ABG sampling provides valuable information relating to a patient's respiratory, metabolic acid‐base and electrolyte status (Table 13.8). It allows the partial pressure of oxygen (PaO₂), the partial pressure of carbon dioxide (PaCO₂) and the acid‐base balance (pH) to be measured. It is also used to measure electrolytes and haemoglobin (for example) at a specific point in the course of a patient's illness (Keogh 2017, Pagana and Pagana 2017). The sample is often taken during a period of acute deterioration (Pagana and Pagana 2017, Welch and Black 2017).

There are two methods for sampling an ABG: arterial puncture and intra‐arterial cannula. The latter method is usually used in the critical care unit, intensive care unit or high‐dependency unit. In practice, interpreting the abnormalities in each value that may be compensated will help to determine the treatment plan. Compensation occurs when the body is able to respond to the acid‐base imbalance to normalize the pH via the respiratory and/or metabolic systems (Table 13.9).

Arterial puncture may result in spasm, intraluminal clotting or bleeding, haematoma formation, or a transient obstruction of blood flow. These factors may decrease the arterial flow in distal tissues unless adequate collateral arterial vessels are available. Traditionally, palpation is the method used to locate the radial artery; however, Doppler ultrasound has also been used (Ueda et al. [245]). The radial artery at the wrist is the best site for obtaining an arterial sample because it is near the surface, is relatively easy to palpate and stabilize, and usually has good collateral supply from the ulnar arteries. This can be confirmed via a modified Allen test (Figure 13.11).

Multiple puncture attempts will increase the risk of patient discomfort, localized injury and haematoma, and delayed interventions, and may also contribute to arterial spasm (Ueda et al. [245]). There are various factors that are associated with failed arterial puncture or cannulation and they include obesity, oedema, arterial scarring, hypotension and atherosclerosis (White et al. [256]). If the patient requires repetitive blood sampling, it may be more appropriate to admit them to a more clinically suitable area where intra‐arterial cannulation is possible. This not only minimizes both the distress to the patient and the time taken by skilled personnel in obtaining each sample but also reduces the risks associated with multiple punctures (Brzezinski et al. [25]). It is worth noting that morbidity associated with arterial cannulation is less than that associated with five or more arterial punctures (Bersten et al. [13]; see also Chapter c17: Vascular access devices: insertion and management). Radial artery puncture and cannulation are relatively safe procedures with severe complications occurring rarely; however, there can be infections, thrombotic complications or mechanical complications in 1% of patients. Although the anatomy of the arteries in the forearm and hand is variable, adequate collateral flow in the event of radial artery thrombosis is present in most patients (Ueda et al. [245], White et al. [256]).