YOU’RE WORKING IN THEemergency department whenDoug Phillips arrives complain-ing of shortness of breath. Mr.Phillips, 67, has a history ofchronic obstructive pulmonarydisease (COPD). A physicalexam reveals a barrel-shapedchest, slightly cyanotic nail bedswith slow capillary refill time,and digital clubbing. His breath

sounds are distant with inspira-tory crackles. He sits on theedge of his chair, leaning for-ward, with both hands on hisknees. You draw a blood samplefor arterial blood gas (ABG)analysis. What will it show?

In this article, we’ll describe astep-by-step approach to inter-preting ABG results, which tellyou about oxygenation and

acid-base balance in thepatient’s blood and guide treat-ment decisions. First, though,let’s take a closer look at each ofthe values measured by ABGanalysis.

Five componentsArterial blood gas values have fivebasic components that you use toassess patients: percentage of

50 Nursing2004, Volume 34, Number 8 www.nursing2004.com

Take this step-by-step approach to demystify the parameters of oxygenation, ventilation, and acid-base balance. BY WILLIAM C. PRUITT, RRT, CPFT, MBA, AND MICHAEL JACOBS, RN, CCRN, CEN, MSN

I N T E R P R ET I N G A R T E R I A L B L O O DG A S E S : EASY AS

B

C

A

hemoglobin saturated with oxygenin arterial blood (SaO2), partialpressure of oxygen dissolved inarterial blood (PaO2), arterial bloodacidity or alkalinity (pH), partialpressure of carbon dioxide in arte-rial blood (PaCO2), and concentra-tion of bicarbonate ions in arterialblood (HCO3

–). Let’s look at whateach one tells you about yourpatient’s condition. (For normaltest parameters, see ABGs in Adults:What’s Normal?)• SaO2 and PaO2: An oxygenationreview. Oxygen is transported inthe blood in two forms. Oxyhemo-globin (oxygen bound to hemoglo-bin molecules in red blood cells[RBCs]) accounts for about 97% ofthe oxygen in the blood and ismeasured as SaO2. A normal oxy-hemoglobin saturation should begreater than 95%; if the valuedrops to 90% or less, immediatelyassess the patient and administeroxygen.

The remaining 3% of oxygen inthe blood travels as oxygen mole-cules dissolved in the blood and ismeasured as PaO2. The measuredPaO2 is related to the patient’s SaO2level: As oxygen is dissolving intothe blood, it’s also combining withhemoglobin in the RBCs. With ahigher PaO2, hemoglobin quicklytakes up oxygen molecules until thehemoglobin is saturated. At thatpoint, the SaO2 is 100%. (Note thatmore oxygen can still dissolve intothe blood, so the PaO2 can climbhigher than normal. For example, ina young person with no lungdisease breathing 100% oxy-gen for a short period, the PaO2could reach 600 mm Hg.)

The relationship betweenPaO2 and SaO2 is shown bythe S-shaped oxyhemoglobindissociation curve. Changesin certain parameters thatoccur in the body will cause a

shift of the S-shaped curve to theleft or right. A shift to the left,which indicates hemoglobin’sincreased affinity for oxygen(inhibiting oxygen release to thecells), can be caused by increasedpH, decreased temperature, ordecreased PaCO2. A shift to theright, which indicates hemoglo-bin’s decreased affinity for oxygenand easier movement of oxygeninto cells, can be caused bydecreased pH, increased tempera-ture, and increased PaCO2.

If the patient is hypoxemic, thelow oxygen content in his bloodwill be reflected in low PaO2 andSaO2 values. Mild hypoxemia isdefined as a PaO2 of 60 to 79 mmHg; moderate hypoxemia, 40 to 59mm Hg; and severe hypoxemia,less than 40 mm Hg.

Prolonged or severe hypoxemialeads to tissue hypoxia and anaero-bic metabolism, altering thepatient’s acid-base status.Administering supplemental oxy-gen to a patient who’s hypoxemicor hypoxic may prevent largechanges in acid-base status.• pH: Acid or base? The acidity oralkalinity of a solution is measuredby its pH: The more hydrogen ionsin a solution, the more acidic it is.The normal range for pH is narrow(7.35 to 7.45); below 6.8 or above7.8, the body’s metabolic processesfail and the patient dies.• PaCO2: A respiratory parame-ter. The PaCO2 is a measure of thepartial pressure that dissolved car-

bon dioxide exerts in the plasmaand is directly related to theamount of carbon dioxide beingproduced by the cells. The PaCO2is regulated by the lungs and canbe used to determine if an acid-base disturbance is respiratory inorigin. This value is inverselyrelated to the rate of alveolar venti-lation, so a patient with bradypnearetains carbon dioxide. Increasedventilation reduces PaCO2, anddecreased ventilation raises PaCO2.A PaCO2 level below 35 mm Hgcauses alkalosis, and a level above45 mm Hg causes acidosis. Thebody can adjust the level of PaCO2in the body in a matter of minutesby increasing or decreasing respi-ratory rate or the volume of airbreathed. • HCO3

–: Metabolic parameter.The bicarbonate ion (HCO3

–) isthe acid-base component regu-lated by the kidneys. Acting asthe body’s buffer system, thekidneys retain or excrete thealkalotic bicarbonate ion asneeded. You can use the HCO3

–

value to determine if the sourceof an acid-base disturbance isrespiratory or metabolic. AnHCO3

– level below 22 mEq/literindicates acidosis; above 26mEq/liter indicates alkalosis.Unlike the respiratory system,which can make a quick adjust-ment to change PaCO2 levels, therenal system needs much moretime to alter HCO3

– levels. In aperson with normal renal func-

tion, HCO3– adjustments

may take several hours. Insomeone who’s elderly orwho has decreased renalfunction, HCO3

– adjust-ments may take severaldays.

Acute causes of changes inacid-base balance includeoversedation and head trau-

www.nursing2004.com Nursing2004, August 51

ABGs in adults: What’s normal?ABG component

pH

PaCO2

PaO2

HCO3–

SaO2

Normal range

7.35-7.45

35-45 mm Hg

80-100 mm Hg

22-26 mEq/liter

95%-100%

Test your ABG interpretation skills

ma (resulting in respiratory acido-sis), anxiety and anemia (resultingin respiratory alkalosis), starva-tion and diabetic ketoacidosis(resulting in metabolic acidosis),and vomiting and prolonged naso-gastric tube suctioning (resultingin metabolic alkalosis).

Complicating things with compensationCompensation is the body’sattempt to maintain a normalpH level. The respiratory sys-tem controls the carbon dioxidelevel, and the renal system con-trols the bicarbonate level. The

body uses these two systems tooppose each other in order tomaintain a normal pH. Forexample, if one system changesin the acidic direction, theother will compensate in thealkalotic direction.

A patient who’s breathing

52 Nursing2004, Volume 34, Number 8 www.nursing2004.com

Step 1: The PaO2 and SaO2 indicate mild hypoxemia. Administer supplemental oxygen andcontinue to monitor the patient’s oxygenation status.

Step 2: The pH indicates acidosis.Step 3: The PaCO2 level indicates alkalosis in the respiratory component of the ABG.Step 4: The HCO3

– indicates acidosis in the metabolic component of the ABG.Step 5: This patient is in acidosis because the pH is below normal. The origin of the acido-

sis is metabolic because the HCO3– value matches the acid-base status of the pH.

Step 6: The PaCO2 isn’t within normal limits, and neither is the pH, so the patient is partially compensated.

Step 7: The patient is in partially compensated metabolic acidosis with mild hypoxemia.

Step 1: The PaO2 and SaO2 indicate no hypoxemia.

Step 2: The pH indicates acidosis.

Step 3: The PaCO2 level indicates alkalosis in the respiratory component of the ABG.

Step 4: The HCO3– level indicates acidosis in the metabolic component of the ABG.

Step 5: The patient is in acidosis because the pH is on the low side of the normal range.

The origin of the acidosis is metabolic because the HCO3– matches the acid-base

status of the pH.

Step 6: The PaCO2 isn’t within normal limits, but the pH is, so the patient is fully compensated.

Step 7: The patient is in fully compensated metabolic acidosis with normal oxygenation.

Step 1: The PaO2 and SaO2 indicate mild hypoxemia. Administer oxygen and continue tomonitor the patient’s oxygenation status.

Step 2: The pH indicates acidosis.Step 3: The PaCO2 level indicates acidosis in the respiratory component of the ABG.Step 4: The HCO3

– level indicates alkalosis in the metabolic component of the ABG.Step 5: The patient is in acidosis because the pH is on the low side of the normal range.

The origin of the acidosis is respiratory because the PaCO2 matches the acid-basestatus of the pH.

Step 6: The HCO3– isn’t within normal limits, but the pH is, so the patient is fully compen-

sated.Step 7: The patient is in fully compensated respiratory acidosis with mild hypoxemia. This is

a typical ABG for a stable patient with chronic obstructive pulmonary disease(COPD) and is a commonly seen acid-base disturbance, given the many Americanswith COPD.

Case 1The patient’s values are: pH, 7.32 PaCO2, 31 mm Hg HCO3

–, 19 mEq/liter PaO2, 78 mm Hg SaO2, 89%

Case 2The patient’s values are: pH, 7.36 PaCO2, 29 mm Hg HCO3

–, 20 mEq/liter PaO2, 108 mm Hg SaO2, 99%

Case 3The patient’s values are: pH, 7.37 PaCO2, 58 mm Hg HCO3

–, 29 mEq/literPaO2, 65 mm Hg SaO2, 87%

www.nursing2004.com Nursing2004, August 53

rapidly blows off too much car-bon dioxide, reducing his PaCO2and increasing the pH of arterialblood. The body tries to com-pensate for this alkalosis byusing the kidneys to excretemore bicarbonate, which makesarterial blood more acidic.

An uncompensated statusindicates that one of the bodysystems (respiratory or kidneys)has made no attempt to com-pensate for the changing pH. Apartially compensated statusindicates that the opposingbody system is attempting tocompensate but hasn’t changedenough to bring the pH back tonormal limits. In this case, theopposing body system valuewill be outside its normal rangein the direction opposite to thecause of the problem.

A fully compensated statusconsists of pH within normallimits and values for the respi-ratory and metabolic compo-nents that are outside theirnormal ranges but in oppositedirections. Remember thatpatients with COPD often haveABG results that show fullycompensated respiratoryacidosis.

As carbon dioxide levels inthe blood increase, more hydro-gen ions are generated and thepH will drop, resulting in acido-sis. As carbon dioxide decreas-es, fewer hydrogen ions are gen-erated and the pH increases,resulting in alkalosis. Increasesin HCO3

– in the blood takehydrogen ions out of circula-tion, resulting in alkalosis;decreases in HCO3

– leave morehydrogen ions in circulation,resulting in acidosis.

Now let’s apply these princi-ples to interpreting a patient’sABG results.

Taking a systematic approachSuppose your patient’s ABG resultsare as follows: pH, 7.52; PaCO2,30 mm Hg; HCO3

–, 24 mEq/liter;PaO2, 89 mm Hg; and SaO2, 96%.You can see immediately that thepH is elevated, the PaCO2 is low,and the remaining values are with-in normal limits. How do youassess what these values tell youabout the patient’s condition?Follow these steps:

Step 1: Examine the PaO2 andthe SaO2 levels to determine ifhypoxemia exists and intervene ifnecessary. In our example, bothvalues are within normal limits, sothe patient isn’t hypoxemic. Con-tinue to monitor his oxygenationstatus.

Step 2: Examine the pH anddetermine if it indicates acidosis oralkalosis and circle the correctterm. Note that a pH between 7.35and 7.40 is considered normalacidic; a pH between 7.41 and 7.45is considered normal alkalotic. Inthe example, the pH of 7.52 indi-cates a clear alkalosis.

Step 3: Examine the PaCO2 anddetermine if it indicates acidosis oralkalosis. In this example, thePaCO2 is low, so the respiratorycomponent indicates alkalosis.

Step 4: Examine the HCO3– and

determine if it indicates acidosis oralkalosis. In the example, thismetabolic component is normal.

Step 5: Identify the origin of theacid-base disturbance as respirato-ry or metabolic. Circle the acidosisor alkalosis that matches the pH.In this case, the PaCO2 matches thepH, indicating respiratory alkalo-sis.

Step 6: Now determine whetherthe patient is in compensation. Isthe pH within normal limits? If so,the patient is fully compensated. Ifnot, look at the value you didn’tcircle (the one that didn’t match

the pH)—the HCO3– in our exam-ple. It’s within normal limits, so thepatient is uncompensated. If thisvalue had been outside the normallimits on the acidic side (becausethe pH was outside normal), thepatient would be partially compen-sated.

Step 7: Put it all together: Thepatient is in uncompensated respi-ratory alkalosis with normal oxy-genation.

For more practice, see Test YourABG Interpretation Skills.

Easy does itWith practice and careful thought,you can improve your skill andaccuracy at ABG interpretation. Byadding your knowledge of yourpatient’s clinical condition to what’shappening with his oxygenation,ventilation, and acid-base balance,you can intervene correctly anddeliver better patient care.

SELECTED REFERENCES Hogan, M., and Wane, D.: Fluids, Electrolytes, & Acid-Base Balance. Upper Saddle River, N.J.,Prentice Hall, 2002.

Huang, Y.: “Arterial Blood Gases,” in RespiratoryCare: Principles and Practices, D. Hess, et al.(eds). Philadelphia, Pa., W.B. Saunders Co.,2001.

Smeltzer, S., and Bare, B. (eds): Brunner and Sud-darth’s Textbook of Medical-Surgical Nursing, 10thedition. Philadelphia, Pa., Lippincott Williams &Wilkins, 2003.

William C. Pruitt is an instructor in the cardiorespira-tory care department in the College of Allied Healthat the University of South Alabama in Mobile. Healso works p.r.n. as a respiratory therapist atSpringhill Medical Center in Mobile. Michael Jacobsis a clinical assistant professor in the adult healthdepartment in the College of Nursing at theUniversity of South Alabama and a p.r.n. nursingsupervisor and emergency department staff nurse atOcean Springs (Miss.) Hospital. He also is a doctoralcandidate in nursing at Louisiana State UniversityHealth Sciences Center in New Orleans.

Virtual Hospital

An Approach to the Analysis of ArterialBlood Gases and Acid-Base Disorders

http://www.vh.org/adult/provider/internalmedicine/bloodgases

Last accessed on July 1, 2004.

SELECTED WEB S ITE