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Acid base balance 341

Learn to interpret and diagnose acid-base disorders with this tutorial, covering normal values, differential diagnoses, and steps in analysis. Case studies included.

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Acid base balance 341

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  1. Acid base balance341 Mohammed Al-Ghonaim, MBBS,FRCPC,FACP

  2. Objective • At the end of this tutorial you will be able to: • State the normal value for PH,PCO2,HCO3 • Interpret basic acid base disturbance • List common differential diagnosis for different acid base disorder

  3. Steps in Acid-Base Analysis • Step 1: Acidemic or Alkalemic? • Step 2: Is the primary disturbance respiratory or metabolic? • Step 3: Is there appropriate compensation for the primary disorder? • Metabolic acidosis: PCO2 = [1.5 x (serum HCO3)] + 8 (±2) • Metabolic alkalosis: ↑PCO2 = 0.6 x ↑HCO3 (±2) • Respiratory acidosis: ↑PCO2 10, ↑ HCO3 by 1 (acute) or 3-3.5 (chronic) • Respiratory alkalosis: ↓PCO2 10, ↓ HCO3 by 2 (acute) or 4-5 (chronic)

  4. Steps in Acid-Base Analysis • Step 4: Is the respiratory disturbance acute or chronic? • Step 5: For a metabolic acidosis, is there an increased anion gap? • Anion gap = [Sodium] - ([Chloride] + [Bicarbonate]) OrAG = [Na+] - ([Cl-] + [HCO3-]). • Normal AG 8-16 • Serum Osmolality = (2 x (Na + K)) + (BUN ) + (glucose )

  5. Step 1: Acidemic or Alkalemic? • The pH of the arterial blood gas measurement identifies the disorder as alkalemic or acidemic. • Normal arterial blood pH = 7.35 – 7.45 • Acidemic: pH < 7.35 • Alkalemic: pH > 7.45 • Bicarbonate - (HCO3): 22 - 26 mEq/liter • Partial pressure of carbon dioxide (PaCO2): 35 - 45 mmHg

  6. Step 2: Is the primary disturbance respiratory or metabolic? • To determine whether the disturbance affects primarily • The arterial PaCO2 or • The serum HCO3-. • Respiratory disturbances alter the arterial PaCO2 (normal value 35-45) • Metabolic disturbances alter the serum HCO3- (normal value 22-26)

  7. primary disturbance

  8. Case study: (1) • A 22 year-old woman presents with 4 hours of numbness in both hands and SOB • ABG: pH 7.48, pCO2 30 mmHg, pO2 86 mmHg • Na 140, Cl 110, HCO3 22 • Step 1: Alkalosis • Step 2: Respiratory • Step 3: Acute. Drop in the pCO2 by 10 corresponds to a drop in the HCO3 by 2 if acute

  9. Respiratory Alkalosis • Pain • Drugs • Sepsis • Fever • Thyrotoxicosis • Pregnancy • Overaggressive mechanical ventilation • Hepatic failure • Anxiety • Hypoxemia • Restrictive lung disease • Severe congestive heart failure • Pulmonary emboli

  10. Respiratory acidosis • Is it acute or chronic? • Note that the PH is abnormal • Note the HCO2 is with in normal • Remember: • Acute respiratory acidosis: • HCO3- increase by 1 mEq/l for every 10 mmHg increase in PaCO2 • Chronic respiratory acidosis: • HCO3- increase by 3-3.5 mEq/l for every 10 mmHg increase in PaCO2

  11. Respiratory acidosis • Primary mechanism: Hypoventilation • CNS • Peripheral nerve • Neuro muscular junction • Chest wall • Bronchial tree

  12. Case study: (2) • pH =7.2, pCO2 = 60, HCO2 = 24. • What it is the primary problem? Compensation? • Differential diagnosis? • Treatment ?

  13. Case study: (3) • What do you expect the ABG in the following patients to be: • 24 years old male with acute SOB, and wheezes for 2days. • Past hx: Bronchial asthma • 67 years old women, HTN,DM II, COPD presenting with cough and SOB

  14. Case study: (4) • pH: 7.25 • [HCO3-]: 20 mEq/L • PaCO2: 52 mmHg • What it is the primary problem? Compensation? • Differential diagnosis?

  15. Case study: (5) • pH: 7.32 • [HCO3-]: 19 mEq/L • PaCO2: 55 mmHg • What it is the primary problem? Compensation? • Differential diagnosis? • What other investigation you want to do?

  16. Step 4: Is the respiratory disturbance acute or chronic? • Acute respiratory acidosis: • HCO3- increase by 1 mEq/l for every 10 mmHg increase in PaCO2 • Chronic respiratory acidosis: • HCO3- increase by 3-3.5 mEq/l for every 10 mmHg increase in PaCO2 • Acute respiratory alkalosis: • HCO3- decrease by 2 mEq/l for every 10 mmHg decrease in PaCO2 • Chronic respiratory alkalosis: • HCO3- decrease by 4-5 mEq/l for every 10 mmHg decrease in PaCO2

  17. Case study: (6) • A 70 year-old smoker presents with an acute onset of shortness of breath. • ABG: pH 7.30, pCO2 = 60 mmHg, pO2 60 mmHg • Na 135, Cl 100, HCO3 30 • Step 1: Acidosis • Step 2: Respiratory • Step 3: Acute on chronic. pCO2 increased by 20, therefore the HCO3 should increase by 2 if acute and 8 if chronic. Because the HCO3 increased from 24 to 30 (6), an acute on chronic respiratory acidosis is present.

  18. Step 5: For a metabolic acidosis, is there an increased anion gap? • Anion gap = [Sodium] - ([Chloride] + [Bicarbonate]) OrAG = [Na+] - ([Cl-] + [HCO3-]). • Normal AG 8-16 • Serum Osmolality = (2 x (Na + K)) + (BUN ) + (glucose )

  19. Case study: (7) • 32 year old man with depression and alcohol abuse presents with altered mental status. • ABG: pH 6.9, pCO2 29, pO2 100 • Na 140, Cl 101, HCO3 5 • Step 1: Acidosis • Step 2: Metabolic • Step 3: pCO2 = 1.5(HCO3) + 8 = 15, but the patient’s pCO2 is higher than 15. Therefore, a respiratory acidosis is also present, possibly secondary to CNS depression. • Step 4: AG = 140 – (101 + 5) = 34

  20. Increased anion gap metabolic acidosis • Methanol other alcohols, and ethylene glycol intoxicationUremia (renal failure)Lactic acidosisEthanol Paraldehyde and other drugsAspirin Ketones (starvation, alcoholic and diabetic ketoacidosis)

  21. Case study: (8) • A 68 year old man who recently took antibiotics for a skin infection presents with 10 episodes of watery diarrhea per day for the last 5 days. • ABG: pH 7.34, pCO2 34, pO2 80 • Na 135, Cl 108, HCO3 18 • Step 1: Acidosis • Step 2: Metabolic • Step 3: pCO2 = 1.5(HCO3) + 8 = 35 • Step 4: AG = 135 – (108 + 18) = 9 • NAGMA (non anion gap metabolic acidosis)

  22. Is the respiratory system compensating adequately for a metabolic disturbance Metabolic acidosis: PCO2 decreases by 1 mmHg for every 1 mEq/l decrease in HCO3 OR PCO2 = [1.5 x (serum HCO3)] + 8 (±2) Metabolic alkalosis: PCO2 increases by 0.6 mmHg for every 1 mEq/l increases in HCO3 OR ↑PCO2 = 0.6 x ↑HCO3 (±2)

  23. Case study: (9) • pH =7.1, pCO2 = 42, HCO3- = 12.

  24. Case study: (10) • HCO3= 39; pCO2 = 40; pH = 7.6; K = 3.0. The patient has been vomiting. • pH = 7.6 and K = 3.0

  25. Case study: (11) • pH =7.58, pCO2 = 27, HCO3- = 24. • What is the primary cause of the condition? • Hyperventilation - excess loss of CO2 gas

  26. Case study: (12) • 56 yo M with Hx of COPD is admitted with 1-wk Hx of dyspnea, productive cough and diarrhea (Na) 125, (Cl) 103 , (BUN) 42, (Glucose) 100, (K) 3.5, (HCO3-) 10, (Creat) 1.4 • ABG 7.14 pCO2 30 pO2 50 • What is the predominant acid base disorder ? Acidosis

  27. Case study: (12) continue • What pCO2 is expected with normal respiratory compensation ? 40 – (1.2 * (24-10)) = 23.2, this is not full compensation b/c pCO2 is 30 – indicates an underlying primary respiratory acidosis, suggested by the Hx of COPD, dyspnea, and productive cough (lungs not able to appropriately compensate) • What is the Anion Gap ? 125 – (103+10) = 12 – normal AG  etiology is either diarrhea or RTA – most likely diarrhea b/c of the history

  28. Case study: (13) • 32 y/o male present w/ 2d Hx of intractable vomiting. ; pH 7.51, pCO2 41 Na132, Cl 90 32 K3.4 HCo2= 33 creatinine1.6  • What is the predominant acid-base disorder? Alkalosis

  29. Etiologies of Metabolic Alkalosis

  30. Case study: (13) contunue • What pCO2 is expected w/ normal respiratory compensation? = 40 + (32 – 24) * (~ 0.6  0.7) = 44.8  45.6 mmHg; since the measured pCO2 < 44.8  45.6, there is also a primary respiratory alkalosis (inappropriate hyperventilation)  • Tx: Isotonic saline to correct for volume depletion –

  31. Case study: (14) • A 58- year old man presents to the Emergency Department with abdominal pain and hypotension. Investigation reveal the following: • Na 140 K 4 Cl 90 HCO3 = 5 PH 6.8 PCO2 36 PO2 7 • Analyze the acid-base disorder(s) seen in the patient.

  32. Case study: (14) continue • Acidemia • Anion Gap = 45

  33. Case study: (14) • A 42-year-old man to the ER after he was found unconscious. • O/E • 120/80, HR110/min, RR 28/min, and Temp 37°C. • Unresponsive • Pupils minimally reactive to light • Bibasilar crackles • Deep tendon reflexes were brisk and symmetric

  34. Case study: (14) continue • Lab: • pH 7.1 • PaCO2, 35 mm Hg • PaO2, 90 mm Hg at room air • Na 145 mEq/L; K 5 mEq/L; Cl 97 mEq/L; HCO3 12 mEq/L • blood urea nitrogen, 10 mmol/l; creatinine, 140 umol/l; and glucose, 6.0 mmol/l.

  35. Case study: (14) continue • Acidemic the change in PaCO2 equaled about 14 mm Hg (ie, 1.2 x 12). • Therefore, the expected PaCO2 was 26 mm Hg (ie, 40 - 14 ). • the measured PaCO2 of 35 mm Hg was much higher than expected

  36. Case 11 continue • Concomitant respiratory acidosis • AG= 145 – (97+12) = 36 • Measured osmolality= 350 • Calculated osmolality = (2 x 145)+6+10= 306 • Osmole Gap=44 • Causes of increased Gap: • Methanol • Ethylene glycol • Ethanol • Isopropyl alcohol

  37. Case study: (14) continue • lactate, 1 mEq/L • ketones, negative • Salicylate, negative • Urinalysis showed calcium oxalate crystals • +ve ethylene glycol • Final Dx • High AG Met acidosis • Ethylene Glycol • Respiratory Acidosis • Decreased level of consciousness • Metabolic alkalosis due to vomiting

  38. Summary • First, does the patient have an acidosis or an alkalosis • Look at the pH • Second, what is the primary problem – metabolic or respiratory • Look at the pCO2 • If the pCO2 change is in the opposite direction of the pH change, the primary problem is respiratory

  39. Summary • Third, is there any compensation by the patient - do the calculations • For a primary respiratory problem, is the pH change completely accounted for by the change in pCO2 • if yes, then there is no metabolic compensation • if not, then there is either partial compensation or concomitant metabolic problem

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