Arterial Puncture for Blood Gas Analysis
Kimberly A. Robinson
Deborah H. Markowitz
Richard S. Irwin
Analysis of a sample of arterial blood for pHa, partial arterial carbon dioxide pressure (Paco2), partial arterial oxygen pressure (Pao2), bicarbonate, and percentage oxyhemoglobin saturation is performed with an arterial blood gas (ABG) analysis. Because an ABG can be safely and easily obtained and furnishes rapid and accurate information on how well the lungs and kidneys are working [1,2 and 3], it is the single most useful laboratory test in managing patients with respiratory and metabolic disorders. One should not rely on oximetry alone to evaluate arterial oxygen saturation (Sao2) fully. Given the shape of the oxyhemoglobin saturation curve, there must be a substantial fall in Pao2 before Sao2 is altered to any appreciable degree and it is not possible to predict the level of Pao2 and Paco2 reliably using physical signs such as cyanosis [4] and depth of breathing [5]. Additionally, a discrepancy between Sao2 measured by pulse oximetry and that calculated by the ABG can aid in the diagnosis of carboxyhemoglobinemia and methemoglobinemia.
Unsuspected hypoxemia or hypercapnia (acidemia) can cause a constellation of central nervous system and cardiovascular signs and symptoms. The clinician should have a high index of suspicion that a respiratory or metabolic disorder, or both, is present in patients with these findings and is most appropriately evaluated by obtaining an ABG. Although acute hypercapnia to 70 mm Hg (pH 7.16) and hypoxemia to less than 30 mm Hg may lead to coma and circulatory collapse, chronic exposures permit adaptation with more subtle effects [6]. Thus, the ABG provides the most important way of making a diagnostic assessment regarding the nature and severity of a respiratory or metabolic disturbance and of following its course over time.
Drawing the Arterial Blood Gas Specimen
Percutaneous Arterial Puncture
The conventional technique of sampling arterial blood using a glass syringe is described in detail, because it is the standard to which all other methods are compared. The pulsatile arterial vessel is easily palpated in most cases. If a large enough needle is used, entry is apparent as the syringe fills spontaneously by the pressurized arterial flow of blood, without the need for applying a vacuum or using a vacuum-sealed collecting tube. It is logical to preferentially enter arteries that have the best collateral circulation so that, if spasm or clotting occurs, the distal tissue is not deprived of perfusion. Logic also dictates that puncture of a site where the artery is superficial is preferable, because entry is easiest and pain is minimized. The radial artery best fulfills the above criteria; it is very superficial at the wrist, and the collateral circulation to the hand by the ulnar artery provides sufficient collateral blood flow in approximately 92% of normal adults in the event of total occlusion of the radial artery [9].
The absence of a report of total occlusion of the radial artery after puncture for ABG in an adult with normal hemostasis and the absence of significant peripheral vascular disease attests to the safety of the percutaneous arterial puncture. It also suggests that determining the adequacy of collateral flow to the superficial palmar arch by Allen’s test [10], a modification of Allen’s test [11] (see Chapter 3), or Doppler ultrasound [9] before puncture is not routinely necessary in those patients with normal hemostasis and the absence of significant peripheral vascular disease. If radial artery sites are not accessible, dorsalis pedis, posterior tibial, superficial temporal (in infants), brachial, and femoral arteries are alternatives (see Chapter 3).
Contraindications
Brachial and especially femoral artery punctures are not advised in patients with abnormal hemostatic mechanisms, because adequate vessel tamponade may not be possible in that these vessels are not located superficially, risking greater chance of complications [12]. If frequent sampling of superficial arteries in the same situation becomes necessary, arterial cannulation is recommended (see Chapter 3). Moreover, any vessel that has been reconstructed surgically should not be punctured for fear of forming a pseudoaneurysm, compromising the integrity of an artificial graft site or seeding the foreign body that could become a nidus for infection. This should also include avoidance of a femoral arterial puncture on the same side as a transplanted kidney.
The conventional recommended radial artery technique is as follows:
Put on protective gloves and sit in a comfortable position facing the patient.
With the patient’s palm up, slightly hyperextend the wrist and palpate the radial artery. Severe hyperextension may obliterate the pulse.
Cleanse the skin with an alcohol swab.
With a 25-gauge needle, inject enough 1% lidocaine intradermally to raise a small wheal at the point where the skin puncture is to be made. The local anesthetic makes subsequent needle puncture with a 22-gauge needle less painful and often painless [13]. If local anesthesia is not given, however, the potential pain and anxiety, if associated with breath holding, may cause substantial blood gas changes. Thirty-five seconds of breath holding in normal subjects has been associated with a fall in Pao2 of 50 mm Hg and a pH of 0.07 and a rise in Paco2 of 10 mm Hg [14].
Attach a needle no smaller than 22 gauge to a glass syringe that can accept 5 mL blood.
Wet the needle and syringe with a sodium heparin solution (1,000 units per mL). Express all excess solution.
With the needle, enter the artery at an angle of approximately 30 degrees to the long axis of the vessel. This insertion angle minimizes the pain due to inadvertently scraping the periosteum below the artery.
As soon as the artery is entered, blood appears in the syringe. Allow the arterial pressure to fill the syringe with at least 3 mL blood [15]. Do not apply suction by pulling on the syringe plunger.
Immediately after obtaining the specimen, expel any tiny air bubbles to ensure that the specimen will be anaerobic; then cap the syringe.
Roll the blood sample between both palms for 5 to 15 seconds to mix the heparin and blood. Apply pressure to the puncture site for 5 minutes or longer, depending on the presence of a coagulopathy. If the arterial sample was obtained from the brachial artery, compress this vessel so that the radial pulse cannot be palpated.
Immerse the capped sample in a bag of ice and water (slush) and immediately transport it to the blood gas laboratory.
Write on the ABG slip the time of the drawing and the conditions under which it was drawn (e.g., fraction of inspired oxygen, ventilator settings, position of the patient).
Deviations from these recommended techniques may introduce the following errors:
The syringe material may influence the results of Pao2 (16,17 and 18). The most accurate results have been consistently obtained using a glass syringe. If plastic is used, the following errors may occur: (a) falsely low Pao2 values may be obtained because plastic allows oxygen to diffuse to the atmosphere from the sample whenever the Po2 exceeds 221 mm Hg; (b) plastic syringes with high surface area to volume ratios (e.g., 1-mL tuberculin syringes) worsen gas permeability errors as compared to standard 3-mL syringes. For this reason, butterfly infusion kits with their long thin tubing should not be used [19]; (c) plastic syringes tenaciously retain air bubbles, and extra effort is necessary to remove them [17]; (d) plastic impedes smooth movement of the plunger, which could have an impact on the clinician’s confidence that arterial rather than venous blood has been sampled.Related posts:
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