ABG

APPROACH TO INTERPRETATION OF ABG

• Know the primary disorder• Compute for the range of compensation• For metabolic acidosis get anion gap• For high anion gap get change in anion gap

and compare with change in bicarbonate

COMPENSATORY RESPONSEDisorder pH Primary Disturbance Compensatory

Response

Metabolic AcidosisDec HCO3

(loss of HCO3 or gain in H+)

Dec pCO2 (hyperventilation)

Respiratory Acidosis Inc pCO2(hypoventilation)

Inc HCO3(HCO3 generation)

Metabolic AlkalosisInc HCO3

(gain in HCO3 or loss of H+)

Inc pCO2(hypoventilation)

Respiratory Alkalosis Dec pCO2(hyperventilation)

Dec HCO3(HCO3 consumption)

COMPENSATORY RESPONSESDISORDER PRIMARY COMPENSATORY

RESPONSEMetabolic

Acidosis↓HCO3 1.2 mmHg decrease in PCO2 for every

1meq/L fall in HCO3

MetabolicAlkalosis

↑HCO3 0.7 mmHg elevation in PCO2 for every 1meq/L rise in HCO3

RespiratoryAcidosis

↑PCO2 1meq/L increase in HCO3 for every 10mmHg rise in PCO2

RespiratoryAlkalosis

↓PCO2 2meq/L reduction in HCO3 for every 10mmHg fall in PCO2

Case 1• A 55 year-old female, a known diabetic for 10 years, came in at

the ER for a 3 day history of fever and productive cough with anasarca, abdominal pain and vomiting

• Labs:– RBS – 22 (396)– BUN – 3.6– Crea- 323 mmol/L– Ca – 1.88– Alb – 22– Urine ketone +++– Na - 129– K – 3.8– Cl – 98– Phos – 3.2– USG – 1.030

ABG:pH – 7.135pCO2 – 20.2pO2 – 89HCO3 – 12.4BE – -9.0O2 sats – 98%

Case 1

• Interpret ABG– Look at the pH

• Acidotic – What caused the pH

• Metabolic acidosis– Compute for compensation

• Decrease in pCO2 = 1.2 x change in bicarbonate– Expected pCO2:

• Expected pCO2 = 26• Expected pCO2 is higher than actual pCO2 (20.2), therefore

there is concomitant respiratory alkalosis

ABG:pH – 7.135pCO2 – 20.2pO2 – 89HCO3 – 12.4BE – -9.0O2 sats – 98%Room air

Case 1:

• Interpret ABG– If metabolic, compute the anion gap

• Get the corrected Na:– Corrected Na = (RBS – 100) x 0.016 + Na– Corrected Na = 134

• Compute the anion gap:– Anion gap = Na – HCO3 – Cl – Anion gap = 23.6– HAGMA

Case 1:

• Interpret ABG– Get delta-delta:

• Change in anion gap– Change = AG - 12– Change = 11.6

• Change in bicarbonate– Change = 24 – bicarbonate– Change = 11.6

• Change in AG = change in bicarbonate– Therefore: pure HAGMA

• Otherwise:– Change in AG > change in bicarbonate: HAGMA + metab alkalosis– Change in AG < change in bicarbonate: HAGMA + NAGMA

Case 1:

• Interpret ABG– Check for pO2

• Correlate with FiO2– LPM to FiO2 at nasal cannula: 4 x LPM + 20

• Hypoxemia (pO2 at FiO2 21%)– Mild: 80-90– Moderate: 60-80– Severe: < 60

ABG:pH – 7.135pCO2 – 20.2pO2 – 89HCO3 – 12.4BE – -9.0O2 sats – 98%Room air

Case 1:

• Final ABG interpretation:– High anion gap metabolic acidosis with

concomitant respiratory alkalosis, mild hypoxemia

Case 1:• Conditions that lead to metabolic acidosis:

– HAGMA• DKA• Alcoholic ketoacidosis• Lactic acidosis• Renal insufficiency• Starvation• Salicylate intoxication• Methanol intoxication• Ethylene glycol intoxication

– NAGMA• Diarrhea• Pancreatic fluid loss• Ileostomy• Carbonic anhydrase inhibitors• RTA• Arginine and lysine in parenteral nutrition

Case 2:• Case 2: A 55 year-old female, a known diabetic for 10 years came in at the ER for a 3-

day history of fever and productive cough with anasarca, abdominal pain and vomiting

• RBS – 22• BUN – 13.2• Crea – 323• Ca – 1.62• Alb – 22• Urine ketone +• Na – 117• K – 2.8• Cl – 98• Phos – 6.2• USG – 1.025

ABG:pH – 7.535pCO2 – 45.22pO2 – 89.00HCO3 – 30.40BE – 6.0O2 sats – 98%

Case 2:

• Interpret ABG– Look at the pH

• Alkalotic – What caused the pH

• Metabolic alkalosis– Compute for compensation

• increase in pCO2 = 0.7 x change in bicarbonate– Expected pCO2:

• Expected pCO2 = 44.48• Expected PCO2 is near actual pCO2. Therefore, patient has

compensated metabolic alkalosis

ABG:pH – 7.535pCO2 – 45.22pO2 – 89.00HCO3 – 30.40BE – 6.0O2 sats – 98%10 lpm

Case 2:

• Interpret ABG– Check for pO2

• Correlate with FiO2– LPM to FiO2 at nasal cannula: 4 x LPM + 20

• Hypoxemia (pO2 at FiO2 21%)– At FiO2 21%, pO2 should be at least 90– Target pO2 = 90 / 0.21 x FiO2– Target pO2 - 257– hypoxemia

ABG:pH – 7.535pCO2 – 45.22pO2 – 89.00HCO3 – 30.40BE – 6.0O2 sats – 98%10 lpm

Case 2

• Final ABG interpretation:– Compensated metabolic alkalosis, with hypoxemia

Case 2:• Conditions associated with metabolic alkalosis:

– Diuretic therapy– Posthypercapnia– Vomiting– Nasogastric suction– NaHCO3– Refeeding alkalosis– Primary aldosteronism– Hyperreninism– Liddle’s syndrome– Licorice (exogenous mineralocorticoids)

Case 3:• A 50 year old male, with a 50 pack year smoking history presents at the ER for chronic cough,

weight loss, with a 3 day history of high-grade fever and dyspnea for which he took cotrimoxazole 800/160 mg TID. Early this morning, he was found unresponsive and was then brought to the ER. On PE, there was note of tachypnea, poor air entry and ocassional wheezing all over

• RBS – 6.3• BUN – 8.3• Crea- 96• Ca – 1.92• Alb – 22• USG – 1.035• Na – 163• K – 7.8• Cl – 98• Phos – 4.6• 12L ECG – ST, NA, NSSTWCs

ABG:pH – 7.135pCO2 – 70.2pO2 – 49.00HCO3 – 24.20BE – -2.0O2 sats – 78%

Case 3:

• Interpret ABG– Look at the pH

• acidotic– What caused the pH

• Respiratory acidosis– Compute for compensation

• increase in HCO3 = 1/10 x change in pCO2 (acute)• Increase in HCO3 = 3/10 x change in pCO2 (chronic)

– Expected HCO3: (acute)• Expected HCO3 = 27• Expected HCO3 is higher than actual HCO3 (24.20). Therefore,

patient has uncompensated respiratory acidosis

ABG:pH – 7.135pCO2 – 70.2pO2 – 49.00HCO3 – 24.20BE – -2.0O2 sats – 78%5 LPM

Case 3:

• Interpret ABG– Check for pO2

• Correlate with FiO2– LPM to FiO2 at nasal cannula: 4 x LPM + 20

• Hypoxemia (pO2 at FiO2 21%)– Expected pO2 = 90/0.21 x FiO2– Expected pO2 = 171– hypoxemia

ABG:pH – 7.135pCO2 – 70.2pO2 – 49.00HCO3 – 24.20BE – -2.0O2 sats – 78%5 LPM

Case 3

• Final ABG interpretation:– Uncompensated respiratory acidosis with

hypoxemia

Case 3

• Conditions associated respiratory acidosis:– Airway obstruction– Respiratory center depression– Neuromuscular defects– Restrictive defect

Case 4:• A 50 year old male, with a 50 pack-year smoking history presents at the ER for chronic cough,

weight loss, with 3 day history of low grade fever and dyspnea for which he took cotrimoxazole 800/160 mg TID. Early this morning, he was found unresponsive and was then brought to the ER. On PE, there was note of tachypnea, poor air entry and occasional wheezing all over

• RBS – 6.3• BUN – 14.3• Crea – 96• Ca - 2.42 • Alb – 22• USG – 1.015• Na – 113• K – 7.8• Cl – 98• Phos – 4.6• 12L ECG – ST, NA, shortened Qt, peaked T waves

ABG:pH – 7.48pCO2 – 30.2pO2 – 49.00HCO3 – 25.50BE – -1.8O2 sats – 78%

Case 4:

• Interpret ABG– Look at the pH

• alkalosis– What caused the pH

• Respiratory alkalosis– Compute for compensation

• decrease in HCO3 = 2/10 x change in pCO2 (acute)• decrease in HCO3 = 5/10 x change in pCO2 (chronic)

– Expected HCO3: (acute)• Expected HCO3 = 22.04• Expected HCO3 is lower than actual HCO3 (25.50). Therefore, patient

has concomitant metabolic alkalosis

ABG:pH – 7.48pCO2 – 30.2pO2 – 49.00HCO3 – 25.50BE – -1.8O2 sats – 78%6 LPM

Case 4:

• Interpret ABG– Check for pO2

• Correlate with FiO2– LPM to FiO2 at nasal cannula: 4 x LPM + 20

• Hypoxemia (pO2 at FiO2 21%)– Target pO2 = 90/0.21 x FiO2– Target pO2 = 189– hypoxemia

ABG:pH – 7.48pCO2 – 30.2pO2 – 49.00HCO3 – 25.50BE – -1.8O2 sats – 78%6 LPM

Case 4:

• Final ABG interpretation:– Respiratory alkalosis with concomitant metabolic

alkalosis

Case 4:• Conditions associated with respiratory alkalosis:

– High altitude– V/Q mismatch– Severe anemia– Infection – trauma– Hepatic failure– Gram negative septicemia– Interstitial lung disease– Pneumonia– Pulmonary embolism– Pulmonary edema