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Acid-Base Physiology 2012. Objectives  Understand normal mechanisms and regulation of acid-base balance  Interpret blood gases  Understand the effects.

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Presentation on theme: "Acid-Base Physiology 2012. Objectives  Understand normal mechanisms and regulation of acid-base balance  Interpret blood gases  Understand the effects."— Presentation transcript:

1 Acid-Base Physiology 2012

2 Objectives  Understand normal mechanisms and regulation of acid-base balance  Interpret blood gases  Understand the effects of acidosis and alkalosis  Evaluate and manage acidosis and alkalosis

3 Normal Physiology  Acid-base balance maintained by normal pulmonary excretion of CO2 and renal excretion of acid  Organic buffers: HCO3-, HPO4, protein anions, carbonate  90% of bicarb is reabsorbed by kidney  Renal excretion – H+ combines with urinary titratable acids (phosphates) or ammonia to form ammonium  Henderson-Hasselbach equation: pH = log (HCO3 ÷ (0.03 x PCO2))

4 Compensatory Mechanisms  pH is determined by ratio of HCO3 and PCO2  Body responds to changes in pH by attempting to normalize the pH  Buffering  Respiratory – alterations in paCO2  Renal – alterations in bicarbonate excretion

5 Compensatory Mechanisms  Compensated metabolic acidosis:  1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3  Compensated metabolic alkalosis:  0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3  Compensated respiratory acidosis:  Acute- 1 meq/L  for every 10 mmHg  in pCO2  Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2  Compensated respiratory alkalosis:  Acute- 2 meq/L  for every 10 mmHg  in pCO2  Chronic- 4 meq/L  for every 10 mmHg  in pCO2

6 Blood Gas Interpretation  General guidelines:  1) Is it acidosis or alkalosis?  Acidosis – pH < 7.38  Alkalosis – pH > 7.42  2) Is it primary respiratory or metabolic?  Evaluate paCO2 and bicarbonate  3) Is there compensation?  Calculations from previous slides

7 Blood Gas Interpretation  4) If respiratory disturbance, is it acute or chronic?  Respiratory acidosis:  Acute  decrease in pH = 0.08 x (paCO2-40)/10  Chronic  decrease in pH = 0.03 x (paCO2-40)/10  Respiratory alkalosis  Acute  increase in pH = 0.08 x (40-paCO2)/10  Chronic  increase in pH = 0.03 x (40-paCO2)/10  5) If metabolic disturbance, is there an anion gap?  Check serum Na, Cl, HCO3

8 Case #1  pH 7.16, pCO2 70, HCO3 24  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical Scenario?

9 Case #1  pH 7.16, pCO2 70, HCO3 24  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical Scenario?

10 Case #1  pH 7.16, pCO2 70, HCO3 24  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical Scenario?

11 Case #1  pH 7.16, pCO2 70, HCO3 24  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  No … likely acute  Clinical Scenario?

12 Case #1  pH 7.16, pCO2 70, HCO3 24  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical Scenario?  2 yo receiving deep sedation by the adult ED attending who gives him 4 mg morphine, respiratory rate is 6

13 Acute Respiratory Acidosis  Respiratory pathophysiology – airway obstruction, severe pneumonia, chest trauma, pneumothorax  Acute drug intoxication (narcotics, sedatives)  Residual neuromuscular blockade  CNS disease (head trauma, decreased consciousness)  Bicarbonate is often normal, kidneys have not had time to compensate

14 Case #2  pH 7.6, pCO2 23, HCO3 22  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

15 Case #2  pH 7.6, pCO2 23, HCO3 22  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

16 Case #2  pH 7.6, pCO2 23, HCO3 22  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

17 Case #2  pH 7.6, pCO2 23, HCO3 22  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  No … likely acute  Clinical scenario?

18 Case #2  pH 7.6, pCO2 23, HCO3 22  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?  4 mo mechanically ventilated pt who was bagged during transport to CT scan by an overeager intern

19 Respiratory Alkalosis  Pain, anxiety  Hypoxemia  Interstitial lung disease  Severe congestive heart failure (pulmonary edema)  Pulmonary emboli  Drugs – salicylates, methylxanthines, nicotine  Sepsis, fever  Hepatic failure – encephalopathy  Pregnancy  Thyrotoxicosis  CNS hemorrhage  Overagressive mechanical ventilation

20 Case #3  pH 7.29, pCO2 26, HCO3 12  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

21 Case #3  pH 7.29, pCO2 26, HCO3 12  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

22 Case #3  pH 7.29, pCO2 26, HCO3 12  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

23 Compensatory Mechanisms  Compensated metabolic acidosis:  1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3  Compensated metabolic alkalosis:  0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3  Compensated respiratory acidosis:  Acute- 1 meq/L  for every 10 mmHg  in pCO2  Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2  Compensated respiratory alkalosis:  Acute- 2 meq/L  for every 10 mmHg  in pCO2  Chronic- 4 meq/L  for every 10 mmHg  in pCO2

24 Case #3  pH 7.29, pCO2 26, HCO3 12  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Yes … 1.2 mmHg decrease in pCO2 for every 1 meq/L decrease in HCO3  Clinical scenario?

25 Case #3  pH 7.29, pCO2 26, HCO3 12  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?  10 yo dev delayed pt admitted with diarrhea, med list reveals mom has been giving Miralax every 4 hours

26 Metabolic Acidosis  Anion gap

27 Metabolic Acidosis  Anion gap  Lactic acidosis  DKA  Toxic ingestions (salicylates, ethylene glycol, ethanol, isopropyl alcohol, paraldehyde, methanol)  Renal failure – uremia

28 Metabolic Acidosis  Nonanion gap

29 Metabolic Acidosis  Nonanion gap  RTA  Diarrhea  Hypoaldosteronism  Potassium-sparing diuretics  Pancreatic loss of bicarbonate  Ureteral diversion  Carbonic anhydrase inhibitors  Acid administration (ArgCl, NaCl)

30 Case #4  pH 7.47, pCO2 46, HCO3 32  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

31 Case #4  pH 7.47, pCO2 46, HCO3 32  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

32 Case #4  pH 7.47, pCO2 46, HCO3 32  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

33 Compensatory Mechanisms  Compensated metabolic acidosis:  1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3  Compensated metabolic alkalosis:  0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3  Compensated respiratory acidosis:  Acute- 1 meq/L  for every 10 mmHg  in pCO2  Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2  Compensated respiratory alkalosis:  Acute- 2 meq/L  for every 10 mmHg  in pCO2  Chronic- 4 meq/L  for every 10 mmHg  in pCO2

34 Case #4  pH 7.47, pCO2 46, HCO3 32  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Yes … 0.7 mmHg increase in pCO2 for every 1 meq/L increase in HCO3  Clinical scenario?

35 Case #4  pH 7.47, pCO2 46, HCO3 32  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?  5 yo s/p appendectomy with NG tube left to suction on 7CH for 5 days

36 Metabolic Alkalosis  Chloride-responsive (urine Cl < 10 meq/L)  Gastric acid loss (vomiting, NG suction)  Contraction alkalosis (often due to loop or thiazide diuretics)  Posthypercapnia syndrome

37 Metabolic Alkalosis  Chloride-resistant  Mineralocorticoid excess  Renal chloride wasting (Bartter syndrome)  Exogenous alkali (milk-alkali syndrome, massive blood transfusion)  Hypokalemia

38 Case #5  pH 7.30, pCO2 89, HCO3 42  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clincial scenario?

39 Case #5  pH 7.30, pCO2 89, HCO3 42  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clincial scenario?

40 Case #5  pH 7.30, pCO2 89, HCO3 42  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clincial scenario?

41 Compensatory Mechanisms  Compensated metabolic acidosis:  1.2 mmHg  in pCO2 for every 1 meq/L  in HCO3  Compensated metabolic alkalosis:  0.7 mmHg  in pCO2 for every 1 meq/L  in HCO3  Compensated respiratory acidosis:  Acute- 1 meq/L  for every 10 mmHg  in pCO2  Chronic- 3.5 meq/L  for every 10 mmHg  in pCO2  Compensated respiratory alkalosis:  Acute- 2 meq/L  for every 10 mmHg  in pCO2  Chronic- 4 meq/L  for every 10 mmHg  in pCO2

42 Case #5  pH 7.30, pCO2 89, HCO3 42  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Yes … 3.5 meq/L increase in HCO3 for every 10 mmHg increase in CO2  Clincial scenario?

43 Case #5  pH 7.30, pCO2 89, HCO3 42  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clincial scenario?  35 yo CF patient on the Peds floor with end-stage lung disease

44 Chronic Respiratory Acidosis  Chronic lung diseases (BPD, CF)  Neuromuscular disorders  Severe restrictive lung disease  Severe obesity

45 Case #6  pH 6.84, pCO2 82, HCO3 14  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

46 Case #6  pH 6.84, pCO2 82, HCO3 14  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

47 Blood Gas Interpretation  4) If respiratory acidosis or alkalosis, is it acute or chronic?  Respiratory acidosis:  Acute  decrease in pH = 0.08 x (paCO2-40)/10  Chronic  decrease in pH = 0.03 x (paCO2-40)/10  Respiratory alkalosis  Acute  increase in pH = 0.08 x (40-paCO2)/10  Chronic  increase in pH = 0.03 x (40-paCO2)/10

48 Case #6  pH 6.84, pCO2 82, HCO3 14  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

49 Case #6  pH 6.84, pCO2 82, HCO3 14  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  No …  Combined acidosis  Clinical scenario?

50 Case #6  pH 6.84, pCO2 82, HCO3 14  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?  2 mo found down at home, hypoperfusion leading to lactic acidosis, hypoventilation leading to respiratory acidosis

51 Case # 7  pH 7.46, pCO2 24, HCO3 16  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

52 Case # 7  pH 7.46, pCO2 24, HCO3 16  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

53 Case # 7  pH 7.46, pCO2 24, HCO3 16  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

54 Case # 7  pH 7.46, pCO2 24, HCO3 16  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Well … 2 meq/L decrease in HCO3 for every 10 mmHg decrease in CO2  Combined respiratory alkalosis & metabolic acidosis  Clinical scenario?

55 Case # 7  pH 7.46, pCO2 24, HCO3 16  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?  Anxious 5 yo who is hyperventilating and has a history of RTA

56 Case #8  pH 7.45, pCO2 54, HCO3 36  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

57 Case #8  pH 7.45, pCO2 54, HCO3 36  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

58 Case #8  pH 7.45, pCO2 54, HCO3 36  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?

59 Case #8  pH 7.45, pCO2 54, HCO3 36  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  No … 0.7 mmHg increase for every 1 meq/L predicts pCO2 of 48  Combined metabolic alkalosis and respiratory acidosis  Clinical scenario?

60 Case #8  pH 7.45, pCO2 54, HCO3 36  Acidosis or alkalosis?  Respiratory or metabolic?  Compensated?  Clinical scenario?  1 yo with vomiting for 3 days who presents to the ED with lethargy and decreased arousal, hypoventilating

61 Physiologic Effects of Acidosis  Shifts the oxygen-hemoglobin dissociation curve to the right  Decreased affinity for O2

62 Physiologic Effects of Acidosis  Pulmonary effects – vasoconstriction decreases pulmonary blood flow  Cardiac effects – depressed contractility  Neurologic effects – increased cerebral blood flow, increased ICP  Extracellular shift of K+  hyperkalemia  Sympathetic overactivity, resistance to catecholamines

63 Physiologic Effects of Alkalosis  Shifts the oxygen-hemoglobin dissociation curve to the left  Stronger bond between Hb and O2  Decreased O2 delivery to tissues

64 Physiologic Effects of Alkalosis  Cardiac arrhythmias  Lungs – vasodilation increases pulmonary blood flow  Neurologic effects – headache, seizures, altered mental status  Decreased cerebral blood flow from vasoconstriction  Decreased levels of ionized Ca++  Intracellular shift of potassium  severe hypokalemia

65 Management - Respiratory Acidosis  Treat the underlying disorder  Assist or increase ventilation  Secure airway if necessary  Increase tidal volume or respiratory rate if mechanically ventilated  Noninvasive ventilation  Bronchodilators  Reverse sedative medications

66 Management – Respiratory Alkalosis  Treat the underlying disorder  Decrease ventilation  Decrease respiratory rate  Decrease tidal volume  Sedation and pain control  Reassurance for anxious patients

67 Management - Metabolic Disorders  Acidosis  Treat the underlying disorder  Consider bicarb administration depending on etiology  Dialysis in the setting of renal failure  Alkalosis  Treat the underlying disorder  Chloride-responsive: replete chloride (NaCl, KCl, ArgCl)  Carbonic-anydrase inhibitors if diuresis needed

68 Conclusion  The body has compensatory mechanisms to maintain acid-base balance.  Blood gases should be interpreted in a systematic way to determine the etiology of the acid-base disturbance.  Acidosis causes pulmonary vasoconstriction, cardiac depression, arterial vasodilation, & decreased O2 affinity.  Alkalosis causes pulmonary vasodilation, arterial vasoconstriction, & increased O2 affinity.  Management of acid-base disorders primarily involves treatment of the underlying disorder.

69 Everyone always has slides of their kids …

70 QUESTIONS?

71 References    Morganroth ML. Six steps to acid-base analysis: clinical applications. The Journal of Critical Illness. 1990;5:  Morganroth ML. An analytic approach to diagnosing acid- base disorders. The Journal of Critical Illness. 1990;5:


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