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The arterial blood gas (ABG)analysis is a lab test that mea-sures the acid-base balance (pH)and oxygenation of an arterialblood sample, usually obtainedby direct arterial puncture. Forthe patient in critical care requir-ing multiple blood draws, anarterial line should be used.Nurses can learn valuable infor-mation about their patients byanalyzing the ABG results. Forexample, subtle changes in thepH may signal hemodynamicdecompensation, and improve-ments in oxygen saturation maybe related to improved perfusion.

Like many other lab tests, theABG analysis is a tool to helpnurses provide better care fortheir patients.

Measure for measureThe blood pH is a measurementof the acid content of the blood;

specifically, the partial pressureof hydrogen ions in the blood.Too many hydrogen ions in theblood lower the partial pressureand decrease the pH, causingacidosis. Conversely, too fewhydrogen ions increase the par-tial pressure and the pH levelrises, causing the patient tobecome alkalotic.1 Because thehuman body is sensitive tochanges in pH, the normal rangeis narrow.

The ABG analysis can mea-sure two factors that affect thepH: the partial pressure of car-bon dioxide (PaCO2) and bicar-bonate (HCO3) levels.1 ThePaCO2 measures carbon dioxide(CO2) in the blood; it’s affectedby CO2 removal in the lungs.(Carbon dioxide is produced bybody tissues as a by-product ofmetabolism.) Respiratory disor-ders like emphysema will affect

the lungs’ ability to removePaCO2. The PaCO2 is the respira-tory component of the ABG.(See Glossary of terms used inABG analysis.)

The HCO3 measures thebicarbonate content of theblood, and it’s affected by renalproduction of bicarbonate. If thebody produces more acid thanthe kidneys can buffer withbicarbonate, the patient willdevelop acidosis. If, on the otherhand, too much bicarbonate isproduced, alkalosis develops.The HCO3 is the metabolic com-ponent of the ABG analysis.

The ABG measurement alsoassesses oxygenation, as men-tioned earlier. The partial pres-sure of oxygen (PaO2) measuresthe amount of oxygen dissolvedin the blood. After oxygen dis-solves in the blood, it attaches tohemoglobin. The number of

ABGABGSix steps to

hemoglobin binding sites thathave oxygen attached to them iscalled the oxygen saturation(SaO2).

1 An SaO2 of 95% meansthat 95% of the available hemo-globin binding sites have oxygenattached. The SaO2 is dependenton the PaO2. Oxygen has to firstdissolve in the blood before itcan bind to hemoglobin. Bodytemperature, hydrogen ion con-centration, 2,3-diphosphoglycer-ate, and CO2 levels can affecthow easily oxygen attaches tohemoglobin and will, therefore,affect the SaO2.

2 (See Oxyhemo-globin dissociation curve.)

Choosing the targetArterial blood gas samples mustbe drawn from an artery that’sclose to the skin and has ade-quate redundant circulation.The radial artery is generallythe preferred site because it’s

readily accessible and its redun-dant circulation comes from theulnar artery. Care should betaken when drawing a bloodsample from the wrist of apatient with carpal tunnel syn-drome; the condition may makehim more susceptible to risk of injury of the underlyingnerves.3

If, for whatever reason, the

radial artery can’t be used todraw the blood sample, thefemoral artery is the secondchoice. It, too, is readily accessi-ble and has redundant circula-tion. The downside is that thissite is more prone to infection.

The choice of last resort is thebrachial artery. This vessel oftenlacks redundant circulation, anddamage to the brachial artery

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Refresh your understanding of arterial blood gas measurements and what they tell you about your patient.By David W. Woodruff, RN, CCRN, CNS, MSNBGBGanalysis

Glossary of terms used in ABG analysis

pH Acid content of the bloodPaCO2 Carbon dioxide content of the bloodPaO2 Oxygen content of the bloodHCO3 Bicarbonate content of the bloodSaO2 Percentage of hemoglobin saturated with oxygen Hypoxia Inadequate oxygenation of the tissueHypoxemia Low oxygen content in the bloodHypercarbia High carbon dioxide contentAcidemia Too much acid in the bloodAlkalemia Too many buffers in the bloodCompensation Ability of the body to stabilize acid-base imbalances

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can result in ischemia of theforearm and hand.3

Sampling refresherAn arterial line should be usedfor obtaining ABGs in the patientwhose condition requires multi-ple testing. A peripheral punctureis performed if multiple blooddraws aren’t necessary. Drawingan ABG sample is similar todrawing a venous blood sample.Follow your facility’s policy andprocedure. Perform Allen’s testprior to obtaining the ABG sam-ple. (See Allen’s test.) Put onclean gloves and then prepare thesite using an antimicrobial solu-tion (such as 2% chlorhexidinegluconate or alcohol swab).Because the artery often isn’t vis-ible, you’ll have to palpate it.Once you locate it, you make thepuncture.

Syringes used for arterial sam-ples are different from those usedfor venous samples. An arterialsyringe usually has a small-boreneedle attached (22-gauge or

smaller), and the syringe containsheparin to prevent clotting. Oncethe artery is punctured, bloodwill start flowing into the sy-ringe. Pressure in the arterial sys-tem usually provides a brisk,sometimes pulsatile, flow. Becareful not to introduce air bub-bles into the sample becausethey’ll alter oxygen readings.

The sampling syringe ismarked to show when the re-quired amount of blood is drawn(usually 1 to 1.5 mL). Once thecorrect amount of blood isdrawn, the needle should bewithdrawn rapidly and pressureapplied immediately. If bleedingoccurs at the puncture site, itmay be quite brisk and couldcause a hematoma or, rarely, pri-mary compartment syndrome.Maintain pressure on the punc-ture site for a minimum of 5 min-utes, longer if the patient has anelevated prothrombin time/acti-vated partial thromboplastin timeor if he’s taking anticoagulants.Apply a pressure dressing to pre-

vent oozing orrebleeding.When you docu-ment the proce-dure, be sure toinclude the timethe specimenwas drawn, per-centage of oxy-gen therapy,arterial puncturesite, results ofAllen’s test, anydifficultiesencounteredduring the pro-cedure, applica-tion of pressure,what type ofdressing wasused, and the

patient’s response.3 Follow yourfacility’s policy and procedureregarding the use of ice for ABGspecimens.

Six-step programAn ABG result is best analyzedby dividing it into the two majorcomponents discussed at thebeginning of this article: acid-base balance and oxygenation.This process can be described bythe following six steps. (See A 6-step program for ABG analysis.)1. Analyze the pH. Although7.4 is the optimal blood pH, thebody will tolerate a pH from7.35 to 7.45.4 If the pH is lowerthan 7.35, the patient is acidotic;if it’s higher than 7.45, he’s alka-lotic. If the pH is in the normalrange, look to see which side of7.4 it lies on. If the pH is 7.37,it’s said to be normal lying onthe acidotic side. This indicatesthat the patient may be acidotic,but he’s compensating to makethe pH closer to normal.5

2. Analyze the PaCO2. Remem-ber, CO2 is produced in the tis-sues of the body and eliminated

L=leftN=normalR=rightNL=normal

Shift to right ↓ Hgb affinity O2

Shift to left ↑ Hgb affinity O2

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Adapted from: Smeltzer S, Bare B. Brunner & Suddarth’s Textbook ofMedical-Surgical Nursing. 10th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2004.

Allen’s testThe Allen’s test is used to con-firm redundant circulation. Toperform it, occlude the radial andulnar arteries by applying firmpressure to the inner and outeraspects of the wrist. Maintain thepressure until the hand turnspale; then release the pressureon the ulnar artery. The handshould “pink up.” If the handremains pale, insufficient redun-dant circulation is present anddamage to the radial artery couldresult in ischemia of the hand.Another site should be con-sidered to draw an ABG sample.

Oxyhemoglobin dissociation curve

Arterial blood gas

in the lungs. Changes in thePaCO2 level reflect lung function.Normal PaCO2 levels range from35 to 45 mm Hg.4 A PaCO2 levelbelow 35 mm Hg can be causedby hyperventilation—basicallyblowing off CO2—which willmake the patient alkalotic. Whenthe PaCO2 level rises above 45mm Hg and the patient retainsCO2, he’s said to be acidotic.3. Analyze the HCO3. Bicar-bonate is produced by the kid-neys and represents the metabol-ic component of the blood gas.Normal levels are from 22 to 26mEq/L.4 An HCO3 level below 22mEq/L indicates acidosis, and alevel above 26 mEq/L indicatesalkalosis.4. Match either the PaCO2 or theHCO3 with the pH. If the pH islow and the PaCO2 is high, thepatient has respiratory acidosis.The patient has respiratory alka-losis if the pH is high and thePaCO2 is low. If the pH and HCO3are high but the PaCO2 is normal,the patient has metabolic alkalo-sis. The patient has metabolic aci-dosis if the pH and HCO3 are lowand the PaCO2 is normal. 5. Determine whether thePaCO2 or the HCO3 go in theopposite direction of the pH. Ifso, then the patient has compen-sation. Compensation is the abil-ity of one system to attempt tobalance the pH when the othersystem is causing an imbalance.For example, when the respira-tory system (CO2) becomes aci-dotic, the metabolic system(HCO3) will become alkalotic toattempt to bring the pH back tonormal. The respiratory systemcan compensate within seconds,but it may take hours for themetabolic system to fully com-pensate.4

6. Analyze the PaO2 and SaO2for hypoxemia. If the PaO2 is lessthan 80 mm Hg, or the SaO2 isless than 95%, the patient hashypoxemia. A patient on supple-mental oxygen may have a PaO2of more than 100 mm Hg.1

Evaluating resultsConsider three examples of ABGresults and what they tell youabout your patient’s condition.For instance:• pH, 7.28• PaCO2, 56 mm Hg• PaO2, 70 mm Hg• HCO3, 25 mEq/L• SaO2, 89%.

What do these numbers tellyou about the patient? The pH isless than 7.35, indicating acido-sis. The PaCO2 is higher than 45mm Hg, indi-cating acido-sis. The PaCO2matches thepH, making ita respiratoryacidosis. TheHCO3 is nor-mal, indicat-ing there’s nocompensa-tion. The PaO2and SaO2 are low, indicatinghypoxemia.

The full diagnosis for apatient with these ABG resultsis uncompensated respiratoryacidosis with hypoxemia. Thispatient may be suffering frompneumonia, chronic obstructivepulmonary disease (COPD), orsome other primary respiratorydisorder. Treatment will consistof administering oxygen toimprove his oxygenation anddecrease his PaCO2 by improvinghis ventilation. That’s an ency-clopedia of knowledge to get out

of a small amount of blood.Another example could be:

• pH, 7.5• PaCO2, 36 mm Hg• PaO2, 92 mm Hg• HCO3, 27 mEq/L• SaO2, 97%.

The pH is above 7.45, indicat-ing alkalosis. The PaCO2 is nor-mal, with no compensation. TheHCO3 is above 26 mEq/L, whichis alkalotic; it matches the pH,indicating metabolic alkalosis.The PaO2 and SaO2 are normal.(See Normal values for ABGs.)

The full diagnosis indicatedby this ABG analysis is uncom-pensated metabolic alkalosis.The patient is losing acid fromthe body, probably from vomit-ing or loss from a nasogastric(NG) tube. Treatment should be

aimed at limiting gastrointesti-nal (GI) loss and giving intra-venous (I.V.) fluids to replacevolume and restore pHbalance.4

And lastly:• pH, 7.37• PaCO2, 66 mm Hg• PaO2, 70 mm Hg• HCO3, 37 mEq/L• SaO2, 93%.

Although the pH is normal,it’s less than 7.4. So it’s on theacidotic side. The PaCO2 is above45 mm Hg, which is acidotic; itmatches the pH, indicating res-

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A 6-step program for ABG analysis

1. Analyze the pH.2. Analyze the PaCO2.3. Analyze the HCO3.4. Match either the PaCO2 or the HCO3 with the pH.5. Does either the PaCO2 or the HCO3 go in the opposite

direction of the pH?6. Analyze the PaO2 and SaO2.

1.2.3.4.5.

6.

piratory acidosis. The HCO3 isabove 26 mEq/L, which is alka-lotic; it goes the opposite direc-tion, indicating compensation.Because the pH is adjusted backinto the normal range, it’s calledfull compensation. Both thePaO2 and the oxygen saturationare low, indicating hypoxemia.

The full diagnosis for thepatient with this ABG analysis isfully compensated respiratoryacidosis with hypoxemia. Com-pensation from the kidneystakes several hours, indicatingthat this problem is probablychronic.4 Treatment will likelyinclude oxygen administration.The PaCO2 will remain uncor-rected if the problem is in factchronic, such as in COPD.Trying to correct PaCO2 isn’tadvised because the patient willsimply resume retaining CO2once treatment stops.

Acidosis vs. alkalosisRespiratory acidosis is caused bythe lungs’ inability to effective-ly remove the CO2 produced bymetabolism.1 It’s most oftencaused by a pulmonary disor-der, like COPD, asthma, pneu-monia, or pulmonary edema. Toremove the excess CO2, thepatient will have to move moreair through his lungs. This canbe accomplished by using bron-chodilators to open up the air-ways or by using a bilevel posi-tive airway pressure (BiPAP)machine or mechanical ventila-tion to increase tidal volume.The metabolic system compen-sates for respiratory acidosis byproducing more HCO3.

1 Thisprocess is slow, and full com-pensation by the kidneys indi-cates a chronic condition.

As mentioned above, respira-

tory alkalosis is caused by blow-ing off CO2, usually by hyper-ventilation. Encourage thepatient to slow his breathing. Insome cases, it’s helpful to havethe patient breathe into a paperbag; this allows the rebreathingof CO2. If a patient on a BiPAPmachine or a ventilator devel-ops respiratory alkalosis, hisrespiratory rate or tidal volumeis probably set too high andneeds to be adjusted. In respira-tory alkalosis, the metabolicsystem compensates by lower-ing the HCO3.

Metabolic acidosis can bebrought on by a variety of con-ditions, ranging from kidneyfailure, poisoning (especiallywith antifreeze or aspirin over-dose), diarrhea, or shock to dia-betic ketoacidosis.4 Treatment ofthe underlying condition shouldcome first. If that doesn’t fullyresolve the acidosis, then admin-istration of sodium bicarbonatemay be appropriate. The pul-monary system compensates formetabolic acidosis by increasingthe respiratory rate, thus in-creasing CO2 removal.

Metabolic alkalosis can be theresult of loss of acid from thestomach through vomiting orexcess NG suction.4 If vomitingcan be controlled or NG suction

slowed down, alkalosis mayresolve spontaneously. If not,I.V. fluids are typically given forvolume replacement and correc-tion of the imbalance. The pul-monary system compensates formetabolic alkalosis by decreas-ing the respiratory rate andretaining CO2.

6

Further stepsIf no compensation is found inthe blood gas analysis, the prob-lem is likely to be acute; thepatient’s acid-base imbalancemay cause respiratory, cardiac,or GI dysfunction. Treatmentgoals include managing theunderlying disorder to correctthe pH.

If, on the other hand, compen-sation is indicated by the testresults, the disorder may bechronic and the acid-base imbal-ance may persist despite treat-ment. Instruct the patient on howto manage the underlying disor-der so the imbalance doesn’tbecome worse. v

REFERENCES1. Guyton AC, Hall JE. Regulation of acid-base balance. In: Textbook of Medical Physi-ology, 10th ed. Philadelphia, Pa: W.B. Saun-ders; 2000.

2. Varjavand N, et al. The interactive oxy-hemoglobin dissociation curve. Available at: http://www.ventworld.com/resources/oxydisso/dissoc.html. Accessed December 4,2006.

3. Bucher L. Arterial puncture. In: Lynn-McHale Wiegand DJ, Carlson KK, eds.AACN Procedure Manual for Critical Care,5th ed. Philadelphia, Pa: Elsevier Saunders;2005.

4. Dufour DR. Clinical Use of LaboratoryData. Philadelphia, Pa: Lippincott Williams& Wilkins; 1998.

5. Woodruff D. Take these 6 easy steps toABG analysis. Nurs Made Inc Easy! 2006;4(1):4-7.

6. Martin L. All You Really Need to Know to Interpret Arterial Blood Gases, 2nd ed.Philadelphia, Pa: Lippincott Williams &Wilkins; 1999.

David W.Woodruff is president, Ed4Nurses.com,Macedonia, Ohio.

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Normal values forABGs

pH 7.35 to 7.45PaCO2 35 to 45 mm HgPaO2 80 to 100 mm HgHCO3 22 to 26 mEq/LSaO2 95 to 100%

Note: These are normal values at sealevel.

Arterial blood gas