Blood Gas Analysis

MOTIVATION FOR LEARNING ABOUT BLOOD GAS INTERPRETATION

From: THE ICU BOOK - 2nd Ed. (1998) MOTIVATION In a survey conducted at a university teaching hospital, 70% of the participating physicians claimed that they were well versed in the diagnosis of acid-base disorders and that they needed no assistance in the interpretation of arterial blood gases (ABGs). These same physicians were then given a series of ABG measurements to interpret, and they correctly interpreted only 40% of the test samples. Hingston DM. A computerized interpretation of arterial pH and blood gas data: do physicians need it? Respir Care 1982;27:809-815. From: THE ICU BOOK - 2nd Ed. (1998)

MOTIVATION A survey at another teaching hospital revealed that incorrect acid-base interpretations led to errors in patient management in one-third of the ABG samples analyzed. Broughton JO, Kennedy TC. Interpretation of arterial blood gases by computer. Chest 1984;85:148-149. From: THE ICU BOOK - 2nd Ed. (1998)

MOTIVATION These surveys reveal serious deficiencies in an area that tends to be ignored. This can cause trouble in the ICU, where 9 of every 10 patients may have an acid-base disorder. Gilfix BM, Bique M, Magder S. A physical chemical approach to the analysis of acid-base balance in the clinical setting. J Crit Care 1993;8:187-197. From: THE ICU BOOK - 2nd Ed. (1998)

Getting an arterial blood gas sample

Ulnar Artery Radial Artery

What Is An ABG? pH [H+] PCO2 Partial pressure PO2 Partial pressure HCO3 Bicarbonate BE Base excess SaO2 Oxygen Saturation

Normal Ranges Parameters Normal Range pH 7.35 - 7.45 PaCO2 35- 45 mmHg PaO2 80-100 mmHg HCO3 22-26 mEq/liter O2sat 95-100% Base Excess –2 to +2 mEq/liter

Acid-base Balance Henderson-Hasselbalch Equation [HCO3-] pH = pK + log ---------------- .03 [PaCO2] For teaching purposes, the H-H equation can be shortened to its basic relationships: HCO3- pH ~ --------- PaCO2

Primary Acid-base Disorders: Respiratory Alkalosis Respiratory alkalosis - A primary disorder where the first change is a lowering of PaCO2, resulting in an elevated pH. Compensation (bringing the pH back down toward normal) is a secondary lowering of bicarbonate (HCO3) by the kidneys; this reduction in HCO3- is not metabolic acidosis, since it is not a primary process. Primary Event Compensatory Event HCO3- ↓HCO3- ↑ pH ~ ------- ↑ pH ~ -------- ↓ PaCO2 ↓ PaCO2

Primary Acid-base Disorders: Respiratory Acidosis Respiratory acidosis - A primary disorder where the first change is an elevation of PaCO2, resulting in decreased pH. Compensation (bringing pH back up toward normal) is a secondary retention of bicarbonate by the kidneys; this elevation of HCO3- is not metabolic alkalosis since it is not a primary process. Primary Event Compensatory Event HCO3- ↑ HCO3- ↓ pH ~ --------- ↓ pH ~ --------- ↑PaCO2 ↑ PaCO2

Primary Acid-base Disorders: Metabolic Acidosis Metabolic acidosis - A primary acid-base disorder where the first change is a lowering of HCO3-, resulting in decreased pH. Compensation (bringing pH back up toward normal) is a secondary hyperventilation; this lowering of PaCO2 is not respiratory alkalosis since it is not a primary process. Primary Event Compensatory Event ↓ HCO3- ↓HCO3- ↓ pH ~ ------------ ↓ pH ~ ------------ PaCO2 ↓ PaCO2

Primary Acid-base Disorders: Metabolic Alkalosis Metabolic alkalosis - A primary acid-base disorder where the first change is an elevation of HCO3-, resulting in increased pH. Compensation is a secondary hypoventilation (increased PaCO2), which is not respiratory acidosis since it is not a primary process. Compensation for metabolic alkalosis (attempting to bring pH back down toward normal) is less predictable than for the other three acid-base disorders. Primary Event Compensatory Event ↑ HCO3- ↑HCO3- ↑ pH ~ ------------ ↑ pH ~ --------- PaCO2 ↑PaCO2

Steps to an Arterial Blood Gas Interpretation Acid-base evaluation requires a focus on three of the reported components: pH PaCO2 HCO3

Case 1 ABGs obtained in the ICU pH 7.18 PCO2 20 mmHg HCO3 7 mEq/L

Case 2 Arterial blood gas sample is taken, revealing the following: pH : 7.30 PCO2 : 65 mm Hg

Case 2 What is the acid-base disorder?

Example 1 Jane Doe is a 45-year-old female admitted to the nursing unit with a severe asthma attack. She has been experiencing increasing shortness of breath since admission three hours ago. Her arterial blood gas result is as follows: Result Normal range pH 7.22 7.35 – 7.45 PaCO2 55 35 - 45 mmHg HCO3 25 22 - 26mEq/L

Step One Assess the pH to determine if the blood is within normal range, alkalotic or acidotic. above 7.45 : alkalotic below 7.35 : acidotic

Example 1 Acidosis Result Normal range pH 7.22 7.35 – 7.45 PaCO2 55 35 - 45 mmHg HCO3 25 22 - 26mEq/L PaO2 75 80 – 100 mmHg Acidosis

Step Two Determine if it is caused primarily by a : respiratory or metabolic problem. To do this, assess the PaCO2 level. Remember that with a respiratory problem: pH decreases below 7.35, the PaCO2 rise. pH rises above 7.45, the PaCO2 fall. If pH and PaCO2 are indeed moving in opposite directions, then the problem is : respiratory

Example 1 Acidosis Respiratory Result Normal range pH 7.22 7.35 – 7.45 PaCO2 55 35 - 45 mmHg HCO3 25 22 - 26mEq/L PaO2 75 80 – 100 mmHg Acidosis Respiratory

Step Three Assess the HCO3 value. Remember that with with a metabolic problem: pH increases, the HCO3 also increase. pH decreases, the HCO3 also decreases If pH and HCO3 are moving in the same direction, then the problem is metabolic.

Step Four Look for Compensation When a patient develops an acid-base imbalance, the body attempts to compensate. If PaCO2 rise → HCO3 will rise too → pH return to normal If PaCO2 fall → HCO3 will fall too → pH return to normal If HCO3 rise → PaCO2 will rise too → pH return to normal If HCO3 fall → PaCO2 will fall too → pH return to normal

Uncompensated, partially compensated, or fully compensated In uncompensated or partially compensated: pH remains outside the normal range. In fully compensated states : pH has returned to within the normal range. If pH in normal range but > 7,4 → Alkalosis fully compensated If pH in normal range but < 7,4 → Acidosis fully compensated

Example 1 Acidosis Respiratory uncompensated Result Normal range pH 7.22 7.35 – 7.45 PaCO2 55 35 - 45 mmHg HCO3 25 22 - 26mEq/L PaO2 75 80 – 100 mmHg Acidosis Respiratory uncompensated

Step Five Assess the PaO2. A value below 80 mm Hg can indicate hypoxemia, depending on the age of the patient. If PaO2 < 80 mmHg : Hypoxemia If PaO2 > 100 mmHg : Hyperoxemia

Example 1 Acidosis Respiratory uncompensated with hypoxemia Result Normal range pH 7.22 7.35 – 7.45 PaCO2 55 35 - 45 mmHg HCO3 25 22 - 26mEq/L PaO2 75 80 – 100 mmHg Acidosis Respiratory uncompensated with hypoxemia

PaO2 [oxygen tension] SaO2 [oxygen saturation] a = arterial

Pulse Oximeter Measures SaO2

Example 2 Jane Doe is a 54-year-old female admitted for an ileus. She had been experiencing nausea and vomiting. An NG tube has been in place for the last 24 hours. Here are the last ABG results: Result Normal range pH 7.43 7.35 – 7.45 PaCO2 48 35 - 45 mmHg HCO3 36 22 - 26mEq/L PaO2 110 80 – 100 mmHg alkalosis metabolic fully compensated With hyperoxemia

Example 3 John Doe is a trauma patient with an altered mental status. His initial arterial blood gas result is as follows: Result Normal range pH 7.33 7.35 – 7.45 PaCO2 62 35 - 45 mmHg HCO3 35 22 - 26mEq/L PaO2 60 80 – 100 mmHg acidosis respiratory partially compensated With hypoxemia

Example 4 John Doe is admitted to the hospital. He is a kidney dialysis patient who has missed his last two appointments at the dialysis center. His arterial blood gas values are reported as follows: Result Normal range pH 7.32 7.35 – 7.45 PaCO2 32 35 - 45 mmHg HCO3 18 22 - 26mEq/L PaO2 77 80 – 100 mmHg acidosis metabolic partially compensated With hypoxemia

Exercise 1 Result Normal range pH 7.33 7.35 – 7.45 PaCO2 60 35 - 45 mmHg HCO3 34 22 - 26mEq/L PaO2 70 80 – 100 mmHg Interpretation? Acidosis respiratory partially compensated with hypoxemia

Exercise 2 Result Normal range pH 7.48 7.35 – 7.45 PaCO2 42 35 - 45 mmHg HCO3 30 22 - 26mEq/L PaO2 120 80 – 100 mmHg Interpretation? Alkalosis metabolic uncompensated with hyperoxemia

Exercise 3 Interpretation? Normal Result Normal range pH 7.38 7.35 – 7.45 PaCO2 38 35 - 45 mmHg HCO3 24 22 - 26mEq/L PaO2 90 80 – 100 mmHg Interpretation? Normal

Exercise 4 Result Normal range pH 7.21 7.35 – 7.45 PaCO2 60 35 - 45 mmHg HCO3 24 22 - 26mEq/L PaO2 80 – 100 mmHg Interpretation? Acidosis respiratory uncompensated with hypoxemia

Exercise 5 Result Normal range pH 7.48 7.35 – 7.45 PaCO2 28 35 - 45 mmHg HCO3 20 22 - 26mEq/L PaO2 110 80 – 100 mmHg Interpretation? Alkalosis respiratory partially compensated with hyperoxemia

Exercise 6 Interpretation? Alkalosis respiratory uncompensated Result Normal range pH 7.5 7.35 – 7.45 PaCO2 29 35 - 45 mmHg HCO3 24 22 - 26mEq/L PaO2 100 80 – 100 mmHg Interpretation? Alkalosis respiratory uncompensated

Exercise 7 Result Normal range pH 7.28 7.35 – 7.45 PaCO2 40 35 - 45 mmHg HCO3 18 22 - 26mEq/L PaO2 50 80 – 100 mmHg Interpretation? Acidosis metabolic uncompensated with hypoxemia

Exercise 8 Interpretation? Alkalosis respiratory fully compensated Result Normal range pH 7.45 7.35 – 7.45 PaCO2 26 35 - 45 mmHg HCO3 16 22 - 26mEq/L PaO2 85 80 – 100 mmHg Interpretation? Alkalosis respiratory fully compensated

Practice ABG’s PaO2 90 SaO2 95 pH 7.48 PaCO2 32 HCO3 24

Answers to Practice ABG’s uncompensated Respiratory alkalosis uncompensated Respiratory acidosis , Hypoxemia uncompensated Metabolic acidosis Fully Compensated Respiratory acidosis uncompensated Metabolic alkalosis Fully Compensated Respiratory acidosis Hypoxemia Fully Compensated Metabolic alkalosis uncompensated Respiratory acidosis Hypoxemia uncompensated Metabolic alkalosis Hyperoxemia

Mixed respiratory and metabolic acid-base disorder

Anion Gap = Na - (Cl + HCO3) Normal = 12 mmol/L The anion gap, is really not a gap at all, it just represents the anions we don’t usually measure. Anion Gap = Na - (Cl + HCO3) Normal = 12 mmol/L if the anion gap is >/= 20mmol/L, there is a primary metabolic acidosis regardless of the pH or serum bicarbonate concentration.

To see if there is more going on (ie an underlying metabolic akalosis or an additional non-gap met acidosis, calculate the corrected serum bicarb: 1) Excess anion gap = measured gap - normal gap (usually 12) 2) corrected bicarb = excess gap + measured HCO3

Analysis : 1) In a simple gap acidosis, the HCO3 falls by Analysis : 1) In a simple gap acidosis, the HCO3 falls by an amount equal to the excess gap (the excess acid consumes the bicarb) and the corrected bicarb will be normal 22-26 2) if corrected bicarb is high (>30), there is an underlying metabolic alkalosis also 3) if corrected bicarb is low (<23), there is an additional non-anion gap metabolic acidosis

Example Consider a patient with a pH of 7.50, a Pco2 of 20 mmHg, a bicarbonate 15 mmol/L, a sodium:140 mmol/L, and a chloride: 103 mmol/L. This person is alkalemic with a low Pco2 and a low bicarbonate concentration. Because the pH is high, the low Pco2 represents a primary disorder, and a respiratory alkalosis is present. At first inspection, the low bicarbonate level appears to be in metabolic compensation for a chronic alkalosis.

Follow the second rule calculate the anion gap: 140 - (103 + 15) = 22 mmol/L. A gap of 22 mmol per liter is greater than would be expected solely in compensation for a chronic alkalosis and suggests that a second primary disorder, an anion gap metabolic acidosis, is also present. If the anion gap had not been calculated, the underlying metabolic acidosis would have been missed.

proceed to the third rule calculate the excess anion gap: 22 - 12 = 10 mmol/L, and add it to the measured bicarbonate level: 15 mmol per liter. corrected bicarb: 10 + 15 = 25 mmol/L

Result: normal, indicating that no further primary abnormalities are present. This patient had ingested a large quantity of aspirin and displayed the centrally mediated respiratory alkalosis and the anion gap metabolic acidosis associated with salicylate overdose.

Example 2 Patient with: pH : 7.40 PCO2 : 40 mmHg bicarbonate : 24 mmol/L sodium: 145 mmol/L chloride: 100 mmol/L

This is a seemingly normal set of values until the anion gap is calculated: 145 - (100 + 24) = 21 mmol/L The increased gap defines a metabolic acidosis even though the pH is normal.

Now calculate the excess anion gap: 21 - 12 = 9 mmo/L, and add it to the measured bicarbonate level: 24 mmol per liter. corrected bicarb: 9 + 24 = 33 mmol/L

The sum, 33 mmol/L, is higher than a normal bicarbonate concentration, indicating a metabolic alkalosis is also present. Result: Metabolic Acidosis and Metabolic Alkalosis These laboratory values are from a patient with chronic renal failure (causing the metabolic acidosis) who began vomiting (hence the metabolic alkalosis) as his uremia worsened.

The acute alkalosis of vomiting off set the chronic acidosis of renal failure, resulting in normal pH. Without a systematic approach to acid-base disorders, including calculation of the anion gap and of the excess anion gap, these mixed acid-base disorders could easily have been overlooked.

Example 3 Patient with: pH : 7.50 PCO2 : 20 mmHg bicarbonate : 15 mmol/L sodium: 145 mmol/L chloride: 100 mmol/L

The pH is high, the PCO2 and bicarbonate values are low. Because the pH is above 7.40, the low PCO2 is a primary abnormality, and the patient has a respiratory alkalosis.

Anion gap: 145 - (100 + 15) = 30 mmol/L anion gap metabolic acidosis

excess anion gap: 30 - 12 = 18 mmo/L, corrected bicarb: 18 + 15 = 33 mmol/L metabolic alkalosis Result: Respiratory Alkalosis, Metabolic Acidosis, and Metabolic Alkalosis

The near-normal pH reflects the competing effects of these three primary disorders. This person had a history of vomiting (the metabolic alkalosis), evidence of alcoholic ketoacidosis (causing metabolic acidosis), and findings compatible with a bacterial pneumonia (hence the respiratory alkalosis). These three independent disorders can occur concurrently in other clinical settings. Four primary acid-base disorders cannot coexist, as a patient cannot hypoventilate and hyperventilate at the same time.

Example 4 Patient with: pH : 7.15 PCO2 : 15 mmHg bicarbonate : 5 mmol/L sodium: 140 mmol/L chloride: 110 mmol/L

Anion gap: 140 - (110 + 5) = 25 mmol/L anion gap metabolic acidosis

excess anion gap: 25 - 12 = 13 mmo/L, corrected bicarb: 13 + 5 = 18 mmol/L nonanion gap metabolic acidosis Result: Anion Gap and Nonanion Gap Metabolic Acidoses

suggesting that additional gastrointestinal or renal loss of bicarbonate has occurred and that both an anion gap and a nonanion gap metabolic acidosis are present. Diabetic ketoacidosis was responsible for the anion gap acidosis in this patient. The nonanion gap (hyperchloremic) acidosis is that phenomenon observed in the recovery phase of diabetic ketoacidosis due to failure to regenerate bicarbonate from ketoacids lost in the urine. If the sum of the excess anion gap and the measured bicarbonate had not been calculated, the underlying nongap acidosis would not have been appreciated.

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