Chapter 19 Acid Base Review Practice Questions

Study Guide NURS 2140

Chapter 19 – Acid Base Review Practice Questions

“Arterial blood gas analysis is an essential part of diagnosing and managing a patient’s

Oxygenation status and acid-base balance. The usefulness of this diagnostic tool is dependent

on being able to correctly interpret the results. This self-learning packet will examine the

components of an arterial blood gas, what each component represents and the interpretation of

these values to determine the patient’s condition and treatment.”

The Basics explained:

The pH is a measurement of the acidity or alkalinity of the blood. It is inversely proportional to

the number of hydrogen ions (H+) in the blood. The more H+ present, the lower the pH will be.

Likewise, the fewer H+ present, the higher the pH will be. The pH of a solution is measured on

a scale from 1 (very acidic) to 14 (very alkalotic). A liquid with a pH of 7, such as water, is

neutral (neither acidic nor alkalotic).

1 714

Very Acidic NeutralVery Alkalotic (Base)

The normal blood pH range is 7.35 to 7.45. In order for normal metabolism to take place, the

body must maintain this narrow range at all times. When the pH is below 7.35, the blood is said

to be acidic. Changes in body system functions that occur in an acidic state include a decrease

in the force of cardiac contractions, a decrease in the vascular response to catecholamines, and

a diminished response to the effects and actions of certain medications. When the pH is above

7.45, the blood is said to be alkalotic. An alkalotic state interferes with tissue oxygenation and

normal neurological and muscular functioning. Significant changes in the blood pH above 7.8

or below 6.8 will interfere with cellular functioning, and if uncorrected, will lead to death.

Key Concepts:

The only 2 ways an acidotic state can exist is from either toomuch PaCO2 or too little HCO3.
The only 2 ways an alkalotic state can exist is from either too little PaCO2 or too much HCO3.

The body regulates acid base in order to maintain normal pH (7.35-7.45) by using delicate buffer mechanisms between the respiratory and renal systems.

The Respiratory (Lungs) Buffer Response

A normal by-product of cellular metabolism is carbon dioxide (CO2). CO2 is carried in the

blood to the lungs, where excess CO2 combines with water (H2O) to form carbonic acid

(H2CO3). The blood pH will change according to the level of carbonic acid present. This

triggers the lungs to either increase or decrease the rate and depth of ventilation until the

appropriate amount of CO2 has been re-established. Activation of the lungs to compensate for

an imbalance starts to occur within 1 to 3 minutes.

The Renal (Metabolic) Buffer Response

In an effort to maintain the pH of the blood within its normal range, the kidneys excrete or

retain bicarbonate (HCO3-). As the blood pH decreases, the kidneys will compensate by

retaining HCO3- and as the pH rises, the kidneys excrete HCO3- through the urine. Although the

kidneys provide an excellent means of regulating acid-base balance, the system may take from

hours to days to correct the imbalance. When the respiratory and renal systems are working

together, they are able to keep the blood pH balanced by maintaining 1 part acid to 20 parts

base.

ACID BASE DISORDERS

Respiratory Acidosis

Respiratory acidosis is defined as a pH less than 7.35 with a PaCO2 greater than 45 mm Hg.

Acidosis is caused by an accumulation of CO2 which combines with water in the body to

produce carbonic acid, thus, lowering the pH of the blood. Any condition that results in

hypoventilation can cause respiratory acidosis. These conditions include:

 Central nervous system depression related to head injury

 Central nervous system depression related to medications such as narcotics, sedatives, oranesthesia

 Impaired respiratory muscle function related to spinal cord injury, neuromuscular diseases,or neuromuscular blocking drugs

 Pulmonary disorders such as atelectasis, pneumonia, pneumothorax, pulmonary edema, orbronchial obstruction

 Massive pulmonary embolus

 Hypoventilation due to pain, chest wall injury/deformity, or abdominal distension.

SIGNS AND SYMPTOMS OF RESPIRATORY ACIDOSIS
Pulmonary / Dyspnea
Respiratory distress
Shallow and/or slow respirations
Cardiovascular / Tachycardia
Dysrhythmias
Neurological / Headache
Restlessness
Confusion

CLINICAL APPLICATION:

If the CO2 becomes extremely high, drowsiness and unresponsiveness may be noted.

Increasing ventilation(minute volume) will correct respiratory acidosis. The method for achieving this will varywith the cause of hypoventilation. If the patient is unstable, manual ventilation with a bag mask (ambu bag) is indicated until the underlying problem can be addressed. After stabilization, rapidly resolvable causes are addressed immediately. Causes that can be treated rapidly includepneumothorax, pain, and CNS depression related to medications (i.e.: narcotic overdose-narcan). If the cause cannot be readily resolved, the patient may require mechanical ventilation while treatment is rendered. Although patients with hypoventilation often require supplemental oxygen, it is important to remember that oxygen alone will not correct the problem.

Respiratory Alkalosis

Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 35 mm Hg.

Any condition that causes hyperventilation can result in respiratory alkalosis. These conditions

include:

 Psychological responses, such as anxiety or fear

 Pain

 Increased metabolic demands, such as fever, sepsis, pregnancy, or thyrotoxicosis (condition caused by overactive thyroid)

 Medications, such as respiratory stimulants

 Central nervous system lesions (abnormality in CNS caused by disease or trauma)

SIGNS AND SYMPTOMS OF RESPIRATORY ALKALOSIS
Cardiovascular / Dysrhythmias
Palpitations
Diaphoresis
Neurological / Light-headedness
Numbness and tingling
Confusion
Inability to concentrate
Blurred vision
Miscellaneous / Dry mouth
Tetanic spasms of the arms and legs

CLINICAL APPLICATION:

Treatment of respiratory alkalosis centers on resolving the underlying problem.

Patients presenting with respiratory alkalosis have dramatically increased work of

breathing and must be monitored closely for respiratory muscle fatigue. When the

respiratory muscles become exhausted, acute respiratory failure may ensue.

Metabolic Acidosis

Metabolic acidosis is defined as a bicarbonate level of less than 22 mEq/L with a pH of less

than 7.35. Metabolic acidosis is caused by either a deficit of base in the bloodstream or an

excess of acids, other than CO2. Diarrhea and intestinal fistulas may cause decreased levels of

base. Causes of increased acids include:

Renal failure

Diabetic ketoacidosis

Anaerobic metabolism

Starvation

Salicylate (aspirin) intoxication

SIGNS AND SYMPTOMS OF METALBOLIC ACIDOSIS
Pulmonary / Kussmaul’s respirations (deep and labored breathing)
Cardiovascular / Dysrhythmias
Warm, flushed skin
Neurological / Headache
Confusion
Restlessness
Lethargy
Stupor or coma
Gastrointestinal / Nausea and vomiting

As with most acid-base imbalances, the treatment of metabolic acidosis is dependent upon the

cause. The presence of metabolic acidosis should spur a search for hypoxic tissue somewhere

in the body. Hypoxemia can lead to anaerobic metabolism system-wide, but hypoxia of any

tissue bed (brain, heart, kidneys, liver, pancreas, etc.) will produce metabolic acids as a result of anaerobic metabolism even if the PaO2 is normal. The only appropriate way to treat this source of acidosis is to restore tissue perfusion to the hypoxic tissues. Other causes of metabolic acidosis should be considered after the possibility of tissue hypoxia has been addressed.

CLINICAL APPLICATION:

Current research has shown that the use of sodium bicarbonate is indicated only for known

bicarbonate-responsive acidosis, such as that seen with renal failure. Routine use of sodium

bicarbonate to treat metabolic acidosis results in subsequent metabolic alkalosis with

hypernatremia and should be avoided.

Metabolic Alkalosis

Metabolic alkalosis is defined as a bicarbonate level greater than 26 mEq/liter with a pH greater

than 7.45. Either an excess of base or a loss of acid within the body can cause metabolic

alkalosis. Excess base occurs from

ingestion of antacids
excessive use of bicarbonate
use of lactate in dialysis
vomiting
gastric suction
hypochloremia (low chloride)
excess administration of diuretics
high levels of aldosterone

SIGNS AND SYMPTOMS OF METALBOLIC ALKALOSIS
Pulmonary / Respiratory depression
Musculoskeletal / Weakness
Muscle twitching
Muscle cramps
Tetany
Neurological / Dizziness
Lethargy
Disorientation
Seizures
Coma
Gastrointestinal / Nausea
Vomiting

Metabolic alkalosis is one of the most difficult acid-base imbalances to treat. Bicarbonate

excretion through the kidneys can be stimulated with drugs such as acetazolamide (Diamox®),

but resolution of the imbalance will be slow. In severe cases, IV administration of acids may

be used.

CLINICAL APPLICATION:

It is significant to note that metabolic alkalosis in hospitalized patients is usually

iatrogenic (brought on about unintentionally by treatment of other medical conditions).

COMPONENTS OF THE ARTERIAL BLOOD GAS

The arterial blood gas provides the following values:

pH:

Measurement of acidity or alkalinity, based on the hydrogen (H+) ions present.

The normal range is 7.35 to 7.45

Remember:

pH > 7.45 = alkalosis

pH< 7.35 = acidosis

PaO2:

The partial pressure of oxygen that is dissolved in arterial blood.

The normal range is 80 to 100 mm Hg.

SaO2:

The arterial oxygen saturation.

The normal range is 95% to 100%.

PaCO2:

The amount of carbon dioxide dissolved in arterial blood.

The normal range is 35 to 45 mm Hg.

Remember:

pCO2 >45 = acidosis

pCO2 <35 = alkalosis

HCO3

The calculated value of the amount of bicarbonate in the bloodstream.

The normal range is 22 to 26 mEq/liter

Remember:

HCO3 > 26 = alkalosis

HCO3 < 22 = acidosis

B.E.

The base excess indicates the amount of excess or insufficient level of bicarbonate in the

system.

The normal range is -2 to +2 mEq/liter. (-3 to +3 in Osborn book)

Remember:

A negative base excess indicates a base deficit in the blood.

The 6 Easy Steps to ABGAnalysis:

Step 1: Analyze the pH

The first step in analyzing ABGs is to look at the pH. Normal blood pH is

7.4, plus or minus 0.05, forming the range 7.35 to 7.45. If blood pH falls below 7.35 it is acidic. If blood pH rises above 7.45, it is alkalotic. If it falls into the normal range, note what side of 7.4 it falls on. Lower than 7.4 is normal/acidic, higher than 7.4 is normal/alkalotic.

Step2:AnalyzetheCO2

ThesecondstepistoexaminethepCO2.NormalpCO2levelsare35 ‐45mmHg.Below35isalkalotic,above45isacidic.

Step3:AnalyzetheHCO3

Thethirdstepistolook at theHCO3level.AnormalHCO3levelis22--‐26mEq/L.IftheHCO3isbelow22,thepatientisacidotic.IftheHCO3isabove26,thepatientisalkalotic.

Step4:MatchtheCO2ortheHCO3withthepH

NextmatcheitherthepCO2ortheHCO3withthepHtodeterminetheacid--‐basedisorder.Forexample,ifthepHisacidotic,andtheCO2isacidotic,thentheacid--‐basedisturbanceisbeingcausedbytherespiratorysystem.Therefore,wecallitarespiratoryacidosis.However,ifthepHisalkaloticandtheHCO3isalkalotic,theacid--‐basedisturbanceisbeingcausedbythemetabolic(orrenal)system.Therefore,itwillbeametabolicalkalosis.

Step5:DoestheCO2orHCO3gotheoppositedirectionofthepH?

Fifth,doeseithertheCO2orHCO3gointheoppositedirectionofthepH?Ifso,thereiscompensationbythatsystem.Forexample,thepHisacidotic,theCO2isacidotic,andtheHCO3isalkalotic.TheCO2matchesthepHmakingtheprimaryacid--‐basedisorderrespiratoryacidosis.TheHCO3isoppositeofthepHandwouldbeevidenceofcompensationfromthemetabolicsystem. (explained more in next section).

Step6:AnalyzethepO2andtheO2saturation.

Finally,evaluatethePaO2andO2sat.Iftheyarebelownormalthereisevidenceofhypoxemia.

ABG Test / Normal Range / Decreased Value / Increased Value
pH / 7.35 – 7.45 / Acidosis / Alkalosis
PaCO2 / 34 – 45 / Alkalosis / Acidosis
HCO3 / 22-26 / Acidosis / Alkalosis
PaO2 (dissolved O2) / 80-100 / Hypoxemia / O2 Therapy
SaO2 (saturation) / 95-100% / Hypoxemia / ------

Notice that if the pH is lower than 7.35 it indicates acidosis, if the pH is higher than 7.45 it indicates alkalosis. The HCO3 is also acidotic if it is low: less than 22 indicate acidosis. If the HCO3 is higher than 26 it indicates alkalosis. However, if the CO2 is lower than 35 it indicates alkalosis, and if the CO2 is higher than 45 Itindicatesacidosis.Onewaytorememberthisrelationshipistousetheacronym. ROME.

Respiratory

Opposite

Metabolic

Equal

The PaCO2istherespiratorycomponentoftheABG,andifitislowandthepHishighthepatientwouldhavearespiratoryalkalosis.Theymoveinoppositedirectionstomatch.

TheHCO3isthemetaboliccomponentoftheABG.IftheHCO3islowandthepHislowthepatientwouldhavemetabolicacidosis.Theymoveinthesamedirectiontomatch.

STEP 5 REFERS TO COMPENSATION.

Compensation is the attempt by the body to maintain homeostasis by correcting the pH. The opposite system will do this. The component of the respiratory system that balances the pH is the dissolved carbon dioxide(PaCO2) that is produced by the cellular processes and removed by the lungs. The component of the renal system that balances the pH is the dissolved bicarbonate (HCO3) produced by the kidneys. ThekidneysalsohelpcontrolpHbyeliminatinghydrogen(H+)ions.Thewaythetwosystemsinteractisthroughtheformationofcarbonicacid(H2CO3).

Movementthroughthecarbonicacidsystemisfluidandconstant.Whatthismeansisthatwater(H2O)cancombinewithCO2andformcarbonicacid.Ifnecessary,carbonicacid(H2CO3)canthenbreakuptoformhydrogenions(H+)andbicarbonate(HCO3).Thissystemworksinbothdirections.Bybalancingbackandforth,anormalpHisachieved.TherespiratorysystembalancesthepHbyincreasingordecreasingtherespiratoryrate,therebymanipulatingthe PaCO2level.Fastanddeepbreathing (Increased minute volume) “blowsoff” PaCO2.Conversely,slowandshallowbreathing (decreased minute volume) “retains” PaCO2.TherenalsystembalancespHbyproducingHCO3orbyeliminatinghydrogenions(H+).Therenalsystemwillreflectchangesinmetabolicactivitywithinthebody.Forexample,apatientinshockwillundergoanaerobicmetabolism,whichproduceslacticacid.TheproductionoflacticacidwillbindoruseupavailableHCO3andwillbemanifestedbyadecreaseintheHCO3level.Therefore,theHCO3levelisanindicatorofmetabolic acid-basebalance.

Balance must always be achieved by the opposing system. Therefore, our body regulates pH by using the opposing system to balance pH. So if the pH is out of balance because of a respiratory disorder, it will be the renal system that makes the corrections to balance the pH. Conversely, if the renal system is to blame for the pH disorder, the respiratory system will have to compensate. This process is called compensation. Compensation may not always be complete.

Complete or Full compensation returns the pH balance to normal. TherearetimeswhentheimbalanceistoolargeforcompensationtorestorethepHtonormal.Thisiscalledpartialcompensation.Liketheseesaw,compensationmustcomefromtheoppositesystem.

Step5analyzescompensationbylookingforthesystemthatisgoingtheoppositedirectionofthe pH. Opposite in terms of acidosis and alkalosis. (i.e.: pH is acid (low), HCO3 is acid (low) but PaCO2 is alkalotic (high) – would be Metabolic Acidosis with partial compensation from the respiratory system).

Examples of interpretation ABG are using the six steps:

Example 1:

pH /
7.27 / acidotic
PaCO2 /
53 / acidotic
PaO2 / 50 / low
SaO2 / 79% / low
HCO3 / 24 / normal

Step 1: The pH is less than 7.35, so it is acidotic.

Step 2: The CO2 is greater than 45, so it is acidotic.

Step 3: The HCO3 is normal.

Step 4: The PaCO2 matches the pH, because they are both acidotic. Therefore the imbalance is respiratory acidosis. It is acidotic because the pH is acidotic, it is respiratory because the PaCO2 moved opposite (ROME) and matches the pH.

Step 5: The HCO3 is normal, therefore no compensation. If the HCO3 was alkalotic (high) (moved in opposite direction to pH) then a compensation (partial) would be present.

Step 6: Lastly, the PaO2 and SaO2 are both low indicating hypoxemia.

The Full Interpretation for this ABG is: Uncompensated Respiratory Acidosis with hypoxemia.

This patient has an acute respiratory disorder. Caused by hypoventilation. Retaining CO2. Hypoventilation is slow shallow breathing (decreased Minute Volume) and decreased Alveolar Minute Volume (areas that actually participate in gas exchange in the lungs). These conditions are: COPD, Drug overdoses (narcotics, opiates), obstructed airway, neuromuscular diseases that affect breathing, chest trauma, high spinal column injuries and pulmonary edema.

Example 2:

pH /
7.52 / alkalotic
PaCO2 /
29 / alkalotic
PaO2 / 100 / normal
SaO2 / 98% / normal
HCO3 / 23 / normal

Step 1: The pH is greater than 7.45, so it is alkalotic

Step 2: The PaCO2 is less than 35, so it is alkalotic.

Step 3: The HCO3 is normal.

Step 4: The PaCO2 matches the pH, because they are both alkalotic. Therefore the imbalance is respiratory alkalosis. It is alkalotic because the pH is alkalotic; it is respiratory because the PaCO2 moved opposite (ROME) and matches the pH.

Step 5: The HCO3 is normal, therefore there is no compensation. If the HCO3 was acidotic (low) (opposite of the pH) then compensation (partial) would be present.

Step 6: Lastly, the PaO2 and SaO2 are normal indicating normal oxygenation (no hypoxemia).

The Full Interpretation for this ABG is: Uncompensated respiratory alkalosis.

This patient is hyperventilating. “Blowing off CO2”. Hyperventilation is fast and deep breathing (increased Minute Volume) and increased Alveolar Minute Volume (areas that actually participate in gas exchange in the lungs). Conditions that can cause this are: Anxiety, Pain, High Altitudes, Fever, initial stages of Pulmonary embolism, hypoxia. Treatment for this patient would be to treat the condition: anxiety medications, pain medications, fever reducers, etc.), encourage patient to breath slow, possible rebreathing in paper bag to restore CO2.

Example 3:

pH /
7.18 / acidotic
PaCO2 / 44 / normal
PaO2 / 92 / normal
SaO2 / 95% / normal
HCO3 /
16 / acidotic

Step 1: The pH is less than 7.35, so it is acidotic.

Step 2: The PaCO2 is Normal.

Step 3: The HCO3 is less than 22, so it is acidotic.

Step4: The HCO3 matches the pH, because they are both acidotic. Therefore the imbalance is metabolic acidosis. It is acidotic because the pH is acidotic; it is metabolic because the HCO3 matches the pH.

Step 5: The PaCO2 is normal, therefore, no compensation.

Step 6: Lastly, the PaO2 and SaO2 are normal indicating normal oxygenation (no hypoxemia)

The full interpretation of this ABG is: Uncompensated Metabolic acidosis.

The patient has an acute metabolic disorder such as Diabetic Ketoacidosis (DKA), severe starvation, Salisylate (Aspirin) Overdose, Shock, Sepsis, severe diarrhea, renal failure. Treatment is to treat the underlying disorder, administer medication and fluids, replace electrolytes, anddialysis.

Example 4.

pH /
7.30 / acidotic
PaCO2 /
30 / alkalotic
PaO2 / 68 / low
SaO2 / 92% / low
HCO3 / 14 / acidotic

Step 1: The pH is less than 7.35, so it is acidotic

Step 2: The PaCO2 is less than 35, so it is alkalotic.

Step 3: The HCO3 is less than 22, so it is acidotic.

Step 4: The HCO3 matches the pH, because both are acidotic. Therefore the imbalance is a metabolic acidosis. It is acidotic because the pH is acidotic; it is metabolic because the HCO3 matches the pH.

Step 5: The PaCO2 is alkalotic and goes the opposite direction of the pH, so therefore there is compensation. Because the pH is not in the normal range (7.35 – 7.45) the compensation is called partial.

Step 6: Lastly, the PaO2 and the SaO2 are low indicating hypoxemia. (inadequate oxygenation).

The full interpretation for this ABG is: Partially-compensated metabolic acidosis with hypoxemia.

There are a number of conditions that can cause metabolic acidosis: renal failure, severe diarrhea, Aspirin overdose, DKA, Shock, and Sepsis. This patient is probably in shock because the metabolic acidosis is associated with poor oxygenation.

Example 5:

pH /
7.60 / alkalotic
PaCO2 /
56 / acidotic
PaO2 / 92 / normal
SaO2 / 98% / normal
HCO3 /
35 / alkalotic

Step 1: The pH is greater than 7.45, so it is alkalotic.

Step 2: The PaCO2 is greater than 45, so it is acidotic.

Step 3: The HCO3 is greater than 26, so it is alkalotic.

Step 4: The HCO3 matches the pH, because both are alkalotic. Therefore the imbalance is a metabolic alkalosis. It is alkalotic because the pH is alkalotic; it is metabolic because the HCO3 matches the pH.

Step 5: The PaCO2 is acidotic and goes the opposite direction of the pH, so therefore there is compensation. Because the pH is not in the normal range (7.35 – 7.45) the compensation is called partial.

Step 6: Lastly, the PaO2 and the SaO2 are normal indicating normal oxygenation (no hypoxemia).

The full interpretation for this ABG is: Partially-compensated metabolic alkalosis.

The patient is probably losing stomach acid from vomiting, NG tube drainage/suctioning, use of anti-acids or medications that reduce stomach acid, or potassium wasting diuretics (thiazides like furosemide (Lasix).

Example 6: (last one) (Full compensation – ohh shit! Now I am confused again!)

pH / 7.38 / normal
PaCO2 /
62 / acidotic
PaO2 / 93 / normal
SaO2 / 97% / normal
HCO3 /
35 / alkalotic

Step 1: The pH is normal but less than 7.40, so it is within range but on the acidotic (low) side of the normal range.