Acid base balancing act

January 2009 American Nurse Today 17

WHEN IT COMES TO ACIDS ANDBASES, the difference between lifeand death is balance. The bodysacid-base balance depends on somedelicately balanced chemical reac-tions. The hydrogen ion (H+)affects pH, and pH regulation influ-ences the speed of cellular reac-tions, cell function, cell permeabili-ty, and the very integrity of cellstructure.When an imbalance develops,

you can detect it quickly by know-ing how to assess your patient andinterpret arterial blood gas (ABG)values. And you can restore thebalance by targeting your interven-tions to the specific acid-base disor-der you find.

Basics of acid-base balanceBefore assessing a patients acid-base balance, you need to under-stand how the H+ affects acids,bases, and pH. An acid is a substance that candonate H+ to a base. Examplesinclude hydrochloric acid, nitricacid, ammonium ion, lactic acid,acetic acid, and carbonic acid(H2CO3).

A base is a substance that canaccept or bind H+. Examples in-clude ammonia, lactate, acetate,and bicarbonate (HCO3-).

pH reflects the overall H+ con-

centration in body fluids. Thehigher the number of H+ in theblood, the lower the pH; and thelower the number of H+, thehigher the pH.A solution containing more base

than acid has fewer H+ and a high-er pH. A solution containing moreacid than base has more H+ and alower pH. The pH of water (H2O),7.4, is considered neutral.The pH of blood is slightly alka-

line and has a normal range of 7.35to 7.45. For normal enzyme andcell function and normal metabo-lism, the bloods pH must remain inthis narrow range. If the blood isacidic, the force of cardiac contrac-tions diminishes. If the blood is al-kaline, neuromuscular function be-comes impaired. A blood pH below6.8 or above 7.8 is usually fatal.

pH also reflects the balance be-tween the percentage of H+ and thepercentage of HCO3-. Generally, pHis maintained at a ratio of 20 partsHCO3- to 1 part H2CO3. (See Fastfacts on acid-base balance.)

Regulating acid-base balanceThree regulating systems maintainthe bodys pH: chemical buffers,the respiratory system, and the re-nal system.Chemical buffers, substances that

combine with excess acids orbases, act immediately to maintainpH and are the bodys most effi-cient pH-balancing force. Thesebuffers appear in blood, intracellu-lar fluid, and extracellular fluid. Themain chemical buffers are bicarbon-ate, phosphate, and protein.The second line of defense

against acid-base imbalances is therespiratory system. The lungs regu-late carbon dioxide (CO2) in theblood, which combines with H2Oto form H2CO3. Chemoreceptors inthe brain sense pH changes andvary the rate and depth of respira-tions to regulate CO2 levels. Faster,deeper breathing eliminates CO2from the lungs, and less H2CO3 isformed, so pH rises. Alternatively,slower, shallower breathing reducesCO2 excretion, so pH falls.The partial pressure of arterial

Perfecting your

acid-basebalancing actHow to detect and correct acid-base disorders

By Michelle Fournier, MN, RN, CCRN

LEARNING OBJECT IVES1. Identify four disturbances of acid-

base balance.2. Discuss nursing interventions for

patients with acid-baseimbalances.

3. Describe how to interpret arterialblood gas values.

CE1.8 contact

hours

18 American Nurse Today Volume 4, Number 1

CO2 (PaCO2) level reflects the levelof CO2 in the blood. Normal PaCO2is 35 to 45 mm Hg. A higher CO2level indicates hypoventilation fromshallow breathing. A lower PaCO2level indicates hyperventilation. Therespiratory system, which can han-dle twice as many acids and basesas the buffer systems, responds inminutes, but compensation is tem-porary. Long-term adjustments re-quire the renal system.The renal system maintains acid-

base balance by absorbing or ex-creting acids and bases. Also, thekidneys can produce HCO3- to re-plenish lost supplies. The normalHCO3- level is 22 to 26 mEq/L.When blood is acidic, the kidneysreabsorb HCO3- and excrete H+.When blood is alkaline, the kidneysexcrete HCO3- and retain H+. Unlikethe lungs, the kidneys may take 24hours before starting to restore nor-mal pH.

Compensating for imbalancesThe two disorders of acid-base bal-ance are acidosis and alkalosis. Inacidosis, the blood has too muchacid (or too little base). In alkalosis,the blood has too much base (ortoo little acid). The cause of theseacid-base disorders is either respira-tory or metabolic. If the respiratorysystem is responsible, youll detectit by reviewing PaCO2 or serum CO2levels. If the metabolic system is re-sponsible, youll detect it by re-viewing serum HCO3- levels.To regain acid-base balance,

the lungs may respond to a meta-bolic disorder, and the kidneys

may respond to a respiratory dis-order. If pH remains abnormal,the respiratory or metabolic re-sponse is called partial compensa-tion. If the pH returns to normal,the response is called completecompensation. Keep in mind thatthe respiratory or renal systemwill never overcompensate. Acompensatory mechanism wontmake an acidotic patient alkaloticor an alkalotic patient acidotic.

Understanding acidosis andalkalosisCaused by hypoventilation, respira-tory acidosis develops when thelungs dont adequately eliminateCO2. The hypoventilation may re-sult from diseases that severely af-fect the lungs, diseases of thenerves and muscles of the chestthat impair the mechanics ofbreathing, or drugs that slow a pa-tients respirations. Respiratory aci-dosis causes a pH below 7.35 and aPaCO2 above 45 mm Hg. HCO3- isnormal. (See Causes of acid-baseimbalances at a glance.)Caused by hyperventilation, res-

piratory alkalosis develops whenthe lungs eliminate too much CO2.The most common cause of hyper-ventilation is anxiety. Respiratoryalkalosis causes a pH above 7.45and a PaCO2 below 35 mm Hg.HCO3- is normal.Metabolic acidosis may result

from: ingestion of an acidic substanceor a substance that can be me-tabolized to an acid

production of excess acid

an inability of the kidneys to ex-crete normal amounts of acid

a loss of base.Metabolic acidosis causes a

HCO3- below 22 mEq/L and a pHbelow 7.35. PaCO2 is normal.Metabolic alkalosis may result

from: loss of stomach acid an excess loss of sodium orpotassium

a renal loss of H+

a gain of base.Metabolic alkalosis causes a

HCO3- above 26 mEq/L and a pHabove 7.45. PaCO2 is normal.

ABG analysis in four stepsABG analysis is a diagnostic testthat helps you assess the effective-ness of your patients ventilationand acid-base balance. The resultsalso help you monitor your pa-tients response to treatment. ABGanalysis provides several test re-sults, but only three are essentialfor evaluating acid-base balance:pH, PaCO2, and HCO3-. Memorizethese normal values for adults: pH: 7.35 to 7.45 PaCO2: 35 to 45 mm Hg HCO3-: 22 to 26 mEq/L.Remember, the key to interpret-

ing ABG values at the bedside isconsistency. Follow these four sim-ple steps every time: Step 1. List the results for thethree essential values: pH, PaCO2,and HCO3-.

Step 2. Compare them withnormal values. If a result indi-cates excessive acid, write anA next to it. If a result indi-cates excessive base, write aB next to it. And if a result in-dicates a normal balance, writean N next to it. The pH willtell you whether the patienthas acidosis or alkalosis.

Step 3. If youve written the sameletter for two or three results, cir-cle them. If you circle pH andPaCO2, your patient has a respira-tory disorder. If you circle pHand HCO3-, your patient has a

Fast facts on acid-base balance The more hydrogen ion (H+) in the blood, the lower the pH. The less H+ in the blood, the higher the pH. When partial pressure of arterial carbon dioxide (PaCO2) rises, pH falls. When PaCO2 falls, pH rises. In respiratory acid-base disorders, pH and PaCO2 move in opposite directions.

Bicarbonate (HCO3-) remains normal until compensation occurs.

In metabolic acid-base disorders, pH and HCO3- move in the same direction.

PaCO2 remains normal until compensation occurs.


metabolic disorder. If you circleall three results, your patient hasa combined respiratory andmetabolic acid-base disturbance.(See Interpreting arterial bloodgas values.)

Step 4. To check for compensa-tion, look at the result you did-nt circle. If it has moved fromthe normal value in the oppo-site direction of those circled,compensation is occurring. Ifthe value remains in the normalrange, no compensation has oc-curred. Once compensation iscomplete, the pH will return tonormal.Keep in mind that several factors

can make ABG results inaccurate: using improper technique todraw the arterial blood sample

drawing venous blood instead ofarterial blood

drawing an ABG sample within20 minutes of a procedure, suchas suctioning or administeringrespiratory treatment

allowing air bubbles in thesample

delaying transport of the sampleto the lab.

Nursing implicationsABG values provide important in-formation about your patients con-dition. But never underestimate theimportance of your clinical assess-ment and judgment. As a nurse,you are the most important advo-cate for your patients because youare constantly at the bedside, moni-toring, assessing, intervening, andreevaluating.Your role begins with identifying

patients at risk for acid-base distur-bances, including those who haveor are at risk for: significant electrolyte imbalances net gain or loss of acids net gain or loss of bases ventilation abnormalities abnormal kidney function.Assess patients carefully to

identify early clues of acid-basedisturbances. Consider what your

Causes of acid-base imbalances at a glanceListed below are specific causes of the four acid-base disorders.

Respiratory acidosisThe primary problem is alveolar hypoventilation (increased partial pressure ofarterial carbon dioxide [PaCO2]), which may result from: acute pulmonary edema central nervous system depression chronic respiratory disease disorders of respiratory muscles and chest wall inadequate mechanical ventilation oversedation severe pulmonary infections.

Respiratory alkalosisThe primary problem is alveolar hyperventilation (decreased PaCO2), which mayresult from: anxiety early sepsis excessive mechanical ventilation exercise fear heart failure hypermetabolic states such as fever hypoxemia liver failure pain.

Metabolic acidosisThe primary problems are increased acid and decreased bicarbonate (HCO3

-).Increased acid results from: anaerobic metabolism hyperalimentation ketoacidosis renal failure salicylate intoxication severe sepsis starvation.

Decreased HCO3- results from:

anhydrase inhibitors such as acetazolamide diarrhea hyperkalemia intestinal fistulas.

Metabolic alkalosisThe primary problems are increased HCO3

- and decreased acid. Increased HCO3-

results from: excessive ingestion of antacids excessive use of bicarbonate lactate administration in dialysis.

Decreased acid results from: hyperaldosteronism hypokalemia hypochloremia loop or thiazide diuretics nasogastric suction steroids vomiting.


patients vital signs are telling you.Count your patients respirationsfor a full minute. What are the rateand the depth? Are they clues toan impending or underlying respi-ratory or metabolic problem? Whatis your patients level of con-sciousness? Confusion can be anearly sign of an acid-base distur-bance. Correlate your patients flu-id balance and creatinine levelswith kidney function. Always cor-relate your assessment findingswith your patients diagnosis. Dothey match? Or is some clue point-ing in a different direction? Besure to double-check the implica-tions and adverse effects of alldrugs you administer.

Treating acid-base imbalancesTreatment for metabolic acidosisfocuses on correcting the underly-ing cause. For a diabetic patient,treatment consists of controllingblood glucose and insulin levels.In a case of poisoning, treatmentfocuses on eliminating the toxinfrom the blood. Correcting the un-derlying cause of sepsis may in-clude antibiotic therapy, fluid ad-ministration, and surgery. You mayalso treat the acidosis directly. Ifits mild, administering I.V. fluidmay correct the problem. If acido-sis is severe, you may give bicar-bonate I.V., as prescribed.Treatment for metabolic alkalosis

also focuses on the underlyingcause. Frequently, an electrolyte im-

balance causes this disorder, sotreatment consists of replacing fluid,sodium, and potassium.The treatment goal for respirato-

ry acidosis is to improve ventila-tion. Expect to administer drugssuch as bronchodilators to improvebreathing and, in severe cases, touse mechanical ventilation. Maintaingood pulmonary hygiene.Usually, the only treatment goal

for respiratory alkalosis is to slowthe breathing rate. If anxiety is thecause, encourage the patient toslow his or her breathing. Somepatients may need an anxiolytic.If pain is causing rapid, shallowbreathing, provide pain relief.Breathing into a paper bag allowsa patient to rebreathe CO2, raisingthe level of CO2 in the blood.

Practice makes perfectUse the case histories below to testyour acid-base knowledge withsome examples. Read each historyand try to determine the cause ofthe signs and symptoms. Then,read the interpretation section tosee how well you did. (See BeyondpH, PaCO2, and HCO3-.)

Case history 1Mary Barker, 34, comes to theemergency department (ED) withacute shortness of breath and painon her right side. She smokes onepack of cigarettes a day and re-cently started taking birth controlpills. Her blood pressure is 140/

80 mm Hg; her pulse is 110beats/minute; and her respiratoryrate is 44 breaths/minute. HerABG values are as follows: pH: 7.50 PaCO2: 29 mm Hg Partial pressure of arterial oxy-gen (PaO2): 64 mm Hg

HCO3-: 24 mm Hg Oxygen saturation (SaO2): 86%.Interpretation: These ABG values

reveal respiratory alkalosis withoutcompensation. The patients pHand PaCO2 are alkalotic, and herHCO3- is normal, indicating nocompensation. You would adminis-ter oxygen (O2) therapy, as or-dered, to increase SaO2 to morethan 95%; encourage the patient tobreathe slowly and regularly to de-crease CO2 loss; administer an anal-gesic, as ordered, to ease pain; andsupport her emotionally to decreaseanxiety. Based on the clues, theprobable underlying cause is pul-monary embolism.

Case history 2John Stewart, 22, is brought to theED for an overdose of a tricyclicantidepressant. Hes unconsciousand has a respiratory rate of 5 to 8breaths/minute. His ABG values areas follows: pH: 7.25 PaCO2: 61 mm Hg PaO2: 76 mm Hg HCO3-: 26 mm Hg SaO2: 89%.Interpretation: These ABG values

reveal respiratory acidosis withoutcompensation. The patients pH andPaCO2 are acidotic, and his HCO3- isnormal, indicating no compensation.You would administer O2, as or-dered. The patient may be intubatedto protect his airway and placed ona mechanical ventilator. You wouldalso treat the underlying cause byperforming gastric lavage and ad-ministering activated charcoal. Thispatients condition may progress tometabolic acidosis. If so, you wouldgive sodium bicarbonate to reversethe acidosis.

The table shows the values for acid-base disorders.

Disorder pH PaCO2 HCO3- Compensation

Respiratory acidosis N HCO3- > 26 mEq/L

Respiratory alkalosis N HCO3- < 21 mEq/L

Metabolic acidosis N PaCO2 > 45 mm Hg

Metabolic alkalosis N PaCO2 < 35 mm Hg

PaCO2 = partial pressure of arterial carbon dioxideHCO3

- = bicarbonate = increased level; = decreased level; N = normal level.

Interpreting arterial blood gas values


Case history 3Steve Burr, 38, has type 1 dia-betes. He hasnt been feeling wellfor the last 3 days and hasnt eat-en or injected his insulin. Hesconfused and lethargic. His respi-ratory rate is 32 breaths/minute,and his breath has a fruity odor.His serum glucose level is 620mg/dL. While receiving 40% O2,his ABG values are: pH: 7.15 PaCO2: 30 mm Hg PaO2: 130 mm Hg HCO3-: 10 mm Hg SaO2: 94%.Interpretation: These ABG val-

ues reveal metabolic acidosis withpartial respiratory compensation.The patients pH and HCO3- indi-cate acidosis. His PaCO2 is lowerthan normal, reflecting the lungsattempt to compensate. BecausepH is abnormal, you know com-

pensation isnt complete.

ABG values onlyTry interpreting this set of ABG val-ues without a clinical scenario: pH: 7.49 PaCO2: 40 mm Hg PaO2: 85 mm Hg HCO3-: 29 mm Hg SaO2: 90%Interpretation: These values re-

veal uncompensated metabolic al-kalosis. The pH and HCO3- indicatealkalosis. PaCO2 is normal, indicat-ing no compensation.Now, interpret these values:

pH: 7.25 PaCO2: 56 mm Hg PaO2: 80 mm Hg HCO3-: 15 mm Hg SaO2 : 93%Interpretation: These values re-

veal mixed acidosis. The pH, HCO3-,and PaCO2 all indicate acidosis.

Back in balanceHow did you do? Whether youaced this practice quiz or not, re-member that integrating your ABGinterpretation skills into your pa-tient assessments takes practice. Bybecoming more adept at identifyingspecific acid-base disorders, youcan ensure that patients receive theappropriate nursing interventionsand get back in balance as quicklyas possible.

Selected referencesAllibone L, Nation N. Guide to regulation ofblood gases: part two. Nurs Times.2006;102(46):48-50.

Ayers P, Warrington L. Diagnosis and treat-ment of simple acid-base disorders. NutrClin Pract. 2008;23(2):122-127.

Morton P, Fontaine D, Hudak C, Gallo B.Critical Care Nursing: A Holistic Approach.8th ed. Philadelphia, PA: Lippincott Williams& Wilkins; 2004.

Price S, Wilson L. Pathophysiology: ClinicalConcepts of Disease Processes. 6th ed. St.Louis, MO: Mosby; 2003.

Rhoades R, Pflanzer R. Human Physiology.4th ed. Fort Worth, TX: Saunders CollegePublishing; 2003.

Simpson H. Interpretation of arterial bloodgases: a clinical guide for nurses. Br J Nurs.2004;13(9):522-528.

Michelle Fournier is Founder and CEO of A ChoiceAbove in Denver, Colorado, and a healthcareconsultant for ja thomas & Associates in Smyrna,Georgia. The planners and author of this CNE activityhave disclosed no relevant financial relationshipswith any commercial companies pertaining to thisactivity.

CE POST-TEST Perfecting your acid-base balancing actInstructionsTo take the post-test for this article and earn contact hour credit,please go to www.AmericanNurseToday.com/ce. Once youvesuccessfully passed the post-test and completed the evaluationform, simply use your Visa or MasterCard to pay the processingfee. (Online: ANA members $15; nonmembers $20.) Youll then beable to print out your certificate immediately.If you are unable to take the post-test online, complete the

print form and mail it to the address at the bottom of the nextpage. (Mail-in test fee: ANA members $20; nonmembers $25.)

Please allow 4 to 6 weeks for CE processing.

Provider accreditationThe American Nurses Association Center for Continuing Educationand Professional Development is accredited as a provider of continu-ing nursing education by the American Nurses Credentialing CentersCommission on Accreditation.

ANA is approved by the California Board of Registered Nursing,Provider Number 6178.Contact hours: 1.8Expiration: 12/31/2010

Purpose/goal: To provide nurses with information so they can bettercare for patients with an acid-base imbalance.

Beyond pH, PaCO2, and HCO3-

To identify acid-base disorders, you need only three arterial blood gas (ABG)valuespH, partial pressure of arterial carbon dioxide (PaCO2), and bicarbonate(HCO3

-). But depending on the circumstances, you may find value in other ABGvalues.

One is a measurement of the partial pressure of arterial oxygen (PaO2). The nor-mal range for PaO2 is 80 to 100 mm Hg. But PaO2 varies with age and decreases af-ter age 60 without signs of hypoxia. PaO2 levels may also be lower in people wholive at higher altitudes.

Another valuable ABG value is oxygen saturation (SaO2), which is a measure ofthe percentage of hemoglobin actually carrying oxygen. The normal range forSaO2 is 95% to 100%.


Please circle the correct answer.1. Which statement about acid is correct?

a. An acid can take hydrogen (H+) from a base.b. An acid can donate H+ to a base.c. Ammonia is an acid.d. Bicarbonate is an acid.

2. Which statement about pH is correct?a. The pH of blood is neutral.b. The pH of blood is slightly acidic.c. pH reflects the overall H+ concentration in body

fluids.d. pH reflects the overall bicarbonate (HCO3

-) con-centration in body fluids.

3. The bodys most efficient pH-balancing force isthe:

a. chemical buffer system.b. respiratory system.c. renal system.d. metabolic system.

4. Which statement correctly describes the kidneysrole in acid-base balance?

a. The kidneys restore balance within minutes.b. The kidneys restore balance within 8 hours.c. When blood is alkaline, the kidneys excrete

HCO3- and retain H+.

d. When blood is alkaline, the kidneys excrete H+

and retain HCO3-.

5. Which statement correctly describes the lungsrole in acid-base balance?

a. A high carbon dioxide (CO2) level indicates hy-poventilation from shallow breathing.

b. A low CO2 level indicates hypoventilation fromshallow breathing.

c. The respiratory system can handle as many acidsand bases as the buffer system.

d. The respiratory system can handle three times as

many acids and bases as the buffer system.

6. An excessive loss of sodium or potassium cancause:

a. metabolic alkalosis.b. metabolic acidosis.c. respiratory alkalosis.d. respiratory acidosis.

7. Excessive ingestion of antacids can cause:a. metabolic alkalosis.b. metabolic acidosis.c. respiratory alkalosis.d. respiratory acidosis.

8. What is the acid-base status of a patientwith thesearterial bloodgas (ABG) results: pH, 7.28; PaCO2, 48mmHg; andHCO3

-, 25mEq/L?a. Metabolic alkalosisb. Metabolic acidosisc. Respiratory alkalosisd. Respiratory acidosis

9. What is the acid-base status of a patient withthese ABG results: pH, 7.26; PaCO2, 40 mm Hg; andHCO3

-, 18 mEq/L?a. Metabolic alkalosisb. Metabolic acidosisc. Respiratory alkalosisd. Respiratory acidosis

10. Which statement about respiratory acid-basedisorders is correct?

a. pH and PaCO2 move in the same direction.b. pH and PaCO2 move in opposite directions.c. HCO3

- increases.d. HCO3

- decreases.

11. What is the acid-base status of a patient withthese ABG results: pH, 7.43; PaCO2, 30 mm Hg; andHCO3

-, 18 mEq/L?a. Uncompensated metabolic alkalosis

b. Compensated metabolic acidosisc. Uncompensated respiratory acidosisd. Compensated respiratory alkalosis

12. An appropriate treatment for a patient with respi-ratory alkalosis is:

a. increasing the breathing rate.b. slowing the breathing rate.c. administering a bronchodilator.d. administering electrolytes.

13. Which statement about patient assessment iscorrect?

a. The patients creatinine level is an important re-flection of lung function.

b. Confusion is usually a late sign of acid-base dis-turbances.

c. Drawing an ABG sample 10 minutes after suc-tioning can lead to inaccurate results.

d. Including an air bubble when drawing an ABGsample ensures accurate results.

14. Whichstatementaboutacid-basebalance is correct?a. An electrolyte imbalance is frequently the cause

of metabolic alkalosis.b. An electrolyte imbalance is frequently the cause

of respiratory acidosis.c. The primary problem in respiratory alkalosis is

alveolar hypoventilation.d. The primary problem in respiratory acidosis is

alveolar hyperventilation.

15. Administering bicarbonate I.V. may be appropri-ate for:

a. mild respiratory acidosis.b. mild respiratory alkalosis.c. mild metabolic acidosis.d. severe metabolic acidosis.

Evaluation form (required)

1. In each blank, rate your achievement of each objective from 1 (low/poor) to 5(high/excellent).

(1.) Identify four disturbances of acid-base balance. ____

(2.) Discuss nursing interventions for patients with acid-baseimbalances. ____

(3.) Describe how to interpret arterial blood gas values. ____

Also rate the following from 1 to 5.2. The relatedness and effectiveness of the purpose, objectives, content, andteaching strategies. ____3. The author(s) competence and effectiveness. ____4. The activity met your personal expectations. ____5. The application to and usefulness of the content in your nursing practice. ____6. Freedom from bias due to conflict of interest, commercial support, productendorsement or unannounced off-label use. ____7. State the number of minutes it took you to read the article and complete thepost-test and evaluation. ____

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