen tension (PvO₂), carbon dioxide tension (PvCO₂), acidity (pH), oxyhemoglobin saturation (SvO₂), base excess (BE), lactate, and serum bicarbonate (HCO₃) concentration:

●PvCO₂, venous pH, BE, and venous serum HCO₃ concentration are used to assess ventilation and/or acid-base status. (See "Simple and mixed acid-base disorders".)

●SvO₂ may be used to guide resuscitation during severe sepsis or septic shock, a process called early goal-directed therapy. (See "Evaluation and management of suspected sepsis and septic shock in adults".)

●PvO₂ has no practical value at this time. It is not useful in assessing oxygenation because oxygen has already been extracted by the tissues by the time the blood reaches the venous circulation.

The inability of a VBG to measure oxygenation is the major drawback compared with an ABG. To overcome this limitation, VBGs are often considered in combination with pulse oximetry.

Interpretation — The usual approach to interpreting a VBG consists of using the venous measurements to estimate the corresponding arterial values, then using these estimated values for clinical decision-making exactly as if an ABG had been performed. The difference between the venous measurements and the arterial measurements depends upon the site of venous sampling and varies among laboratories.

Correlation with arterial blood gases — Venous measurement of PCO₂, pH, and HCO₃ are similar to arterial values with some minor adjustments (table 1) [5-17]:

●The central venous pH is usually 0.03 to 0.05 pH units lower than the arterial pH and the PCO₂ is usually 4 to 5 mmHg higher, with little or no increase in HCO₃ [5,6,16]. Mixed venous blood (ie, SvO₂ drawn from a pulmonary artery catheter) gives results similar to central venous blood (ie, ScvO₂ drawn from a central venous catheter) [7-9].

●The peripheral venous pH is approximately 0.03 to 0.04 pH units lower than the arterial pH, the venous serum HCO₃ concentration is approximately 2 to 3 meq/L higher, and the venous PCO₂ is approximately 3 to 8 mmHg higher [1,2,10-15,18-21].

●The venous BE is approximately 0.15 higher than the arterial BE, and the venous lactate is approximately 0.12 higher than the arterial lactate [22].

There are no venous to arterial conversions for ScvO₂, SvO₂, or peripheral venous oxyhemoglobin saturation (PvO₂).

Importantly, sufficient variability between arterial and venous blood gas values may exist such that periodic correlation between arterial and venous blood gas values is always prudent.

Misleading results — There are conflicting data regarding the correlation between arterial and venous blood gas measurements in patients with hemodynamic instability or during extremes of acid-base disturbance. This observation has two practical consequences. First, clinicians should be wary of VBG results and preferentially base clinical decisions on ABGs in hypotensive patients. Second, periodic correlation of the venous measurements with arterial measurements should be performed whenever venous measurements are used for serial monitoring.

These approaches are supported by the following evidence. In a study of 168 matched sample pairs from 110 patients treated in an intensive care unit, there were only small differences between the arterial and central venous blood samples (pH = 0.03 units, bicarbonate = 0.52 mmol/L, and lactate = 0.08 mmol/L) [23]. In the presence of shock, however, the difference between mixed venous and arterial blood PCO₂ increased by a factor of three. Numerous other studies have similarly found poor correlation of arterial and venous blood gas measurements among patients with shock [24-26] or during extremes of acid-base disturbance [27]. However, in spite of poor correlation, a venous blood gas can still provide useful data in terms of screening for acidosis and hypercarbia but perhaps with less accuracy for the latter.

END-TIDAL CARBON DIOXIDE (CAPNOGRAPHY) — Measurement of end tidal carbon dioxide (PetCO₂) is another method of noninvasively estimating the arterial carbon dioxide tension (PaCO₂). This technique, called capnography, requires a closed system of gas collection, either with a tight-fitting mask or a ventilator circuit. A sample of expired gas is analyzed by infrared or mass spectrometry, and then displayed as a numerical value or a graph. The PetCO₂ is usually within 1 mm of the PaCO₂ in healthy adults, but it is far less accurate in critically ill adults [28] because of the dependence of CO₂ production on cardiac output [29]. The routine use of PetCO₂ has existed primarily in newborn intensive care units, operating rooms, and emergency departments, to provide early warning of endotracheal tube complications. Capnography has now expanded to monitor the effectiveness of cardiopulmonary resuscitation in patients with cardiac arrest [30,31] and to monitor oxygenation during ambulatory transport. PetCO₂ is reviewed in detail separately. (See "Carbon dioxide monitoring (capnography)".)

TRANSCUTANEOUS CARBON DIOXIDE — Systems that measure both transcutaneous carbon dioxide (ptcCO₂) and pulse oximetry are an attractive option because they overcome the major limitations of both ABGs (invasive arterial sampling) and VBGs (lack of information about oxygenation). Such combination systems generally have a heating element that raises the skin temperature to 42 to 45ºC to increase local perfusion, an electrode to measure ptcCO₂, and a light emitter and sensor to measure arterial oxyhemoglobin saturation [32].

Older studies suggested that ptcCO₂ measurements are accurate in neonates, but not critically ill adults because of poor peripheral perfusion (peripheral artery disease, hypotension, vasopressors). These devices were limited because their accuracy diminished when the arterial carbon dioxide tension (PaCO₂) was greater than 56 mmHg [33-37]. Devices have since improved and more recent observational studies suggest that the newer systems may be more accurate in critically ill patients, including those with acute respiratory failure who have a PaCO₂ as high as 89 mmHg [38] and in critically ill patients on vasopressors and vasodilators [33].

Such combination systems have limitations. They may be difficult to keep calibrated, may be difficult to mount in a way that prevents air trapping, and may take up to an hour to sufficiently warm the skin [37]. In addition, the devices must be attached to an ear, which may be difficult in agitated patients or in those who had neurosurgery. Given the limitations of noninvasive monitoring, any persistent or unexpected change in the ptcCO₂ or oxyhemoglobin saturation should be confirmed with an ABG. The use of combined pulse oximetry and ptcCO₂ monitoring is becoming more accurate but remains limited to specific situations and equipment and should not be used routinely outside of well-defined clinical circumstances defined by the device manufacturer, sampling site, the sensor temperature, the clinical diagnosis, and severity of illness [39].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Assessment of oxygenation and gas exchange".)

SUMMARY AND RECOMMENDATIONS

●An arterial blood gas (ABG) is the traditional method of estimating the systemic carbon dioxide tension and pH, usually for the purpose of assessing ventilation and/or acid-base status. However, an ABG requires a sample of arterial blood, which can be difficult to obtain. A venous blood gas (VBG) is an alternative method of estimating systemic carbon dioxide and pH that does not require arterial blood sampling. (See 'Introduction' above.)

●A VBG can be performed using a peripheral venous sample (obtained by venipuncture), central venous sample (obtained from a central venous catheter), or mixed venous sample (obtained from the distal port of a pulmonary artery catheter). (See 'Sampling sites' above.)

●A VBG measures the venous oxygen tension (PvO₂), carbon dioxide tension (PvCO₂), acidity (pH), oxyhemoglobin saturation (SvO₂), base excess, serum lactate, and serum bicarbonate (HCO₃) concentration. The PvCO₂, venous pH, and venous serum HCO₃ concentration are used to assess ventilation and/or acid-base status. The SvO₂ is used to guide resuscitation during severe sepsis or septic shock. The PvO₂ has no practical value. (See 'Measurements' above.)

●The approach to interpreting a VBG consists of using the venous measurements to estimate the corresponding arterial values, then using these estimated values for clinical decision-making exactly as if an ABG had been performed. The central venous pH is usually 0.03 to 0.05 pH units lower than the arterial pH and the PCO₂ is 4 to 5 mmHg higher, with little or no increase in serum HCO₃. Mixed venous blood gives results similar to central venous blood. The peripheral venous pH range is approximately 0.02 to 0.04 pH units lower than the arterial pH, the venous serum HCO₃ concentration is approximately 1 to 2 meq/L higher, and the venous PCO₂ is approximately 3 to 8 mmHg higher. The correlation between arterial and venous blood gas measurements may vary with the hemodynamic stability of the patient and show less correlation at extremes of acid-base disturbance. (See 'Interpretation' above and 'Misleading results' above.)

●Measurement of end tidal carbon dioxide (PetCO₂) is another way of noninvasively estimating the arterial carbon dioxide tension (PaCO₂). PetCO₂ is reviewed in detail separately. (See "Carbon dioxide monitoring (capnography)".)

●Systems that measure both transcutaneous carbon dioxide (ptcCO₂) and pulse oximetry are an attractive option because they overcome the major limitations of both ABGs (invasive arterial sampling) and VBGs (lack of information about oxygenation). However, such combination systems have important limitations and use should be confined to well-defined clinical circumstances with specific equipment. Clinical trials are necessary before combined pulse oximetry and ptcCO₂ monitoring can be used for routine care. (See 'Transcutaneous carbon dioxide' above.)