SUBJECT: Sampling for arterial blood gas analysis
(Blood is drawn anaerobically from a peripheral artery (radial, brachial, femoral, dorsalis pedis, or posterior tibial) via a single percutaneous needle puncture, or from an indwelling arterial cannula or catheter for multiple samples. Either method provides a blood specimen for direct measurement of partial pressures of carbon dioxide (PaCO2) and oxygen (PaO2), hydrogen ion activity (pH), total hemoglobin (Hbtotal), oxyhemoglobin saturation (HbO2), and the dyshemoglobins carboxyhemoglobin (COHb) and methemoglobin (MetHb)).
- To evaluate the adequacy of ventilatory (PaCO2) acid-base (pH and PaCO2), and oxygenation (PaO2 and SaO2) status, and the oxygen-carrying capacity of blood (PaO2, HbO2, Hbtotal, and dyshemoglobins).
- To quantitate the patient's response to therapeutic intervention and/or diagnostic evaluation (e.g., oxygen therapy, exercise testing).
- To monitor severity and progression of a documented disease process.
- Arterial blood gases are done on a physicianís order only.
The following should be monitored as part of arterial blood sampling:
- Sample will not be submitted if it is contaminated by air, improper anticoagulant or inappropriate anticoagulant concentration, flush solution (if sample is drawn from an indwelling catheter), or venous blood. The sample clots because of improper anticoagulation of the collection device, improper mixing, or exposure to air. If there has been delay of > l5 minutes for samples held at room temperature or > 60 minutes for samples held at 2-4įC.
- Inspired oxygen percentage and flowrate
- Proper application of patient device (e.g., venturi mask, partial rebreather or cannula)
- Mode of supported ventilation and relevant ventilator feedback (e.g. actual minute ventilation)
- Pulsatile blood return and appearance of puncture site after direct pressure has been applied and before application of pressure dressing for potential hematoma formation
- Presence or absence of air bubbles or clots in syringe or sample
- Patient's temperature
- Position and /or level of activity (if other than resting)
- Patient's clinical appearance and ease of (or difficulty with) blood sampling.
- Appropriately anticoagulated sterile glass or plastic (low diffusibility) syringe with needle.
- Waste syringe (for 3 ml of waste) if arterial line draw.
- Patient label and lab collection slip.
- 70% isopropyl alcohol or other suitable antiseptic solution.
- Gauze square or similar material (omit if from indwelling catheter).
- Well-fitting non-latex gloves.
- Biohazard laboratory specimen bag.
- Needle cap allowing for single-handed recapping or device designed to allow single-handed insertion of the sample needle point after withdrawal from the artery (this should provide some resistance to insertion and should not allow the needle to completely traverse it).
- Syringe cap.
- Local anesthetic is not generally considered necessary for single punctures (unless for placement of indwelling catheter).
- 'Waste' syringe (if from indwelling catheter).
- Protective eyewear and outerwear (in the anticipation of splashing) (if from indwelling catheter).
- Container with ice deep enough to immerse syringe beyond the level of the specimen (to immerse syringe barrel if specimen will not be analyzed within l5 min).
- Obtain needed equipment.
- Perform Allenís test (both the radial and ulnar arteries should be compressed at a level approximately 1 centimeter proximal to the wrist joint while the patientís hand is squeezed for approximately 5 seconds then relaxed. The palmar surface of the hand should be blanched. Release compression on the ulnar artery. It is normal for the palmar surface to flush within 5 seconds. Prolonged delay before flushing indicates decreased ulnar artery flow. Radial arteries lacking collateral ulnar circulation should be avoided as puncture sites if possible. In adults if the radial artery is unsuitable as a puncture site, the brachial artery is the second choice, followed by the femoral artery).
- The skin over the puncture site is cleaned with 70% isopropyl alcohol or other suitable antiseptic solution.
- Palpate the site trying to stabilize the artery. Slight hyperextension of the wrist or elbow can be achieved by placing a rolled up towel under the joint; this can aid palpation and stabilization of the artery.
- Hold the syringe so the bevel of the needle faces upward, keeping the needle at a 25į to 45į angle to the artery. Insert the needle through the skin into the artery taking care not to puncture the posterior wall of the artery (if any venous blood is obtained the procedure should be restarted with a new syringe). If the artery is not entered immediately the needle may be slightly pulled back then redirected into the artery
- Arterial pressure should cause the blood to flow into the syringe.
- Withdraw the needle when an adequate sample has been obtained. Immediately place dry gauze or cotton over the puncture site and apply pressure.
- Maintain pressure over puncture site for a minimum of 5 minutes (longer if the patient has taken aspirin or anticoagulants).
- Single-handedly cap needle then remove from syringe.
- Expel any air bubbles from the sample and cap the syringe.
- Mix sample by rolling and tilting syringe.
- If not analyzed immediately, store the sample in ice (2-4įC). Iced samples should be analyzed within 3 hours.
- The puncture site should be compressed for a minimum of 5 minutes, longer if the patient is taking anticoagulant therapy, aspirin or has a prolonged prothrombin time. After 5 minutes, the puncture site should be inspected for several seconds to ensure that clotting has taken place. During this inspection, palpate the pulse proximal and distal to the puncture site to assess the presence of arterial spasm.
- A sterile bandage should be placed over the puncture site to keep the puncture site clean while healing. A bandage is not a substitute for compression of the puncture site.
Technique for obtaining sample from infants and children:
- Obtain needed equipment (heparinized syringe should be 1 ml with a 25-guage needle or you may use a 25-guage butterfly infusion kit).
- Perform Allenís test (Radial: The patientís hand is squeezed by the clinician. The clinician then occludes the radial and ulnar arteries by compressing them at the patientís wrist with the middle and index fingers of both hands and forefingers. While both arteries are still occluded, the fist is unclenched. The palmar surface of the hand should be blanched. Release compression on the ulnar artery. It is normal for the palmar surface to flush within 5 seconds. Prolonged delay before flushing indicates decreased ulnar artery flow. Radial arteries lacking collateral ulnar circulation should be avoided as puncture sites if possible. If the radial artery is unsuitable as a puncture site, the dorsalis pedis artery is the second choice, followed by the posterior tibial artery. Femoral artery punctures are performed only in emergency situations in children and never in neonates)(Dorsalis pedis: The foot is elevated and both the dorsalis pedis and posterior tibial arteries are compressed. Pressure is released from the artery that will not be punctured, and the nailbeds and the sole of the foot are assessed for return of blood flow which would confirm collateral circulation).
- Follow number 3-14 in procedure.
Technique for obtaining sample from arterial line:
- Turn stopcock off to patient.
- Remove the sterile cap from stopcock.
- Attach sterile syringe to stopcock.
- Open stopcock to syringe and intra-arterial catheter. Aspirate 3 ml of blood.
- Turn stopcock to the half-closed position, quickly remove syringe, and replace with heparinized syringe.
- Open stopcock to syringe and intra-arterial catheter and obtain arterial blood gas sample.
- Close stopcock to the syringe and remove syringe containing blood sample.
- Activate flush device to clear arterial line.
- Turn stopcock off to the patient and flush side port of stopcock into sterile syringe until all blood is cleared from stopcock.
- Close stopcock and replace sterile protective cap.
- Prepare arterial sample by holding syringe upright and remove air bubbles.
- Immediately seal syringe with cap.
- Roll and tilt syringe gently to ensure heparin mixing.
If not analyzed immediately, store the sample in ice (2-4įC). Iced samples should be analyzed within 3 hours.
Contraindications are absolute unless specified otherwise.
- Abnormal modified Allenís test is indicative of inadequate collateral circulation to the hand and suggests the need to select another extremity as the site for puncture.
- Arterial puncture should not be performed through a lesion or through or distal to a surgical shunt (e.g., as in a dialysis patient). If there is evidence of infection or peripheral vascular disease involving the selected limb, an alternate site should be selected.
- A coagulopathy or medium-to-high-dose anticoagulation therapy (e.g., heparin or coumadin, streptokinase, and tissue plasminogen activator but not necessarily aspirin) may be a relative contraindication for arterial puncture.
- Air or clotted-blood emboli
- Anaphylaxis from local anesthestic
- Introduction of contagion at sampling site and consequent infection in patient; introduction of contagion to sampler by inadvertent needle 'stick.'
- Trauma to the vessel
- Arterial occlusion
- Vasovagal response
- Repeated puncture of a single site increases the likelihood of hematoma, scarring, or laceration of the artery. Care should be exercised to use alternate sites for patients requiring multiple punctures. An indwelling catheter may be indicated when multiple sampling is anticipated.
LIMITATIONS OF METHOD/VALIDATION OF RESULTS:
- Artery may be inaccessible due to periarterial tissues (overlying muscle, connective tissue, or fat).
- Pulse may not be palpable.
- Arteriospasm may preclude collection despite successful introduction of needle into the artery.
- Arterial blood specimens withdrawn from the body only reflect the physiologic condition at the moment of sampling (e.g., pain from the puncture itself may lead to hyperventilation with consequent changes in values).
- Specimens drawn at peak exercise best reflect response to exercise; however, specimens drawn within 15 seconds or less of termination of exercise may be acceptable (otherwise results do not reflect ventilatory status during dynamic activities and may yield false-negatives for hypoxemic events).
- Specimens from mechanically ventilated patients with minimal pulmonary pathology adequately reflect the effects of oxygen concentration change 10 minutes after the change.
- In spontaneously breathing patients, at least 20-30 minutes should elapse following oxygen concentration change (patients with obstructive defects with increased residual volumes may require the full 30 minutes or longer).
- Specimens held at room temperature must be analyzed within 10-15 minutes of drawing; iced samples should be analyzed within 1 hour.' The PaO2 of samples drawn from subjects with elevated white cell counts may decrease very rapidly. Immediate chilling is necessary. Some dual-purpose electrolyte/blood gas analyzers stipulate immediate analysis without chilling because of possible elevations in potassium from chilling; however, the accuracy of the blood gas results should not be affected by the chilling.
Validation of results:
- Sample must be obtained anaerobically and anticoagulated, with immediate expulsion of air bubbles. Sample should be immediately chilled or analyzed within 10-15 minutes if left at room temperature.
- When a sample is obtained, date, time, patient's body temperature, position, activity level, respiratory rate, sample site, results of Allen test, inspired oxygen concentration or supplemental oxygen flow, and mode of supported ventilation should be documented in the patient' s medical record with the results of blood gas analysis.
- Appropriate sample size depends on (1) the anticoagulant used, (2) the requirements of the specific analyzers to be used, and (3) the presence of a need for other assays.
- If liquid heparin (sodium or lithium, 1,000 units/ml of blood) is used, excess heparin (all except that filling the dead space of the syringe and needle) should be expelled and a blood sample of 2-4 ml be drawn (liquid heparin dilutes the specimen and changes PCO2 and PO2 in direct relationship to the heparin volume).
- If lyophilized heparin is used, the minimum volume drawn depends on the design of the analyzers and the need for other assays.
- If other assays are required (e.g., electrolyte determination), the choice of anticoagulant and the volume of the blood sample should be guided by the analyzer manufacturer's recommendations.
- Universal Precautions must be applied in all circumstances involving blood or blood contaminated collection devices in the immediate area.
- Aseptic technique must be employed whenever blood is sampled from an indwelling arterial catheter.
- Prior to a single puncture, the site should be cleaned.
- Blood specimens, contaminated needles, and syringes must be disposed of in appropriate containers.
- Needle sticks are the most frequent source of transmission of blood-borne diseases in health care workers.
- Needles used for blood sampling should be resheathed only with a technique that utilizes a one-hand device or by careful insertion into a cork, rubber plug, or similar device that prevents the sharp point from being accessible.
- The needle should removed from the syringe and the syringe capped.
- Gloves provide little protection from needle punctures but should be worn to prevent splashing of blood on sores or other skin breaks.
- Specimen sampling devices in which the needle retracts after use are recommended when the design does not interfere with obtaining the sample.
- AARC Clinical Practice Guideline - Sampling for Arterial Blood Gas Analysis. Respir Care 1992;37:913-917
- Barnhart SL, Czervinske MP. Perinatal and Pediatric Respiratory Care, 1995; WB Saunders Company