This document provides an overview of blood gas interpretation and acid-base physiology: 1) It discusses factors that can affect blood gas results like improper sampling technique and gives normal ranges for pH, PCO2, and HCO3. 2) Key concepts in acid-base physiology like the Henderson-Hasselbalch equation, bicarbonate buffering system, and renal regulation of bicarbonate are summarized. 3) The document outlines the pathophysiology and clinical effects of various acid-base disturbances like respiratory acidosis and metabolic alkalosis.

1. Blood Gas Interpretation 2005/8/25

2. Before beginning… Allen’s test for radial and ulnar artery Common errors of arterial blood sampling Air in sample: PCO 2 ↓, pH↑, PO 2 ↨ Venous mixture: PCO 2 ↑, pH↓, PO 2 ↓ Excess anticoagulant (dilution): PCO 2 ↓, pH↑, PO 2 ↨ Metabolic effects: PCO 2 ↑, pH↓, PO 2 ↓ Simultaneous electrolytes panel

3. Acid Base Physiology The Law of Mass Action [A] + [B]  [C] + [D] K 1 /K 2 = [C][D]/[A][B] Dissociation constant for an acid Ka = [H + ][A - ]/[HA] K 1 K 2

5. Henderson-Hasselbalch Equation CO 2 + H 2 O  H 2 CO 3  H + + HCO3 - [H + ] = K x [CO 2 ]/[HCO 3 - ] = 24 PCO 2 /[HCO 3 - ] pH = 6.1 + log ([HCO3 - ]/0.0301xPCO 2 )

6. Normal Range pH = 7.35-7.45 PCO 2 = 35-45 mmHg (40 mmHg) HCO 3 - = 22-26 mEq/L (24 mEq/L)

7. Bicarbonate Buffering System CO 2 + H 2 O  H 2 CO 3  H + + HCO3 - Oral intake Kidney Metabolism Oral intake Kidney Stomach Metabolism Lung

8. Acid Production and Elimination Reaction Products Elimination Glucose H + + HCO 3 - Fat H + + HCO 3 - Glucose H + + lactate Cysteine H + + sulfate Phosphoproteins H + + phosphate Anaerobic +O2 +O2 +O2 +O2 Lungs 24,000 mEq/day Volatile acid Kidneys 50-100 mEq/day Non-volatile acid

9. Determinants of CO 2 in the alveolus V A = V E – V D = V T x f (1- V D /V T ) P A CO 2 = k x (VCO 2 /V A ) Physiologic dead space = anatomic dead space + alveolar dead space

10. PaCO 2 PaCO 2 > 40 mmHg, MV = 2x normal PaCO 2 > 80 mmHg  CO2 nacrosis

11. Renal Regulation of Bicarbonate “ Reabsorption“ of filtered HCO 3 - (4000 mmol/day) Formation of titratable acid (4000 mmol/day H + ) Excretion of NH4+ in the urine 80-90% of HCO 3 - : reabsorbed in the proximal tubule Distal tubule: reabsorption of remained bicarbonate and secretion of hydrogen ion

12. Proximal Renal Tubule

13. Distal Renal Tubule

14. Distal Tubule – NH 4 + excretion

15. Acid Base Disturbance Metabolic acidosis: HCO 3 - ↓ Metabolic alkalosis: HCO 3 - ↑ Respiratory acidosis: PCO 2 ↑ Respiratory alkalosis: PCO 2 ↓ Simple Primary Secondary mixed

16. Metabolic Acidosis Indogenous acid production (lactic acidosis, ketoacidosis) Indogenous acid accumulation (renal failure) Loss of bicarbonate (diarrhea) High anion gap Normal (hyperchloremic )

17. Pathophysiologic Effect of Metabolic Acidosis Kussmaul respiration Intrinsic cardiac contractility ↓, normal inotropic function Peripheral vasodilatation Central vasoconstriction  pulmonary edema Depressed CNS function Glucose intolerance

18. Anion Gap AG = Na + - (Cl - + HCO3 - ) Unmeasured anions in plasma (normally 10 to 12 mmol/L) Anionic proteins, phosphate, sulfate, and organic anions Correction: if albumin < 4 Albumin ↓ 1  AG ↓ 2.5

19. Anion Gap Increase Increased unmeasured anions Decreased unmeasured cations (Ca ++ , K + , Mg ++ ) Increase in anionic albumin Decrease Increase in unmeasured cations Addition of abnormal cations Reduction in albumin concentration Decrease in the effective anionic charge on albumin by acidosis Hyperviscosity and severe hyperlipidemia ( underestimation of sodium and chloride concentration)

21. Renal failure (acute and chronic) Starvation Salicylates Alcoholic Methanol Diabetic Ethylene glycol Ketoacidosis Toxins Lactic acidosis Causes of High-Anion-Gap Metabolic Acidosis

22. Metabolic Alkalosis Net gain of [HCO 3 - ] Loss of nonvolatile acid (usually HCl by vomiting) from the extracellular fluid Kidneys fail to compensate by excreting HCO 3 - (volume contraction, a low GFR, or depletion of Cl - or K + )

23. Respiratory Acidosis Severe pulmonary disease Respiratory muscle fatigue Abnormal ventilatory control Acute vs. Chronic (> 24 hrs)

24. Respiratory Acidosis Acute: anxiety, dyspnea, confusion, psychosis, and hallucinations and coma Chronic: sleep disturbances, loss of memory, daytime somnolence, personality changes, impairment of coordination, and motor disturbances such as tremor, myoclonic jerks, and asterixis Headache: vasocontriction

26. Respiratory Alkalosis Strong ventilatory stimulus with alveolar hyperventilation Consuming HCO 3 - > 2-6 hrs: renal compensation (decrease NH4+/acid excretion and bicarbonate re-absorption)

27. Respiratory Alkalosis Reduced cerebral blood flow dizziness, mental confusion, and seizures Minimal cardiovascular effect in normal health Cardiac output and blood pressure may fall in mechanically ventilated patients Bohr effect: left shift of hemoglobin-O 2 dissociation curve  tissue hypoxia (arrhythmia) intracellular shifts of Na + , K + , and PO 4 - and reduces free [Ca 2+ ]

28. Stepwise Approach Do comprehensive history taking and physical examination Order simultaneous arterial blood gas measurement and chemistry profiles Assess accuracy of data Direction of pH: always indicates the primary disturbance Calculate the expected compensation Second or third disorders

29. N Respiratory alkalosis Metabolic alkalsosis Metabolic acidosis Respiratory acidosis 7.4 7.6 7.2 pH 30 40 50 PCO 2 (mmHg) Determination of primary acid-base disorders

31. Compensatory Mechanisms Respiratory compensation Complete within 24 hrs Metabolic compensation Complete within several days Both the respiratory or renal compensation almost never over-compensates

32. [HCO3-] will ↑ 4 mmol/L per 10-mmHg ↑ in PaCO2 Chronic [HCO3-] will ↑ 1 mmol/L per 10-mmHg ↑ in PaCO2 Acute Respiratory acidosis [HCO3-] will ↓ 4 mmol/L per 10-mmHg ↓ in PaCO2 Chronic [HCO3-] will ↓ 2 mmol/L per 10-mmHg ↓ in PaCO2 Acute Respiratory alkalosis PaCO2 = [HCO3-] + 15 PaCO2 will ↑ 6 mmHg per 10-mmol/L ↑ in [HCO3-] or PaCO2 will ↑ 0.75 mmHg per mmol/L ↑ in [HCO3-] or Metabolic alkalosis PaCO2 = [HCO3-] + 15 PaCO2 will ↓ 1.25 mmHg per mmol/L ↓ in [HCO3-] or PaCO2 = (1.5x HCO3-) + 8 or Metabolic acidosis Prediction of Compensation Disorder Prediction of Compensatory Responses on Simple Acid-Base Disturbances

35. Mixed Acid Base Disorders Metabolic alkalosis Metabolic acidosis Respiratory alkalosis Respiratory acidosis    Metabolic alkalosis    Metabolic acidosis   Respiratory alkalosis   Respiratory acidosis Secondary Primary

36. Oxygenation Poor diffusion across alveolar membrane Small pressure gradient between P A O 2 and PaO 2 Large alveolar area is required for gas transfer Hemoglobin carries the majority of oxygen in the blood

38. Oxygenation Ventilation and alveolar disease Ventilation ↓  P A O2 ↓  PaO2 ↓, combined PCO 2 ↑ Alveolar disease Reduced alveolar area Thickened alveolar membrane V/Q mismatch Shunt

39. Alveolar-arterial Oxygen Gradient P A O 2 = F iO2 (P B -P H2O ) – PCO 2 /R = 0.21(760-47) – 40/0.8 = 100 R: respiratory quotient P(A-a)O 2 = P A O 2 – PaO 2 (= Age x 0.4)

41. Oxygen Content and Saturation O 2 content = 1.34 x Hb x Saturation + 0.0031xPO 2

42. Pulse Oximeters Percentage of oxygenated hemoglobin in blood Absorption of light in the red and infra-red spectra Continuous monitor Accurate (  3%) at high saturation, less below 80% Insensitive around the normal PO 2 COHb and MetHb

43. Clinical Example 1 72 y/o male, COPD with acute exacerbation Under O 2 2L/min pH 7.44, PCO 2 54, PO 2 60, HCO 3 36 Metabolic alkalosis with respiratory compensation Mixed respiratory acidosis

44. Clinical Example 2 30 y/o male, sudden onset dyspnea Room air 7.33/24/111/12 Metabolic acidosis Respiratory compensation Normal A-a O2 gradient O2 ↑: hyperventilation

45. Clinical Example 3 70 y/o male, acute hemoptysis and dyspnea Room air 7.50/31/88/24 Respiratory alkalosis Not been renal compensated yet Normal PO2, but A-a O2 gradient ↑

46. Clinical Example 4 18 y/o female, chest tightness and dyspnea for 4 hrs RR 28/min, distressed, widespread wheezing O2 mask 6L/min 7.31/49/115/26 Respiratory acidosis Normal bicarbonate  acute May have problems with oxygenation

47. Clinical Example 5 37 y/o female, mild asthma history Wheezes for 3 weeks, increasing chest tightness and dyspnea for 24 hrs, call for ambulance with Oxygen use RR 18/min, anxious and distressed Room air 7.37/43/97/27 Normal? r/o CO 2 retention Low A-a O 2 : Oxygen use in the ambulance

48. Clinical Example 6 19 y/o male, Duchenne muscular dystrophy on wheelchair for 7 yrs No previous respiratory problems but frequent UTI Room air 7.21/81/44/36 Respiratory acidosis Metabolic compensation Normal A-a O2  pure ventilatory failure

49. Clinical Example 7 57 y/o male, smoker, one week URI then 36 hrs productive cough, fever and dyspnea RR 36/min, distressed, CXR: RLL pneumonia 7.33/27/51/22, 2L/min 7.34/32/58/24, 10L/min mask Early metabolic acidosis Severe hypoxemic respiratory failure Intra-pulmonary shunting

50. Thank you for your attention