ABG

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ABG Interpretation:

ABG Interpretation Prof. AKM Mosharraf Hossain FCPS PhD Respiratory Medicine Wing Bangabandhu Sheikh Mujib Medical University akmmosharrafhossain@yahoo.com www.authorstream.com 31/03/2015

Introduction:

Introduction  Each day, adults generate large amounts of acids that must be expired, excreted, metabolized to non-charged neutral molecules, and/or buffered to avoid fatal acidemia . These acids are of three major classes: Carbonic acid Lactic acid and Citric acid Sulphuric acid Acid-base balance is maintained by normal pulmonary excretion of carbon dioxide, metabolic utilization of organic acids, and renal excretion of nonvolatile acids.

ABG- Basic Concept:

ABG- Basic Concept The anatomy of an arterial blood gas (ABG) as seen most often in the medical record: 7.40 / 40 / 98 / 24 , or pH/PaCO2/PaO2/HCO3- ◊ pH : arterial blood pH ◊ PaCO2 : arterial CO2 pressure, mm Hg (often written more simply as PCO2) ◊ PaO2 : arterial O2 pressure, mm Hg (often written more simply as PO2) ◊ HCO3 -: serum bicarbonate concentration, mEq/liter

Slide 4:

Arterial Blood Gas Values Analysis SI units Non- SIunits HCO3 21-29 mmol/l 21-29 meq/l H ions 37-45 nmol/l pH 7.38-7.42 PaCo2 4.5-6 Kpa 34-45 mmHg PaO2 12-15 Kpa 90-113 mmHg Oxygen sat > 97% Base excess +2 to -2 Anion Gap 3-12

Why ABG Is Necessary? :

Why ABG Is Necessary? The utilization of an ABG analysis becomes necessary in view of the following advantages: Aids in establishing diagnosis. Guides treatment plan. Aids in ventilator management. Improvement in acid/base management; allows for optimal function of medications. Acid/base status may alter electrolyte levels critical to a patient’s status.

Slide 7:

● An abnormal modified Allen test ●Local infection or distorted anatomy at the puncture site ( eg , previous surgical interventions, congenital or acquired malformations, burns, aneurysm, stent, arteriovenous fistula, vascular graft) ●Severe peripheral vascular disease of the artery selected for sampling ●Active Raynaud’s syndrome (particularly sampling at the radial site) Contraindications-ABG Sampling

Types:

Types

What is the primary disorder?:

What is the primary disorder? What disorder is present? pH pCO2 or HCO3 Respiratory Acidosis pH low pCO2 high Metabolic Acidosis pH low HCO3 low Respiratory Alkalosis pH high pCO2 low Metabolic Alkalosis pH high HCO3 high

Respiratory Acidosis:

Respiratory Acidosis Respiratory acidosis results from hypoventilation which is manifested by the accumulation of CO 2 in the blood and a drop in blood pH. Examples: 1 . Central Nervous System Depression (Sedatives, CNS disease, Obesity Hypoventilation syndrome) 2.Pleural Disease (Pneumothorax) 3.Lung Disease (COPD, pneumonia) 4.Musculoskelatal disorders (Kyphoscoliosis, Guillain-Barre, Myasthenia Gravis, Polio)

Respiratory Alkalosis:

Respiratory Alkalosis Respiratory alkalosis results from hyperventilation which is manifested by excess elimination of CO2 from the blood and a rise in the blood pH. Examples: 1. CNS event (CNS hemorrhage) 2. Drugs (salicylates, progesterone) 3. Pregnancy (especially the 3rd trimester) 4. Decreased lung compliance (interstitial lung disease) 5. Liver cirrhosis 6. Anxiety

Metabolic acidosis-anion gap:

Metabolic acidosis-anion gap Anion gap acidosis results from accumulation of acidic metabolites and is manifested by a low HCO3- and an anion gap > 12 [Na-(Cl+HCO3)]. Examples: 1.Uremia 2.Ketoacidosis 3.Alcohol poisons or drug intoxication (methanol, ethylene glycol, paraldehyde, salicylates ) 4.Lactic acidosis (sepsis, left ventricular failure) KULAEM is a mnemonic commonly used ( Ketoacidosis , U remia, L actic acidosis, E thylene glycol intoxication , A spirin, K etoacidosis ).

Metabolic acidosis-nonanion gap:

Metabolic acidosis-nonanion gap Non-anion gap acidosis results from loss of bicarbonate or external acid infusion and is manifested by a low HCO3-, but the anion gap is <12. Examples: 1.GI loss of HCO3- (diarrhea) 2.Renal loss of HCO3- a.Compensation for respiratory alkalosis b.Carbonic anhydrase inhibitor ( Diamox ) c.Renal tubular acidosis d.Ureteral diversion 3.Other causes: HCl or NH 4 Cl infusion, Cl gas inhalation, 4. Hyperalimentation Mnemonic is DRACCUH ( Diarrohea , R enal tubular acidosis, A cid infusion, C ompensation for respiratory alkalosis, C arbonic anhydrase inhibitor, U reteral diversion, hyperalimentation ).

Metabolic alkalosis:

Metabolic alkalosis Metabolic alkalosis results from elevation of serum bicarbonate. Examples of specific causes: 1. Volume contraction (vomiting, overdiuresis , ascites ) 2. Hypokalemia 3. Alkali ingestion (bicarbonate) 4. Excess gluco - or mineralo -corticoids 5. Bartter's syndrome

A-a Oxygen gradient:

A-a Oxygen gradient The PAO2 to PaO2 gradient is normally close to 10 (up to 21 in older individuals) and is written as follows: P(A-a)O2 = 10 mmHg PAO2 = FiO2(700) - (PACO2 x 1.25), or PAO2 = 147 - (PACO2 x 1.25) PACO2 is equal to PaCO2 Average value for arterial PaO2: PaO2 = 104.2 - (0.27 x age) = 100- 1/3 age in yrs

Effect of PACO2 & PAO2:

Effect of PACO2 & PAO2 PACO2 & PaCO2 PAO2 PaO2 40 97 87 64 67 57 80 47 37

pH and H+ conc :

pH and H+ conc pH Approximate [H+] ( mmol /L) 7.00 100 7.05 89 7.10 79 7.15 71 7.20 63 7.25 56 7.30 50 7.35 45 7.40 40 7.45 35 7.50 32 7.55 28 7.60 25 7.65 22

Stepwise Approach of ABG analysis:

Stepwise Approach of ABG analysis Step 1 : Acidemic or Alkalemic? Step 2 : Is the primary disturbance respiratory or metabolic? Step 3 . For a respiratory disturbance, determine whether it is acute or chronic. Step 4. For a metabolic acidosis, determine whether an anion gap is present. Step 5 . Determine whether other metabolic disturbances co-exist with an anion gap acidosis . Step 6. Assess the normal compensation by the respiratory system for a metabolic disturbance.

Assess consistency…:

Assess consistency… Assess the internal consistency of the values using the Henderseon-Hasselbach equation: [H+] = 24(PaCO 2 ) [HCO 3 -] If the pH and the [H+] are inconsistent, the ABG is probably not valid.

Step 1: acidaemic or alkalemic:

Step 1: acidaemic or alkalemic The pH of the arterial blood gas measurement identifies the disorder as alkalemic or acidemic. Normal arterial blood pH = 7.40 ± 0.02 ◊ Acidemic: pH < 7.38 ◊ Alkalemic: pH > 7.42

Step 2:Respiratory or Metabolic:

Step 2:Respiratory or Metabolic The disturbance effects primarily the arterial PaCO2 or the serum HCO3- ◊ A respiratory disturbance alters the arterial PaCO2 (normal value 40, range 38-42). ◊ A metabolic disturbance alters the serum HCO3- (normal value 24, range 22-26). ◊ If HCO3- < 22, metabolic acidosis is present. ◊ If HCO3- > 26, metabolic alkalosis is present,

Step3:Respiratory- acute or chronic:

Step3:Respiratory- acute or chronic ◊ Acute respiratory acidosis: pH decrease = 0.08 x (PaCO2 - 40)/10 ◊Chronic respiratory acidosis: pH decrease = 0.03 x (PaCO2 - 40)/10 ◊ Acute respiratory alkalosis: pH increase = 0.08 x (40 - PaCO2)/10 ◊ Chronic respiratory alkalosis pH increase = 0.03 x (40 - PaCO2)/10

Step4: Metabolic acidosis, anion gap?:

Step4: Metabolic acidosis, anion gap? The normal anion gap is 12 mEq /L. The anion gap is the difference between negatively charged ( anions: HCO3-,Cl-) and positively charged ( cation : Na+) electrolytes, which are measured in routine serum assays. The anion gap is low in hypoalbuminaemia .

Anion gap ions:

Anion gap ions Unmeasured anions Unmeasured cations Proteins, mostly albumin 15 mEq /L Organic acids 5mEq/L Phosphates 2 mEq /L Sulfates 1 mEq /L Totals: 23 mEq /L Calcium 5 mEq /L Potassium 4.5 mEq /L Magnesium 1.5mEq/L   Total 11 mEq /L ◊ Anion gap = Na + - ( Cl - + HCO3 - )=12 ◊ Anion gap metabolic acidosis, anion gap > 12 ◊ Normal or non-anion gap acidosis, anion gap < 12

Step 5: Other metabolic disturbance (if anion gap acidosis)?:

Step 5: Other metabolic disturbance (if anion gap acidosis)? A non-anion gap acidosis or a metabolic alkalosis may exist concurrently with an anion gap acidosis. Corrected HCO3-(Delta Gap) = measured HCO3 - + (anion gap - 12) If the corrected HCO3- is greater than 24, a metabolic alkalosis co-exists. If the corrected HCO3- is less than 24 then a non-anion gap acidosis co-exists.

Nongap metabolic acidosis:

Nongap metabolic acidosis Causes of nongap metabolic acidosis - DURHAM Diarrhea, ileostomy , colostomy, enteric fistulas Ureteral diversions or pancreatic fistulas RTA type I or IV, early renal failure Hyperailmentation , hydrochloric acid administration Acetazolamide , Addison’s Miscellaneous – post - hypocapnia , toulene , sevelamer , cholestyramine ingestion For non-gap metabolic acidosis, calculate the urine anion gap U AG = U NA + U K – U CL If UAG>0: renal problem If UAG<0: nonrenal problem (most commonly GI)

Step 5: Other metabolic disturbence with anion gap acidosis- example:

Step 5: Other metabolic disturbence with anion gap acidosis- example A patient with a anion gap metabolic acidosis has a HCO3- of 10 mEq /L and an anion gap of 26. Corrected HCO3- = 10 + (26 - 12)=24mEq/l, so no other metabolic disturbence . If this patient had a HCO3- of 15 and an anion gap of 26, then Corrected HCO3- = 15 + (26 - 12)=29mEq/l, so more than 24 and metabolic alkalosis coexist.

Step 6: Respiratory compensation for metabolic acidosis:

Step 6: Respiratory compensation for metabolic acidosis The respiratory system responds quickly and most predictably to a metabolic acidosis. The change in PaCO2 exhibits a linear correlation with the change in HCO3-. The equation that predicts the respiratory response to a metabolic acidosis is called Winter's formula : Expected PaCO2 = (1.5 x HCO3-) + (8 ± 2) If the serum HCO3- is 10 mEq /L, then Expected PaCO2 is between 21 and 25. If the measured PaCO2 falls outside of this range, then an additional respiratory disturbance must be occurring concurrently. If the measured PaCO2 is less than 21, then the additional disturbance is a respiratory alkalosis. If the measured PaCO2 is greater than 25, then the additional disturbance is a respiratory acidosis.

Step 6: Respiratory compensation for metabolic alkalosis:

Step 6: Respiratory compensation for metabolic alkalosis The respiratory response to metabolic alkalosis is hypoventilation, but the degree of PaCO2 increase does not exhibit a linear relationship with the HCO3-. a patient will be alkalemic (pH > 7.42) if the PaCO2 is elevated to compensate for a metabolic alkalosis (If the patient is acidemic , pH < 7.38, then an additional respiratory acidosis is present).

Sample analysis (1):

Sample analysis (1) Case: A 23-year-old male, diabetic presents with confusion. He has been suffering from an diarrohea , abdominal pain, vomitting for the last 24 hours. He has not been eating much and stopped taking his insulin. His glucose is high and electrolytes and arterial blood gases as follows: Na+ 130 mEq /L Cl - 80 mEq HCO3-10mEq/L pH 7.20 PaCO2 25 mm Hg PO2 68 mm Hg Interpret the ABG values and determine the acid-base abnormalities.

Determine oxygenation & A-a gradient:

Determine oxygenation & A-a gradient Case: Na+130 , Cl-80 , HCO3-10 . ABG 7.20/25/68 Expected PaO2: 100 - 1/3 of age yrs= 93 mm Hg. So, hypoxemic for age. A-a gradient=[150 - 1.25 (PaCO2 )] - PaO2 =[ 150 - 1.25 (25)] - 68 = 51 This is about 51, which is markedly elevated from normal (about 10) for his age. This suggests that there is a primary lung problem and that his hypoxia is not related to hypoventilation.

Analysis…….:

Analysis……. Patient values: Na+ 130, Cl - 80, HCO3- 10. ABG 7.20/25/68 Step1 : acidosis or alkalosis? The pH is 7.20, which is less than normal (7.38-7.42), so Acidosis. Step 2: Respiratory or Metabolic? The PaCO2 is low (< 40), so the respiratory system is not causing the acidosis. The bicarbonate is low (< 24), which indicates a Metabolic.

Metabolic acidosis, anion gap?:

Metabolic acidosis, anion gap? Step 4: Is there an increased anion gap? Patient values: Na+ 130, Cl - 80, HCO3- 10. ABG 7.20/25/68 The anion gap is 130- (80+10)= 130-90= 40 mEq /liter. So , the patient has a Anion gap M etabolic acidosis.

Any other metabolic disturbence?:

Any other metabolic disturbence? Step 5: Are there other metabolic disturbances present? Patient values: Na+ 130, Cl - 80, HCO3- 10. ABG 7.20/25/68 . Corrected HCO3- = measured HCO3- + (AG-12) = (10) + (40-12) = 38. The corrected HCO3- is much higher than a normal HCO3- (24), suggesting there is a metabolic alkalosis present also.

Any other Respiratory disturbance?:

Any other Respiratory disturbance? Step 6: Is the respiratory system compensating for a metabolic acidosis? Patient values: Na+ 130, Cl - 80, HCO3- 10. ABG 7.20/25/68 Using Winter's formula: Expected PaCO2 = 1.5 (HCO3-) + 8 ± 2 = 1.5 (10) + 8 ± 2 = 23 ± 2 . The PaCO2 is expected to be between 21 and 25, and the measured PaCO2 falls within the expected range, indicating the respiratory system is compensating appropriately.

Total analysis:

Total analysis In summary, the patient is hypoxemic from a primary lung process. He also has a anion gap metabolic acidosis with coexisting metabolic alkalosis , and compensatory respiratory alkalosis

Sample Analysis (2) :

Sample Analysis (2) An obese 70-year-old male has diabetes for 25 years duration complicated by coronary artery disease (CABG x 4 vessels 10 years ago), cerebrovascular disease (carotid artery endarterectomy 3 years ago) and peripheral vascular disease ( Aorto-bifem 2 years ago).

:

ABGs obtained in the ICU pH 7.18 PCO2 20 mmHg HCO3 7 mEq /L What is the primary disorder? What is the physiologic response to this disorder? Patient with Ischemic Bowel

:

Step 1: Acidemic , alkalemic , or normal ? Step 2: Is the primary disturbance respiratory or metabolic ? Step 3: For a primary respiratory disturbance, is it acute or chronic ? Step 4: For a metabolic disturbance, is the respiratory system compensating OK ? Step 5: For a metabolic acidosis, is there an increased anion gap? Step 6: For an increased anion gap metabolic acidosis, are there other derangements? Patient with Ischemic Bowel

Patient with Ischemic Bowel:

Patient with Ischemic Bowel Step 1: Acidemic , alkalemic , or normal? ACIDEMIC

Patient with Ischemic Bowel:

Patient with Ischemic Bowel Step 2: Is the primary disturbance respiratory or metabolic? METABOLIC

Patient with Ischemic Bowel:

Patient with Ischemic Bowel Step 3: For a primary respiratory disturbance, is it acute or chronic? NOT APPLICABLE

Step 4: For a metabolic disturbance, is the respiratory system compensating OK?:

Step 4: For a metabolic disturbance, is the respiratory system compensating OK? DISCUSSION The physiological response to metabolic acidosis is hyperventilation, with a resulting compensatory drop in PCO2 according to "Winter's formula": Expected PCO2 in metabolic acidosis = 1.5 x HCO3 + (8±2 ) If the actual measured PCO2 is much greater than the expected PCO2 from Winter's formula, then the respiratory system is not fully compensating for the metabolic acidosis, and a respiratory acidosis is concurrently present. This may occur, for instance, when respiratory depressants like morphine or fentanyl are administered to the patient to reduce pain.

Step 4: For a metabolic disturbance, is the respiratory system compensating OK?:

Step 4: For a metabolic disturbance, is the respiratory system compensating OK? " Winter's formula": Expected PCO2 in metabolic acidosis = 1.5 x HCO3 + (8±2 ) = 1.5 x 7 + 8 = 18.5 ±2 pH 7.18 PCO2 20 mm Hg HOC3 7 mEq / L

Patient with Ischemic Bowel:

Patient with Ischemic Bowel Step 5: For a metabolic acidosis, is there an increased anion gap? FOR THIS STEP ONE MUST OBTAIN SERUM ELECTROLYTE DATA

Patient with Ischemic Bowel:

Patient with Ischemic Bowel SERUM ELECTROLYTE DATA Serum sodium 135 mEq /L Serum bicarbonate 7 mEq /L Serum chloride 98 mEq /L

Slide 49:

Anion Gap = = 135 - 98 -7 mEq /L = 30 mEq /L (ELEVATED) SERUM ELECTROLYTE DATA Serum sodium 135 mEq /L Serum bicarbonate 7 mEq /L Serum chloride 98 mEq /L Anion Gap = Serum Sodium – Serum Chloride – Serum Bicarbonate

Patient with Ischemic Bowel:

Patient with Ischemic Bowel Step 5: For a metabolic acidosis, is there an increased anion gap? Anion Gap = 135 - 98 -7 mEq /L= 30 mEq /L

Patient with Ischemic Bowel:

Patient with Ischemic Bowel Step 6: For an increased anion gap metabolic acidosis, are there other derangements? To determine if there are other metabolic derangements present we start by determining the “corrected bicarbonate concentration”: Corrected HCO3 = measured HCO3 + (Anion Gap - 12). If the corrected HCO3 is less than normal (under 22mEq/L) then there is an additional metabolic acidosis present. Corrected HCO3 values over 26 mEq /L reflect a co-existing metabolic alkalosis.

Patient with Ischemic Bowel:

Patient with Ischemic Bowel Corrected HCO3 = measured HCO3 + (Anion Gap - 12). Corrected HCO3 = 7 + (30 - 12) = 25 If the corrected HCO3 is less than normal (under 22mEq/L) then there is an additional metabolic acidosis present. Corrected HCO3 values over 26 mEq /L reflect a co-existing metabolic alkalosis.

Patient with Ischemic Bowel:

Patient with Ischemic Bowel Step 6: For an increased anion gap metabolic acidosis, are there other derangements? ANSWER: NO OTHER DERANGEMENTS NOTED

What is the cause of the elevated anion-gap metabolic acidosis?:

What is the cause of the elevated anion-gap metabolic acidosis? The most common etiologies of a metabolic acidosis with an increased anion gap are shown below: Lactic acidosis Ingestion of Ethylene glycol , Methanol, Salicylate , Aspirin  Starvation Renal failure  Ketoacidosis (as in diabetic ketoacidosis )

Slide 55:

“Lactic acidosis is a disease characterized by a pH less than 7.25 and a plasma lactate greater than 5 mmol /L.” “ Hyperlactemia results from abnormal conversion of pyruvate into lactate. Lactic acidosis results from an increase in blood lactate levels when body buffer systems are overcome. This occurs when tissue oxygenation is inadequate to meet energy and oxygen need as a result of either hypoperfusion or hypoxia.” Lactic Acidosis?

Take Home Messages:

Take Home Messages If the pH and the [H+] are inconsistent, the ABG is probably not valid. an acidosis or alkalosis may be present even if the pH is in the normal range (7.35 – 7.45) You will need to check the PaCO 2 , HCO 3 - and anion gap

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