See Marieb & Hoehn 9th ed., Chapter 26 HESC310 12/4/2018 Acid-Base Analysis W. Rose See Marieb & Hoehn 9th ed., Chapter 26 Axial Skeleton

Normal Arterial Blood Gas HESC310 12/4/2018 Normal Arterial Blood Gas pH = 7.35-7.45 PaCO2 = 35-45 mmHg HCO3- = 22-26 mEq/L PaO2 = 80-100 mmHg We will skip base excess and anion gap Axial Skeleton

Arterial pH<7.35: Acidosis Arterial pH >7.45: Alkalosis HESC310 12/4/2018 Abnormal Blood Gases Arterial pH<7.35: Acidosis Arterial pH >7.45: Alkalosis Axial Skeleton

Arterial pH < 7.35: Acidosis HESC310 12/4/2018 Arterial pH < 7.35: Acidosis Is it respiratory, metabolic, or both? Respiratory acidosis = acidosis with high PaCO2. pH<7.35, PaCO2>45 mmHg PaO2 likely to be low (<80 mmHg) Metabolic acidosis = acidosis with low HCO3. pH<7.35, HCO3<22 mEq/L Axial Skeleton

Arterial pH > 7.45: Alkalosis HESC310 12/4/2018 Arterial pH > 7.45: Alkalosis Is it respiratory, metabolic, or both? Respiratory alkalosis = alkalosis with low PaCO2. pH>7.45, PaCO2<35 mmHg Look for hyperventilation PaO2 often high normal (95-100 mmHg) Metabolic alkalosis = alkalosis with high HCO3. pH>7.45, HCO3>26 mEq/L Gain of acid or loss of base Axial Skeleton

Respiratory Alkalosis HESC310 12/4/2018 pH < 7.35: Acidosis pH > 7.45: Alkalosis Respiratory Acidosis PaCO2 > 45 mmHg Respiratory Alkalosis PaCO2 < 35 mmHg Metabolic Acidosis HCO3<22 mEq/L Metabolic Alkalosis HCO3>26 mEq/L Axial Skeleton

Arterial blood gas diagram

HESC310 12/4/2018 Compensation There is immediate but limited compensation from the bicarbonate buffer system of blood. Lungs can compensate (in minutes) for a metabolic disturbance. Increased breathing to compensate for metabolic acidosis is a more robust and reliable response than decreased breathing in response to metabolic alkalosis. Kidneys can compensate (in hours to days) for a respiratory disturbance. Renal compensation for respiratory acidosis is slow but may be nearly complete after 3-4 days; renal compensation for respiratory alkalosis is slow and incomplete. Axial Skeleton

Compensation Abnormally low PaCO2 (<35 mmHg) in HESC310 12/4/2018 Compensation Abnormally low PaCO2 (<35 mmHg) in metabolic acidosis indicates respiratory compensation (hyperventilation). Pure (uncompensated) metabolic acidosis seldom seen since respiratory system compensates quickly. PaO2 is often high normal (95-100 mmHg) when there is respiratory compensation. Compensated (or chronic) metabolic acidosis pH=7.25, PaCO2=25 mmHg, HCO3=10 mEq/L There’s respiratory compensation It’s acidosis The acidosis is metabolic Axial Skeleton

Compensation Abnormally high PaCO2 (>45 mmHg) in HESC310 12/4/2018 Compensation Abnormally high PaCO2 (>45 mmHg) in metabolic alkalosis indicates respiratory compensation (hypoventilation). PaO2 may be normal or slightly below normal (<=80 mmHg) Uncompensated metabolic alkalosis pH=7.58, PaCO2=44 mmHg, HCO3=40 mEq/L Compensated metabolic alkalosis pH=7.50, PaCO2=55 mmHg, HCO3=40 mEq/L It’s alkalosis There’s respiratory compensation The alkalosis is metabolic Axial Skeleton

Compensation Abnormally high HCO3 (>26 mEq/L) in HESC310 12/4/2018 Compensation Abnormally high HCO3 (>26 mEq/L) in respiratory acidosis indicates renal compensation. Uncompensated (or acute) respiratory acidosis pH=7.25, PaCO2=60 mmHg, HCO3=25 mEq/L Compensated (or chronic) respiratory acidosis pH=7.35, PaCO2=60 mmHg, HCO3=32 mEq/L Near normal Abnormally high – this person isn’t getting rid of CO2 Higher than normal – kidneys must be compensating for the high CO2 Axial Skeleton

Compensation Abnormally low HCO3 (<22 mEq/L) in HESC310 12/4/2018 Compensation Abnormally low HCO3 (<22 mEq/L) in respiratory alkalosis indicates renal compensation. Compensated (or chronic) respiratory alkalosis pH=7.44, PaCO2=26 mmHg, HCO3=17 mEq/L, PaO2=53 mmHg, pneumonia It’s alkalosis The alkalosis is not metabolic – quite the opposite The alkalosis is respiratory Axial Skeleton

HESC310 12/4/2018 Sources Rose, W.C., in Fluids and Electrolytes with Clinical Applications, 8th ed., 2009. Chapter 11. Regulatory Mechanisms for pH Control. Chapter 12. Determination of Acid-Base Imbalances. Chapter 13. Metabolic Acidosis and Alkalosis. Chapter 14. Respiratory Acidosis and Alkalosis. Acid-base pHysiology'. K. Brandis, http://www.anaesthesiaMCQ.com GlobalRPh: Arterial blood gases. http://www.globalrph.com/abg_analysis.htm Köse et al. (2014). Acid-Base Disorders in the Emergency Department: Incidence, Etiologies and Outcomes. J. Acad. Emerg. Med. 13: 4-9. Axial Skeleton