by Matija Ambroz and Gregor Prosen
A 23 years old pregnant woman was admitted with a history of polyuria, dysuria, fever, and thirst. She is an insulin dependent diabetic patient. She is febrile. Her chest is clear, and circulation is adequate. Urinalysis shows the presence of ketones, glucose, and leukocytes. Her lab results on admission are:
Na+ 136 mmol/L, K+ 4.8 mmol/L, Cl- 101 mmol/L, Glucose 23.2 mmol/L, Urea 8.1 mmol/L, Creatinine 0.09 mmol/L
Her ABG results are:
pCO2 = 14 mmHg
pO2 = 133 mmHg
HCO3- = 7.1 mmol/L
Low pCO2 and low HCO3- indicates metabolic acidosis. Hyperglycemia, glycosuria, and ketonuria indicate DKA. There might be an underlying UTI that triggered DKA. Respiratory alkalosis is a compensation.
Arterial blood gas (ABG) analysis is an important investigation to monitor the acid-base balance of critically ill patients.
ABG help to determine treatment may indicate the severity of the condition and can help to diagnose a disease. The respiratory status and acid-base equilibrium of individuals with pulmonary disorders, drug overdose, and metabolic disorders may be evaluated through this procedure.
Blood is drawn from a peripheral artery via single percutaneous needle puncture, or from an indwelling arterial cannula or catheter for multiple samples.
Partial pressures of carbon dioxide (PaCO2) and oxygen (PaO2), hydrogen ion activity (pH), total hemoglobin (Hb), oxyhemoglobin saturation (HbO2), and carboxyhemoglobin (COHb) and methemoglobin (MetHb) are directly measured.
Oxygen (O2) and carbon dioxide (CO2) are the most important respiratory gases, and their partial pressures in arterial blood show the overall adequacy of gas exchange. pH, which measures hydrogen ion activity, is a regular part of every arterial blood gas sampling (Figure 1). To learn how to evaluate ABG analysis please click here.
Figure 1: An example of an arterial blood gas analysis result.
ABG Sampling Procedure
- to evaluate ventilation, (PacO2) acid-base status (pH and PaCO2), oxygenation status (PaO2 and SaO2), 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 the progression and severity of the observed disease. We usually evaluate these parameters in patients with multi-organ failure, both chronic and acute respiratory failure, ventilated patients, critically ill trauma patients, septic patients, patients with burns and poisoned patients.
- Inadequate circulation,
- Burger’s disease,
- Raynaud’s syndrome,
- Full-thickness burns. Relative contraindications include:
- Skin infection at the site of puncture,
- Previous surgery in the area,
- Inadequate collateral flow,
- Partial-thickness burns,
- Anticoagulation or coagulopathy*.
*ABG sampling can be performed safely in patients who are on anticoagulants or have other coagulopathies. In patients with severe disseminated coagulopathies, extreme caution is required.
Equipment and Patient Preparation
Equipment used in arterial puncture include;
- ABG syringe, for an adult, use a 20-gauge, 2.5-inch needle for a femoral sample and a 22 gauge, 1.25-inch needle for a radial artery puncture, Also 23 gauge and 25 gauge needle can be used. A 23 gauge syringe may be used as it allows faster filling than 25 gauge one, but does not affect more pain to the patient (Figure 2).
Figure 2: Needle and syringe before assembly.
- 70% isopropyl alcohol or an antiseptic solution,
- gauze or cotton-wool ball to be applied over puncture site,
- well-fitting non-sterile gloves
- puncture-resistant container.
With an adult patient who is conscious, follow the steps below (adapted from W.H.O. best phlebotomy practice guidelines).
- Introduce yourself to the patient and ask their full name.
- Check that the laboratory form matches the patient’s identity.
- Ask whether the patient has allergies, phobias or has ever fainted during previous injections or blood draws.
- Discuss the procedure and obtain verbal consent.
- If the patient is afraid or anxious, help him relax and make him more comfortable.
- Make the patient comfortable in a supine position.
- Place a clean paper or towel under the patient’s arm.
Various arteries can be used for blood collection. The radial, brachial, and femoral arteries are the sites most commonly punctured for blood gas sampling in adults. The first choice is the radial artery due to its superficial anatomical location. It has good collateral circulation and is not surrounded by structures that could be easily damaged by puncturing.
The procedure as defined by W.H.O. guidelines consists of 16 steps for radial artery puncture.
- Approach the patient, introduce yourself and ask the patient to state their full name.
- Place the patient on their back, lying flat. Ask the nurse for assistance if the patient’s position needs to be altered to make them comfortable. If the patient is clenching their fist, holding their breath or crying, this can change breathing and thus alter the test result.
- Locate the radial artery by performing an Allen test for collateral circulation (Video 1). If the test fails to locate the radial artery or collateral flow is inadequate, repeat the test on the other hand.
- Video 1: Modified Allen test; Radial and Ulnar Artery are both pressed to prevent blood flow. Ulnar artery is released after the hand becomes pale. If the hand flushes after 5s – 15s, the ulnar artery has sufficient blood flow and radial artery may be punctured. If it takes more than 15s for hand to flush, the ulnar artery has inadequate blood flow and this hand should not be punctured.
- Perform hand hygiene, clear off a bedside work area and prepare supplies.
- Disinfect the sampling site on the patient with 70% alcohol and allow it to dry.
- Assemble the needle and heparinized syringe and pull the syringe plunger to the required fill level recommended by the local laboratory. (1 – 3 mL)
- Holding the syringe like a dart, use the index finger to locate the pulse again, inform the patient that the skin is about to be pierced then insert the needle at a 45-degree angle, approximately 1 cm distal to the index finger, to avoid contaminating the area where the needle enters the skin.
- Advance the needle into the radial artery until a blood flashback appears, then allow the syringe to fill to the appropriate level. DO NOT pull back the syringe plunger.
- Withdraw the needle and syringe; place a clean, dry piece of gauze or cotton wool over the site and have the patient or an assistant apply firm pressure for sufficient time to stop the bleeding. Check whether bleeding has stopped after 2–3 minutes.
- Activate the mechanisms of a safety needle to cover the needle before placing it in the ice cup.
- Expel air bubbles, cap the syringe and roll the specimen between the hands to gently mix it. Cap the syringe to prevent contact between the arterial blood sample and the air, and to prevent leaking during transport to the laboratory.
- Label the sample syringe.
- Dispose appropriately of all used material and personal protective equipment.
- Remove gloves and wash hands thoroughly with soap and water, then dry using single-use towels; alternatively, use alcohol rub solution.Check the patient site for bleeding and thank the patient.
- Check the patient site for bleeding and thank the patient.
- Transport the sample immediately to the laboratory, following laboratory handling procedures.
ABG Sampling video
Hints and Pitfalls
- Precooling proved to be useful for patients who had problems with anxiety and pain due to arterial blood puncturing. Cryoanalgesia can be provided by ice bag applied to wrist 3 minutes prior to arterial puncture.
- ABG measurements are particularly vulnerable to pre-analytic errors. Problems include air bubbles, improper anticoagulation, delayed analysis, non-arterial samples and other transport or handling related problems.
- After collection, the sample should be analyzed quickly. If a delay of more than 10 minutes is anticipated, the sample must be embedded in an ice bath. Leukocytes and platelets continue to consume oxygen in the sample after it is drawn and can cause a significant fall in PaO2 over time at room temperature, especially in the setting of leukocytosis or thrombocytosis. Cooling decrease the metabolic activity of leukocytes and platelets and thus prevent the clinically important effect of oxygen consumption for at least 1 hour.
- Room air has a PO2, of approximately 150-160 mmHg (at sea level) and a PCO2 of essentially zero. Thus, air bubbles that mix and equilibrate with arterial blood will shift the PaO2 toward 150 mmHg and PaCO2 toward zero.
- Heparin must be added to the syringe as an anticoagulant. Because the pH of heparin is near 7.0, and the PO2 and PCO2 of the heparin solution are near room air values, excess heparin can alter all three ABG measurements. After flushing the syringe with heparin, a sufficient amount usually remains in the dead space of the syringe and needle for anticoagulation without distortion of the ABG determination.
Post Procedure Care and Recommendations
After collection of at least 1 to 2 mL of sample, the needle is removed, and firm pressure is applied at the site of puncture for 3 – 5 minutes. If the patient is on anticoagulants or has any coagulopathy, the pressure is required for 10 – 15 minutes.
There are some potential complications related to arterial blood sampling.
- Temporary arterial occlusion and spasm may be prevented by helping the patient relax. One can achieve this by explaining the procedure, positioning the patient comfortably and using precooling or other forms of analgesia.
- The hematoma is a common complication without a serious sequel. It may be prevented by inserting the needle without puncturing the fat side of the vessel and by applying firm pressure at the site of a puncture. Due to high pressure present in arteries, pressure should be applied for a longer time than in venipuncture.
- Nerve damage may be prevented by choosing an appropriate site for puncturing and avoiding redirection of the needle.
- Fainting may be prevented by ensuring that the patient is lying down with their feet elevated.
Geriatrics, Pediatrics, Pregnant Patients and Other Considerations
Pediatrics should be mentioned as a special consideration due to the challenge that they present in the form of obtaining vascular access and blood samples. Fear and anticipation of pain associated with procedures may the hospital experience traumatic for children. The procedure should be explained before starting and consent taken. Parents may provide comfort to the child, but there is also a potential for parents to faint. Products to decrease the pain may be considered in stable patients. Capillary blood obtained from heel is another option and can be used for gas analysis when arterial access is unavailable or when the clinician is not comfortable obtaining a percutaneous arterial blood sample. Arterial blood may be obtained from radial, brachial, dorsalis pedis and in newborn infants, the umbilical arteries. The radial artery is the site of choice. For arterial puncture in infants and children, a small-gauge butterfly needle is preferable to a needle and syringe as used in adults. In contrast to arterial puncturing in adults, continuous, but gentle suction should be provided in infants. Pulsating blood is a good sign that the radial artery has been punctured.
References and Further Reading
- Kurtz I, Kraut J, Ornekian V, et al. Acid-base analysis: a critique of the Stewart and bicarbonate-centered approaches. Am J Physiol Renal Physiol. 2008 May;294(5)
- American Association for Respiratory Care. AARC Clinical Practice Guideline Samping for Arterial Blood Gas Analysis. Respir Care 1992(8);37:891–897
- Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, And Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 49. Available from: http://www.ncbi.nlm.nih.gov/books/NBK371/
- Kim HT. Arterial Puncture and Cannulation In: Roberts JR, Custalow CB, Thomsen TW, Hedges JR, eds. Roberts and Hedges’ Clinical Procedures in Emergency Medicine. 6th ed. Philadelphia: Saunders; 2013. Chapter 20. 368-384
- Patout M, Lamia B, Lhuillier E, et al. A Randomized Controlled Trial On The Effect Of Needle Gauge On The Pain And Anxiety Experienced During Radial Arterial Puncture. PLoS One. 2015 September; 10(9)
- WHO Guidelines on Drawing Blood: Best Practices in Phlebotomy. Geneva: World Health Organization; 2010. Available from: http://www.ncbi.nlm.nih.gov/books/NBK138650/
- Haynes JM. Randomized Controlled Trial Of Cryoanalgesia (Ice Bag) To Reduce Pain Associated With Arterial Puncture. Respir Care. 2015 Jan;60(1):1-5.
- Santilanes G, Ilene C. Pediatric Vascular Access and Blood Sampling Techniques. In: Roberts JR, Custalow CB, Thomsen TW, Hedges JR, eds. Roberts and Hedges’ Clinical Procedures in Emergency Medicine. 6th ed. Philadelphia: Saunders; 2013. Chapter 19. 341-367
Links to More Information
- Acid – Base physiology crash course and includes clinical cases. http://www.anaesthesiamcq.com/AcidBaseBook/ABindex.php – link
- Quick quiz to test your basic ABG interpretation skills. https://abg.ninja/abg – link
- Quick interpretation of ABGs – http://www.mdcalc.com/arterial-blood-gas-abg-analyzer/ – link
- WHO guidelines on drawing blood: best practices in phlebotomy. Includes step by step pictures for procedure. http://www.ncbi.nlm.nih.gov/books/NBK138650/pdf/Bookshelf_NBK138650.pdf – link