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Arterial Blood Gas AnalysisArterial Blood Gas Analysis IntroductionIntroduction Basic ConceptsBasic Concepts Arterial Blood Gas Gas Exchange Acid-Base Disturbances

Systematic Analysis of Arterial Blood GasesSystematic Analysis of Arterial Blood Gases Oxygenation Stepwise approach to Acid-Base Disorders

Arterial Blood Gas Analysis: Using Cases to Develop Your Skills Arterial Blood Gas Analysis: Using Cases to Develop Your Skills

INTRODUCTION

. Introduction to Arterial Blood Gases

Arterial blood gases are an important diagnostic tool for the evaluation of patients. They are useful for determining both the oxygenation status as well as the acid-base balance of the patient. A background in respiratory physiology is recommended.

BASIC CONCEPTS: ARTERIAL BLOOD GASBASIC CONCEPTS: ARTERIAL BLOOD GAS

In the written medical record, the arterial blood gas often appears as a series of values as below:

7.40 / 40 / 98 / 24, orpH / PaCO2 / PaO2 / HCO3-

wherepH = arterial blood pH

PaCO2 (or PCO2)

= arterial pressure of CO2, in mm Hg

PaO2 (or PO2)= arterial pressure of O2, in mm Hg

HCO3-= serum bicarbonate

concentration, in mEq/liter

The term hypoxia refers to reduced oxygen pressure (partial pressure of oxygen) in the alveolus. The term hypoxemia refers to reduced oxygen content in arterial blood (the suffix "-emia" refers to something "in the blood"). A normal arterial pressure of O2 is dependent on the atmospheric pressure, temperature, inspired O2 content, and the patient's age. Assuming the patient is breathing room air at sea level, a patient's PaO2 may be estimated atPaO2 = 104-(0.27 x age)

BASIC CONCEPTS: ARTERIAL BLOOD GASBASIC CONCEPTS: ARTERIAL BLOOD GAS

Basic Concepts: Acid-Base Basic Concepts: Acid-Base DisturbancesDisturbances

The terms The terms acidemiaacidemia and and alkalemiaalkalemia refer to refer to alterations in blood pH, and are the result alterations in blood pH, and are the result of underlying disturbance(s) (metabolic of underlying disturbance(s) (metabolic and/or respiratory). Again the suffix "-emia" and/or respiratory). Again the suffix "-emia" refers to the blood. The terms refers to the blood. The terms acidosisacidosis and and alkalosisalkalosis refer to the processes that refer to the processes that alter the acid-base status. There can be alter the acid-base status. There can be (and often are) more than one of these (and often are) more than one of these processes simultaneously in a patient. processes simultaneously in a patient.


Processes that alter the acid-base status of a Processes that alter the acid-base status of a patient can be divided into metabolic or patient can be divided into metabolic or respiratory processes.respiratory processes.

MetabolicMetabolic processes are those that primarily processes are those that primarily alter the bicarbonate concentration in the alter the bicarbonate concentration in the bloodblood. . A decrease in serum bicarbonate (an A decrease in serum bicarbonate (an alkali or base) leads to a alkali or base) leads to a metabolic metabolic acidosisacidosis, while an increase in serum , while an increase in serum bicarbonate leads to a bicarbonate leads to a metabolic alkalosismetabolic alkalosis..


RespiratoryRespiratory processes alter the pH of the processes alter the pH of the serum by changing the carbon dioxide levels. serum by changing the carbon dioxide levels. Carbon dioxide accumulation causes an acid Carbon dioxide accumulation causes an acid state in the blood (through carbonic acid), state in the blood (through carbonic acid), and as respirations (respiratory rate and/or and as respirations (respiratory rate and/or tidal volume) increase, the body eliminates tidal volume) increase, the body eliminates more carbon dioxide (acid) and is left with an more carbon dioxide (acid) and is left with an respiratory alkalosisrespiratory alkalosis. .


In conclusion, pH altering processes can be In conclusion, pH altering processes can be one of four types: one of four types: metabolic acidosis, metabolic acidosis, metabolic alkalosis, respiratory acidosis, metabolic alkalosis, respiratory acidosis, or respiratory alkalosisor respiratory alkalosis. Again, one or more . Again, one or more of these processes may be present in a of these processes may be present in a patient with an abnormal acid-base status.patient with an abnormal acid-base status.

Basic Concepts: Gas Basic Concepts: Gas ExchangeExchange

•A patient can be hypoxemic for two basic reasons; oxygen may not be delivered to the alveolar air sacs (hypoventilation) or oxygen in the alveoli may not enter into the blood stream. Hyperoxia is not a term which is clinically used, however, we hope that our patients have normoxia, or normal levels of oxygen (generally attached to hemoglobin) with or without oxygen supplementation.

•A patient can be hypercarbic, or have elevated levels of CO2 in the blood, which is due to an inability to normally exchange gas in the lungs

Systematic Analysis of Arterial Blood Gases

Arterial blood gases are obtained for two basic purposes: to determine oxygenation and to determine acid-base status. To ensure complete interpretation of these points, it is important to systematically examine each component of the arterial blood gas. This module will now demonstrate how to systematically determine oxygenation, and then evaluate the acid-base status.

Determining Oxygenation and (A-a) Gradient{Alveolar:arterial oxygen gradient} An important part of interpreting blood gases is

to assess oxygenation. An arterial oxygen concentration of less than 55 mm Hg, associated with an oxygenation of less than 90%, is poorly tolerated in humans, therefore a PaO2 of less than 55 is termed hypoxemic. However, "normal" oxygenation decreases with age as the lungs become less efficient at diffusing oxygen from the alveolus to the blood. Again, normal oxygenation for age can be estimated

PaO2 = 104.2 - (0.27 x age)

Stepwise Approach to Diagnosing Stepwise Approach to Diagnosing Acid-Base DisordersAcid-Base Disorders

In order to understand the various processes that In order to understand the various processes that can co-exist in a patient, one must systematically can co-exist in a patient, one must systematically evaluate the blood gases and serum electrolytes. evaluate the blood gases and serum electrolytes. This module presents a method described by This module presents a method described by Narins {RG Narins et al, Am J Med 72:496-520, Narins {RG Narins et al, Am J Med 72:496-520, 1982; RG Narins and M Emmet, Medicine 59:161-1982; RG Narins and M Emmet, Medicine 59:161-187, 1980} and refined by Morganroth {ML 187, 1980} and refined by Morganroth {ML Morganrothm J Crit Illness 5:138-150, 1990, 5:460-Morganrothm J Crit Illness 5:138-150, 1990, 5:460-469, 1990}. The method uses 469, 1990}. The method uses 6 simple steps6 simple steps to to determine the acid-base status of the patient.determine the acid-base status of the patient.

Stepwise Approach to Diagnosing Stepwise Approach to Diagnosing Acid-Base DisordersAcid-Base Disorders

Steps in Acid-Base AnalysisSteps in Acid-Base AnalysisStep 1: Acidemic or Alkalemic?Step 2: Is the primary disturbance respiratory or metabolic?Step 3: Is the respiratory disturbance acute or chronic?Step 4: For a metabolic acidosis, is there an increased anion gap?Step 5: Are there other metabolic processes present in a patient with an increased anion gap metabolic acidosis?

Step 6: Is the respiratory system compensating adequately for a metabolic disturbance?

Step 1: Acidemic or Alkalemic?Step 1: Acidemic or Alkalemic?

The pH of the arterial blood gas measurement The pH of the arterial blood gas measurement identifies the disorder as identifies the disorder as alkalemicalkalemic or or acidemicacidemic. .

Normal arterial blood pH = 7.40 ± 0.02 Normal arterial blood pH = 7.40 ± 0.02 Acidemic: pH < 7.38 Acidemic: pH < 7.38 Alkalemic: pH > 7.42 Alkalemic: pH > 7.42

Step 2: Is the primary disturbance Step 2: Is the primary disturbance respiratory or metabolic?respiratory or metabolic?

Step 2 requires one to determine whether the disturbance Step 2 requires one to determine whether the disturbance effects primarily the arterial PaCO2 or the serum HCO3-. effects primarily the arterial PaCO2 or the serum HCO3-.

Respiratory disturbances alter the arterial PaCO2 (normal Respiratory disturbances alter the arterial PaCO2 (normal value 38-42) value 38-42)

Metabolic disturbances alter the serum HCO3- (normal Metabolic disturbances alter the serum HCO3- (normal value 22-26) value 22-26)

If the pH is low (i.e., the primary and controlling If the pH is low (i.e., the primary and controlling disturbance is acidosis causing acidemia) disturbance is acidosis causing acidemia) eithereither the PaCO2 the PaCO2 is high/increased is high/increased oror the HCO3- is low/decreased. (These the HCO3- is low/decreased. (These are the are the onlyonly ways in which the pH can be low). A high ways in which the pH can be low). A high PaCO2 PaCO2 definesdefines a primary respiratory acidosis and a low a primary respiratory acidosis and a low HCO3- HCO3- definesdefines a primary metabolic acidosis. a primary metabolic acidosis.

Conversely, if the pH is high (i.e., the primary Conversely, if the pH is high (i.e., the primary and controlling disturbance is and controlling disturbance is alkalosisalkalosis causing causing alkalemiaalkalemia) ) eithereither the PaCO2 is the PaCO2 is low/decreased low/decreased oror the HCO3- is high/increased. the HCO3- is high/increased. (These are the (These are the onlyonly ways in which the pH can ways in which the pH can be high). A low PaCO2 be high). A low PaCO2 definesdefines a primary a primary respiratory alkalosisrespiratory alkalosis and a high HCO3- and a high HCO3- definesdefines a primary a primary metabolic alkalosis metabolic alkalosis. .

Step 2: Is the primary disturbance Step 2: Is the primary disturbance respiratory or metabolic?respiratory or metabolic?

Step 3: Is the respiratory Step 3: Is the respiratory disturbance acute or chronic?disturbance acute or chronic?

For acute For acute respiratoryrespiratory disturbances, a PaCO2 disturbances, a PaCO2 variation from normal by variation from normal by 10 mm Hg10 mm Hg is is accompanied by a pH shift of accompanied by a pH shift of 0.08 units0.08 units. (For . (For example, in an acute respiratory acidosis, if the example, in an acute respiratory acidosis, if the PCO2 rises from 40 to 50, you would expect PCO2 rises from 40 to 50, you would expect the pH to decline from 7.40 to 7.32; in an the pH to decline from 7.40 to 7.32; in an acute respiratory alkalosis, if the PCO2 falls acute respiratory alkalosis, if the PCO2 falls from 40 to 30, you would expect the pH to rise from 40 to 30, you would expect the pH to rise from 7.40 to 7.48.)from 7.40 to 7.48.)

In chronic In chronic respiratoryrespiratory disturbances, there are renal disturbances, there are renal mediated shifts of bicarbonate that alter and partially mediated shifts of bicarbonate that alter and partially compensate for the pH shift for a change in the compensate for the pH shift for a change in the PaCO2. In chronic disturbances, a PaCO2 variation PaCO2. In chronic disturbances, a PaCO2 variation of of 10 mm Hg 10 mm Hg from normal is accompanied by a pH from normal is accompanied by a pH shift of shift of only 0.03 unitsonly 0.03 units. (For example, in a chronic . (For example, in a chronic respiratory acidosis, if the PCO2 rises from 40 to 50, respiratory acidosis, if the PCO2 rises from 40 to 50, you would expect the pH to decline from 7.40 to you would expect the pH to decline from 7.40 to 7.37; in a chronic respiratory alkalosis, if the PCO2 7.37; in a chronic respiratory alkalosis, if the PCO2 falls from 40 to 30, you would expect the pH to rise falls from 40 to 30, you would expect the pH to rise from 7.40 to 7.43.)from 7.40 to 7.43.)



Acute resp acidosis Chronic respiratory acidosis

pH decrease = 0.08 x (PaCO2 - 40) / 10pH decrease = 0.03 x (PaCO2 - 40) / 10

Acute respiratory alkalosis:

Chronic respiratory alkalosis:

pH increase = 0.08 x (40 - PaCO2) / 10pH increase = 0.03 x (40 - PaCO2) / 10

Step 4: For a metabolic acidosis, is Step 4: For a metabolic acidosis, is there an increased anion gap?there an increased anion gap?

This step is This step is onlyonly necessary if you have already determined that there is a necessary if you have already determined that there is a metabolic acidosis (low HCO3 with an acidosis/acidemia). metabolic acidosis (low HCO3 with an acidosis/acidemia).

Processes that lead to a Processes that lead to a metabolic acidosismetabolic acidosis can be divided into those with can be divided into those with an increased an increased anion gapanion gap and those without an increased gap. The anion gap and those without an increased gap. The anion gap is the difference between the measured serum cations (positively charged is the difference between the measured serum cations (positively charged particles) and the measured serum anions (negatively charged particles). particles) and the measured serum anions (negatively charged particles). (Of course, there is no real gap; in the body the number of positive and (Of course, there is no real gap; in the body the number of positive and negative charges are balanced. The gap refers to the difference in positive negative charges are balanced. The gap refers to the difference in positive and negative charges among cations and anions which are commonly and negative charges among cations and anions which are commonly measured.) The commonly measured cation is sodium. (Some people also measured.) The commonly measured cation is sodium. (Some people also use potassium to calculate the gap; that results in a different range of use potassium to calculate the gap; that results in a different range of normal values, normal values, and we will not use potassium to calculate the gap and we will not use potassium to calculate the gap subsequentlysubsequently.) The measured anions include chloride and bicarbonate. .) The measured anions include chloride and bicarbonate. Thus the anion gap can be summarized as:Thus the anion gap can be summarized as:

Anion gapAnion gap = [Sodium] - ([Chloride] + [Bicarbonate]) = [Sodium] - ([Chloride] + [Bicarbonate])OrOrAG = [Na+] - ([Cl-] + [HCO3-]).AG = [Na+] - ([Cl-] + [HCO3-]).

Step 4: For a metabolic acidosis, is Step 4: For a metabolic acidosis, is there an increased anion gapthere an increased anion gap??

The normal anion gap is 12. An anion gap of greater than 12 is The normal anion gap is 12. An anion gap of greater than 12 is increased.increased. This is important, because it helps to significantly This is important, because it helps to significantly limit the differential diagnosis of a metabolic acidosis. Various limit the differential diagnosis of a metabolic acidosis. Various mnemonics exist for the causes of an increased anion gap mnemonics exist for the causes of an increased anion gap metabolic acidosis (often referred to simply as a gap acidosis), metabolic acidosis (often referred to simply as a gap acidosis), one of which is MULEPAK. The most common etiologies of a one of which is MULEPAK. The most common etiologies of a metabolic acidosis with an increased anion gap include: metabolic acidosis with an increased anion gap include:

Methanol other alcohols, and ethylene glycol intoxicationMethanol other alcohols, and ethylene glycol intoxication Uremia (renal failure)Uremia (renal failure) Lactic acidosisLactic acidosis EthanolEthanol Paraldehyde and other drugsParaldehyde and other drugs Aspirin Aspirin Ketones (starvation, alcoholic and diabetic ketoacidosis) Ketones (starvation, alcoholic and diabetic ketoacidosis)

Step 5: Are there other metabolic Step 5: Are there other metabolic processes present in a patient with an processes present in a patient with an increased anion gap metabolic acidosis?increased anion gap metabolic acidosis? This step is This step is onlyonly necessary if you have already determined that necessary if you have already determined that

there is an increased anion gap metabolic acidosis.there is an increased anion gap metabolic acidosis. In this case, does the increase in anion gap In this case, does the increase in anion gap completelycompletely explain the explain the

acid base disorder, or is there another metabolic process present? acid base disorder, or is there another metabolic process present? Remember that in an anion gap metabolic acidosis, the Remember that in an anion gap metabolic acidosis, the bicarbonate is decreased due to the presence of bicarbonate is decreased due to the presence of unmeasuredunmeasured anions. For every molecule of this anion present, one molecule of anions. For every molecule of this anion present, one molecule of bicarbonate is lost. Thus if the concentration of this unmeasured bicarbonate is lost. Thus if the concentration of this unmeasured anion (the increase in the anion gap above normal) is added to the anion (the increase in the anion gap above normal) is added to the concentration of serum bicarbonate, this "corrected bicarbonate" concentration of serum bicarbonate, this "corrected bicarbonate" value should be equal to a normal serum bicarbonate value should be equal to a normal serum bicarbonate concentration. If the corrected bicarbonate is not normal, an concentration. If the corrected bicarbonate is not normal, an additional metabolic disturbance is present. The corrected additional metabolic disturbance is present. The corrected bicarbonate concentration is calculated:bicarbonate concentration is calculated:

Corrected HCO3- = measured HCO3- + (anion gap - 12). Corrected HCO3- = measured HCO3- + (anion gap - 12).

Step 5: Are there other metabolic Step 5: Are there other metabolic processes present in a patient with an processes present in a patient with an

increased anion gap metabolic acidosisincreased anion gap metabolic acidosis?? If the corrected HCO3- is less than normal (less than If the corrected HCO3- is less than normal (less than

24 mEq/liter) then there is an additional metabolic 24 mEq/liter) then there is an additional metabolic acidosis present. Corrected HCO3- values of greater acidosis present. Corrected HCO3- values of greater than 24 mEq/liter reflects a co-existing metabolic than 24 mEq/liter reflects a co-existing metabolic alkalosis. alkalosis.

For example, in a diabetic ketoacidosis with a For example, in a diabetic ketoacidosis with a measuredmeasured HCO3- of 12, and an HCO3- of 12, and an anion gapanion gap of 24 of 24 [which is increased by 12 over normal], the [which is increased by 12 over normal], the corrected corrected HCO3- HCO3- would = 12 + (24-12) =24, which is normal, would = 12 + (24-12) =24, which is normal, and suggests that no secondary metabolic process was and suggests that no secondary metabolic process was ongoing. ongoing.

Step 5: Are there other metabolic Step 5: Are there other metabolic processes present in a patient with an processes present in a patient with an

increased anion gap metabolic acidosisincreased anion gap metabolic acidosis??

On the other hand, if the diabetic patient also had On the other hand, if the diabetic patient also had gastroparesis and had been vomiting, causing a loss gastroparesis and had been vomiting, causing a loss of gastric acid and therefore a metabolic alkalosis, of gastric acid and therefore a metabolic alkalosis, the measured bicarbonate may be 18 and the anion the measured bicarbonate may be 18 and the anion gap 24. The corrected HCO3- would then = 18 + gap 24. The corrected HCO3- would then = 18 + (24-12) =30, which is elevated, indicating two (24-12) =30, which is elevated, indicating two simultaneous metabolic problems: metabolic simultaneous metabolic problems: metabolic acidosis (from the diabetic ketoacidosis) and acidosis (from the diabetic ketoacidosis) and metabolic alkalosis (from the loss of gastric acids). metabolic alkalosis (from the loss of gastric acids).

Step 6: Is the respiratory system Step 6: Is the respiratory system compensating adequately for a compensating adequately for a

metabolic disturbancemetabolic disturbance?? This step is This step is onlyonly necessary if you have already necessary if you have already

determined that there is a primary metabolic determined that there is a primary metabolic disturbance.disturbance.

The body responds to a metabolic acidosis by The body responds to a metabolic acidosis by increasing respirations and thus decreasing carbon increasing respirations and thus decreasing carbon dioxide (acid) in the blood. This response is dioxide (acid) in the blood. This response is predictable, and there is a linear relationship between predictable, and there is a linear relationship between the PaCO2 and serum bicarbonate. This relationship the PaCO2 and serum bicarbonate. This relationship is expressed in the "is expressed in the "Winter's formulaWinter's formula":": Expected PaCO2 = [1.5 x serum HCO3-] + 8 ± 2.Expected PaCO2 = [1.5 x serum HCO3-] + 8 ± 2.

Step 6: Is the respiratory system Step 6: Is the respiratory system compensating adequately for a compensating adequately for a metabolic disturbance?metabolic disturbance? If the If the measuredmeasured PaCO2 is greater than the PaCO2 is greater than the expectedexpected

PaCO2, then the respiratory system is not PaCO2, then the respiratory system is not compensating for the metabolic acidosis, and a compensating for the metabolic acidosis, and a respiratory acidosisrespiratory acidosis is also present. is also present.

The respiratory system does not respond as The respiratory system does not respond as predictably to a metabolic alkalosis as it does to a predictably to a metabolic alkalosis as it does to a metabolic acidosis. Suffice it to say that (regardless of metabolic acidosis. Suffice it to say that (regardless of the serum HCO3- level) the respiratory system rarely the serum HCO3- level) the respiratory system rarely reacts by retaining CO2 to a pressure greater than 50-reacts by retaining CO2 to a pressure greater than 50-55 mm Hg, and that a value greater than that suggests 55 mm Hg, and that a value greater than that suggests an abnormal (i.e. primary) respiratory acidosis.an abnormal (i.e. primary) respiratory acidosis.

Sample Problem Below is a clinical example of an acid base disorder. You will be led

through the systematic analysis of the disorders present in this patient.

A 23 year old man presents with confusion. He has had diabetes since age 12, and has been suffering from an intestinal flu for the last 24 hours. He has not been eating much, has vague stomach pain, stopped taking his insulin, and has been vomiting. His glucose is high and he has the following electrolytes and arterial blood gases:

Na+ 130 mEq/L Cl- 80 mEq HCO3-10mEq/L pH 7.20 PaCO225 mm Hg PO2 68 mm Hg Interpret the ABG values and determine the acid-base abnormalities.

Sample Problem The following pages provide case scenarios

including arterial blood gases and serum electrolytes. You will be asked to evaluate the oxygenation of the patient as well as identify all acid-base abnormalities present. We suggest that you analyze each problem with regards to: A) Oxygenation (including A-a gradient); B) Acid-Base status (following the 6-step method described above). The first case will guide your thinking, and each additional case will offer less help, allowing you to show off your skills.

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