Interpreting Arterial Blood Gases

Arterial Blood Gases: 

Arterial Blood Gases Ashraf Gomaa

Some Definitions: 

Some Definitions pH: concentration of H+ in terms of 10 -x per litre pH of 7.0 = 10 -7 = 0.0000001 pH of 6.0 = 10 -6 = 0.000001 Acidosis: acidic blood; increase in H+ concentration (therefore decrease in pH) Alkalosis: alkalotic blood; increase in base, therefore decrease in H+

Basic principles: 

Basic principles Human bodies were designed to maintain: pH 7.35-7.45 PaO2 95-100 mmHg PaCO2 35-45 mmHg HCO3 22-26 mmHg

Buffers: 

Buffers Limit pH changes when strong acids/bases are introduced Weak acid and its conjugate base Addition of a strong acid is partly neutralized by the weak base HB (weak acid) H+ (strong acid) B- (conjugate weak base)

Bicarbonate Buffer System: 

Bicarbonate Buffer System CO2 + H20 H2CO3 H+ + HCO3- Main extracellular buffer Also affected by lungs and kidneys

Key Equation: 

Key Equation CO2 + H20 H2CO3 H+ + HCO3- Blood (instantaneously)

Key Equation: 

Key Equation CO2 + H20 H2CO3 H+ + HCO3- Lungs (within minutes)

Key Equation: 

Key Equation CO2 + H20 H2CO3 H+ + HCO3- Excretion via kidneys (hours to days)

Key Equation: 

Key Equation CO2 + H20 H2CO3 H+ + HCO3- Blood (instantaneously) Lungs (within minutes) Excretion via kidneys (hours to days)

Respiratory Acidosis: 

Respiratory Acidosis CO2 + H20 H2CO3 H+ + HCO3- Increase in CO2 “pushes” balance towards producing more H2CO3 Concentration of H+ increases, lowering pH Causes Impaired lung function Decreased respiratory drive

Respiratory Alkalosis: 

Respiratory Alkalosis CO2 + H20 H2CO3 H+ + HCO3- Increased RR causes decreased CO2 Balance shifts towards left decreasing concentration of H+ (therefore increasing pH) Causes Hyperventilation (anxiety)

Metabolic Acidosis: 

Metabolic Acidosis CO2 + H20 H2CO3 H+ + HCO3- Abnormal metabolism releases H+, increasing concentration of pH, and decreasing pH Causes DKA, lactic acidosis (arrest) Toxins Loss of HCO3 in diarrhea

Metabolic Alkalosis: 

Metabolic Alkalosis CO2 + H20 H2CO3 H+ + HCO3- Increased plasma HCO3- neutralizes H+ Decreased concentration of H+ Causes: Loss of H+ in vomit Gain of HCO3 (ingestion, IV)

Interpreting the ABGs: 

Interpreting the ABGs Remember the “normals” pH 7.4, PaCO2 40, HCO3 24 Know the “expected compensation”

Compensation: 

Compensation Disorder Initial change Compensation Metabolic Acidosis Decrease HCO3 1 mmHg decrease in PaCO2 for every 1 decrease in HCO3 Metabolic Alkalosis Increase HCO3 0.3-0.5 mmHg increase in PaCO2 for every 1 increase in HCO3 Respiratory alkalosis Decrease in PaCO2 Acute: 2 decrease in HCO3 for every 10 decrease in PaCO2 Chronic: 4-5 decrease Respiratory acidosis Increase in PaCO2 Acute: 1 increase in HCO3 for every 10 increase in PaCO2 Chronic: 3 increase

Compensation: 

Compensation Disturbance PaCO2 HCO3 Resp acidosis (acute) ↑ 10 ↑ 1 Resp acidosis (chronic) ↑ 10 ↑ 3 Resp alkalosis (acute) ↓ 10 ↓ 2 Resp alkalosis (chronic) ↓ 10 ↓ 4 Metabolic acidosis ↓ 1 ↓ 1 Metabolic alkalosis ↑ 3 ↑ 10

Acid Base determination: 

Acid Base determination The steps: Check the numbers (determine if the data are valid). The measured HCO3 from the chemistry should agree with the calculated HCO3 from the ABG. Determine if it is an acidemia or alkalemia pH greater than or less than 7.40

Acid Base determination: 

Acid Base determination The steps, continued. Determine the primary disorder. If the pH moves in the same direction as the HCO3, then a metabolic disorder is present. If it doesn’t, a respiratory disorder is present. Determine if there is a anion gap. Na - (Cl + HCO3) is normally 12 +/- 2.

Look for other disorders: 

Look for other disorders If an anion gap is present, determine the Delta-Delta. =Measured gap - normal gap + measured HCO3. If there are no other metabolic disorders, then this calculation should be nearly equal to a normal HCO3 (24). Delta-Delta <20, metabolic non gap acidosis present. Delta-Delta >27, metabolic alkalosis present.

Look for other disorders: 

Look for other disorders If metabolic acidosis is present, use Winter’s formula to determine if a respiratory disorder is present. 1.5 x (measured HCO3) + 8 This calculation should equal the PaCO2 +/- 2. If the PaCO2 is higher than this formula predicts, then a respiratory acidosis is present. If PaCO2 is lower, then a respiratory alkalosis is present.

Look for other disorders: 

Look for other disorders If a primary respiratory acidosis is present, determine if it is acute or chronic. 7.40 - (0.8 x (PaCO2 - 40)). If the calculated number agrees with the measured pH, then it is an acute respiratory acidosis. If the pH is higher than the calculated number but less than 7.40, then it is a chronic acidosis. If the pH is 7.40, then a metabolic alkalosis is present

Basic Approach: 

Basic Approach Is the pH acidemic or alkalemic? What is the primary disturbance? Metabolic: change in HCO3 and pH in same direction Respiratory: change in PaCO2 and pH in opposite direction Is the compensation appropriate?

Case 1: 

Case 1 24 yo M hx of drug abuse, brought to ER cyanotic pH 7.08 PaCO2 80 PaO2 37 HCO3 26

Case 1: 

Case 1 24 yo M hx of drug abuse, brought to ER cyanotic pH 7.08 PaCO2 80 PaO2 37 HCO3 26 Acidemic or Alkalemic? Acidemic

Case 1: 

Case 1 24 yo M hx of drug abuse, brought to ER cyanotic pH 7.08 PaCO2 80 PaO2 37 HCO3 26 pH in relation to PaCO2 and HCO3?

Basic Approach: 

Basic Approach Is the pH acidemic or alkalemic? What is the primary disturbance? Metabolic: change in HCO3 and pH in same direction Respiratory: change in PaCO2 and pH in opposite direction Is the compensation appropriate?

Case 1: 

Case 1 24 yo M hx of drug abuse, brought to ER cyanotic pH 7.08 PaCO2 80 PaO2 37 HCO3 26 primarily respiratory

Case 1: 

Case 1 24 yo M hx of drug abuse, brought to ER cyanotic pH 7.08 PaCO2 80 PaO2 37 HCO3 26 Is the compensation adequate?

Case 1: 

Case 1 PaCO2 increased by 40 For every 10 increase you would expect 1 increase in HCO3 Expected HCO3 would be ~28

Case 1: 

Case 1 24 yo M hx of drug abuse, brought to ER cyanotic pH 7.08 PaCO2 80 PaO2 37 HCO3 26 Acidemic, primarily respiratory, but mild component of metabolic Also hypoxemic Narcotic OD

Case 2: 

Case 2 42 F IDDM, presents with 4d hx of unwell pH 7.23 PaCO2 27 PaO2 118 HCO3 12 Acidemia, metabolic DKA, Na 135, Cl 99 AG = Na – Cl – HCO3 = 135 – 111 = 24

Case 3: 

Case 3 71 m hx of COPD, c/o SOB pH 7.21 PaCO2 75 PaO2 41 HCO3 30

PowerPoint Presentation: 

Acidemia, resp (acute on chronic), hypoxic

Case 4: 

Case 4 23 F c/o SOB, lightheaded and perioral tingling pH 7.54 PaCO2 22 PaO2 115 HCO3 21

PowerPoint Presentation: 

Alkalemia, resp (acute)

Case 5: 

Case 5 32 M c/o vomitting x 5d, HR 110 BP 90/50, dry MM pH 7.50 PaCO2 47 PaO2 80 HCO3 38

PowerPoint Presentation: 

Alkalemia, metabolic

Case 6: 

Case 6 ABG pH 7.29 PaCO2 57 HCO3 26 Chemistry Na 139 Cl 99 HCO3 29 Step 1: Borderline for valid data. Step 2: Acidemia. Step 3: Primary respiratory disorder. Step 4: Anion gap is 11 (normal). Step 5: 7.40 - (0.008 x (pCO2-40)) 7.40 - (0.008 x (57-40)=7.264 Interpretation: Acute respiratory acidosis. The pH is close enough to predicted. This patient was unresponsive until narcan was given. She had received too much narcotics.

Case 7: 

Case 7 ABG pH 7.31 PaCO2 10 HCO3 5 Chemistry Na 123 Cl 99 HCO3 5 Step 1: Valid data. Step 2: Acidemia. Step 3: Primary metabolic disorder. Step 4: Anion gap is 19 (elevated). Step 5: Delta-Delta=measured gap - normal gap + measured HCO3 19-12+5=12 Step 6: Winter’s formula 1.5 (5) + 8 = 15.5 Interpretation: Triple acid-base disorder of metabolic gap acidosis, respiratory alkalosis, and metabolic non gap acidosis.

Case 8: 

Case 8 ABG pH 7.20 PaCO2 25 HCO3 10 Chemistry Na 130 Cl 80 HCO3 10 Step 1: Valid data. Step 2: Acidemia. Step 3: Primary metabolic disorder. Step 4: Anion gap is 40 (elevated). Step 5: Delta-Delta=measured gap - normal gap + measured HCO3 40-12+10 = 38 Step 6: Winter’s formula 1.5 (10) + 8 = 23 Interpretation: Metabolic gap acidosis and metabolic alkalosis. There is no respiratory disorder as per step 6.

Case 9: 

Case 9 ABG pH 7.07 PaCO2 28 HCO3 8 Chemistry Na 125 Cl 100 HCO3 8 Step 1: Valid data. Step 2: Acidemia. Step 3: Primary metabolic disorder. Step 4: Anion gap is 17 (elevated). Step 5: Delta-Delta=measured gap - normal gap + measured HCO3 17-12+8 = 13 Step 6: Winter’s formula 1.5 (8) + 8 = 20 Interpretation: Metabolic gap acidosis, metabolic non gap acidosis, and respiratory acidosis. Even though the PaCO2 is less than 40, this is still an acidosis because it should be 20 by the Winter’s formula.

Case 10: 

Case 10 ABG pH 7.18 PaCO2 80 HCO3 30 Chemistry Na 135 Cl 93 HCO3 30 Step 1: Valid data. Step 2: Acidemia. Step 3: Primary respiratory disorder. Step 4: Anion gap is 12 (normal). Step 5: 7.40 - (0.008 x (pCO2-40)) 7.40-(0.008 x (80-40)=7.08 Interpretation: Acute respiratory acidosis with metabolic alkalosis. As predicted by above, the pH is not low enough to be entirely secondary to acute respiratory decompensation. Therefore he has a metabolic alkalosis. This probably represents acute on chronic respiratory acidosis.

Case 11: 

Case 11 ABG pH 7.39 PaCO2 48 HCO3 28.5 Chemistry Na 163 Cl 84 HCO3 29 Step 1: Valid data. Step 2: Acidemia. Step 3: Primary metabolic disorder (because there is a gap). Step 4: Anion gap is 50. Step 5: Delta-Delta=measured gap - normal gap + measured HCO3 50-12+29 = 67 Step 6: Winter’s formula 1.5 (29) + 8 = 51.5 Click mouse for interpretation. This patient has a triple acid-base disorder of respiratory alkalosis, metabolic gap acidosis, and metabolic alkalosis. Just by analyzing the ABG, one may suspect that there was no acid-base disturbance. Once a anion gap is calculated, it is clear that a profound disturbance is present. This patient provides clear evidence that one can not determine acid-base status without a chemistry. In this particular case, it is easier to determine the correct acid base status by starting with a metabolic gap disorder. This patient had taken ethylene glycol in a suicide attempt. Concurrent labs with the ABG were a lactic acid of 32 and ethylene glycol of 0.7 g/l. The profound metabolic alkalosis is secondary to a HCO3 infusion. The patient died 15 minutes after these laboratories were drawn.

Case 12: 

Case 12 ABG pH 6.84 PaCO2 21 HCO3 4 Chemistry Na 147 Cl 110 HCO3 <5 Step 1: Valid data. Step 2: Acidemia. Step 3: Primary metabolic disorder. Step 4: Anion gap is 33 (elevated). Step 5: Delta-Delta=measured gap - normal gap + measured HCO3 33-12+4 = 25 Step 6: Winter’s formula 1.5 (4) + 8 = 14 Interpretation: Metabolic gap acidosis and respiratory acidosis. This patient was septic and had a lactate greater than 12.

Case 13: 

Case 13 ABG pH 7.18 PaCO2 23 HCO3 9 Chemistry Na 140 Cl 102 HCO3 8 Step 1: Valid data. Step 2: Acidemia. Step 3: Primary metabolic disorder. Step 4: Anion gap is 30 (elevated). Step 5: Delta-Delta=measured gap - normal gap + measured HCO3 30-12+8 = 26 Step 6: Winter’s formula 1.5 (8) + 8 = 20 Interpretation: Metabolic gap acidosis and respiratory acidosis. The PaCO2 is very close to an appropriate level. This patient had DKA, with ketones 1:8.

Practice ABG’s: 

Practice ABG’s PaO 2 90 SaO 2 95 pH 7.48 PaCO 2 32 HCO 3 24 PaO 2 60 SaO 2 90 pH 7.32 PaCO 2 48 HCO 3 25 PaO 2 95 SaO 2 100 pH 7.30 PaCO 2 40 HCO 3 18 PaO 2 87 SaO 2 94 pH 7.38 PaCO 2 48 HCO 3 28 PaO 2 94 SaO 2 99 pH 7.49 PaCO 2 40 HCO 3 30 6. PaO 2 62 SaO 2 91 pH 7.35 PaCO 2 48 HCO 3 27 PaO 2 93 SaO 2 97 pH 7.45 PaCO 2 47 HCO 3 29 PaO 2 95 SaO 2 99 pH 7.31 PaCO 2 38 HCO 3 15 PaO 2 65 SaO 2 89 pH 7.30 PaCO 2 50 HCO 3 24 10. PaO 2 110 SaO 2 100 pH 7.48 PaCO 2 40 HCO 3 30