Acid base Imbalance

THIS PRESENTATION WILL COVER THE FOLLOWING AREAS
Definitions
Buffer systems
Regulatory systems
Anion Gap and Osmolar gap
Metabolic acidosis
Metabolic alkalosis
Respiratory acidosis
Respiratory alkalosis

1. ACID BASE IMBALANCE PRESENTER: DR. ORIBA DAN LANGOYA TUTOR: DR. PEACE BAGASHA, NEPHROLOGIST 27 TH NOV, 2021

2. OBJECTIVES Definitions Buffer systems Regulatory systems Anion Gap and Osmolar gap Metabolic acidosis Metabolic alkalosis Respiratory acidosis Respiratory alkalosis

3. Definitions Hydrogen ion: proton released from a hydrogen atom Acid: molecule containing hydrogen atoms that release hydrogen ions Base: An ion or a molecule that can accept an H+. Acidosis: Excess addition of H+ ions into body fluids. Alkalosis: Excess removal of H+ ions from body fluids. Buffer: any substance that reversibly binds H+.

4. pH and hydrogen ions are related by the equation: Normal pH of arterial blood and interstitial fluid is 7.4 Venous blood 7.35 (extra CO2 released from tissues forms carbonic acid) Intracellular pH: ranges from 6-7.4 (metabolism leads to acid production) Urine pH ranges from 4.5-8

5. Regulation of hydrogen ions Done by three major systems Chemical acid base buffer systems of body fluids The respiratory system The kidneys

6. Chemical acid buffer system Buffer is a substance that binds hydrogen ions reversibly Buffer systems include: bicarbonate , phosphate and proteins Bicarbonate buffer system Involves; weak H2CO3 and NaHCO3

7. Chemical acid buffer system Phosphate buffer Plays a role in buffering renal tubular fluid and intracellular fluid Consists of H2PO4 and HPO4 If strong acid is added ; ↑H+ are buffered by HPO4 to form H2PO4–(weak acid) decreasing pH If strong base is added ; OH- is buffered by H2PO4 to form HPO4 (weak base) +H2O

8. Chemical acid buffer system Proteins present in cells 60-70% of chemical buffer occurs intracellularly due to proteins. Commonly is Hb combine with H+ to form HHb All the buffer systems work together, hydrogen ion is common to reactions of all systems Whenever there is a change in H ion concentration in ECF, the balance of all buffer systems changes at the same time ( isohydric principle)

9. Acid base disorders Based on Henderson-Hasselbalch equation

10. Anion Gap Gauge of the unmeasured anions in the blood stream, majority of unmeasured anions in blood is albumin Na + and cations = HCO - 3 and Cl - and anions AG= (Na + ) - (HCO - 3 + Cl - ) Normal range 8-12 mmol/l

11. Albumin-corrected Anion Gap Anion Gap + 2.5 × (Normal albumin – Measured albumin g/dl)

12. Metabolic acidosis Decreased serum HCO3 Decreased pH and ↓ PCO2 [resp compensation] Acute: Pco2 =(1.5)(HCO3)+8 Chronic: Pco2=(HCO3)+15 Compensation; increased ventilation facilitate decrease in PCO2, reduce hydrogen ions Addition of new bicarbonate from the kidneys.

13. Types of metabolic acidosis Normal Anion gap acidosis- diarrhoea, RTA ureterosigmoidostomy Decreased Anion gap acidosis-Myeloma, hypoalbuminemia Increased Anion gap acidosis- LA MUD PIE

14. Increased anion gap acidosis Calculation of the corrected bicarbonate is helpful in assessing the possible presence of coexisting normal anion gap metabolic acidosis/metabolic alkalosis Corrected bicarbonate = 24mEq/l – Δ Anion gap mEq /l Δ Anion gap = increase in albumin-corrected anion gap above normal Measured bicarbonate < Corrected bicarbonate = concomitant normal AG metabolic acidosis Measured bicarbonate > Corrected bicarbonate = concomitant metabolic alkalosis

15. Osmolar Gap In patients with increased anion gap acidosis calculation of Osmolar gap is useful in accessing the presence of unmeasured solutes e.g. methanol, Ethelene glycol Osmolar gap = Measured OG – calculated OG Calculated OG = 2 × serum Na + plasma Glucose/18 + BUN/2.8 Osmolar gap > 10 = Elevated OG (presence of unmeasured solutes)

16. Metabolic alkalosis Increase in serum HCO3 , loss of H ions Compensation; decreased ventilation to increase PCO2 therefore H ions For every ↑ 1.0mmol/l in HCO3 there is a ↑ 0.7mmHg of Pco2 In kidneys increase filtered HCO3 from serum, increase bicarbonate excretion.

17. Acid base disorders Respiratory acidosis Caused by decreased ventilation and increased PCO2 Decreased ventilation results in increased PCO2 reacts with H ions to form H2CO3 Compensation; increase plasma bicarbonate, adding new HCO3 from kidneys

18. Causes of respiratory acidosis Inadequate ventilation e.g. ↓ respiratory drive, neuromuscular dysfunction and musculoskeletal dysfunction Impaired arterial-alveolar gas exchange e.g. pneumonia, haemothorax, pleural effusion, ARDS Airway obstruction e.g. Asthma , COPD

19. Respiratory alkalosis Low PCO2, Compensation; decreased H Ion secretion In urine. Reduced H ion to combine with HCO3 so its excreted in urine

20. Causes of Respiratory Alkalosis Enhanced respiratory drive e.g. sepsis, psychosis, anxiety Hypoxemia e.g. high altitude, severe anaemia Pulmonary disease with thoracic stretch receptor stimulation e.g. pneumonia, ARDS, haemothorax

21. APPROACH TO ABGS PH PACO2 HCO3 ANION GAP DELTA RATIO

22. Summary PH 7.44, PCO2=56, HCO3=37 PH 7.29, PCO2=58 , HCO3=22 PH 7.32, PCO2=34, HCO3=14, Na, 135, cl-109 PH 7.25, PCO2=25, HC03=10, Na=140,CL 77, PH 7.26, PCO2= 58 HCO3=29 PH 7.28, PCO2=26,HCO3= 11, NA=129, CL-100

23. Treatment Correct underlying cause Neutralize excess acid or base in serum Excess acid – use NaHCO3, Na lactate or gluconate Excess base – use ammonium chloride, lysine monohydrochloride

24. References Guyton and Hall; Textbook of Medical Physiology; 11 th Edition, 2006 MKSAP 17.