Acid-Base homeostasis

Normal- pH- 7.35-7.40-7.45pCO2- 36-40-44 mm HgHCO3- 22-24-26 mmol/L

Body’s response

Buffering, in secondsLungs- change in arterial pCO2, in minutes

Kidneys- change in HCO3 excretion, in hours/days

Buffers

Extracellular- HCO3 & NH3Intracellular- proteins & PO4

Approach to A-B disorders Initial clinical assessment- s/s Acid-base diagnosis Arterial pH Pattern of HCO3 & pCO2 Additional clues in other investigations Compensation Formulation Confirmation by further tests Clinical diagnosis

Systematic evaluation History necessary for possible defect Get ABG, electrolytes, RFT

pH<7.35- acidosis; >7.45- alkalosis

Respiratory disorder- CO2 changes Increase- acidosis; Decrease- alkalosis Metabolic disorder- HCO3 changes Decrease- acidosis; Increase- alkalosis

Evaluation- contd. Clues- High anion gap- metabolic acidosis Hyperglycemia +/- urine ketones- DKA Hyperchloremia- normal anion-gap acidosis Elevated creatinine- consider ketoacidosis Elevated BUN/Cr- uremic acidosis/pre-renal ARF Hypokalemia &/or hypochloremia- metabolic alkalosis

Evaluation- contd. Compensation- predictable physiologic consequence of the primary

disturbance Compensation follows the direction of CO2/HCO3 change Compensation brings pH towards normal, but never overshoots

• Respiratory acidosis- CO2 increase- HCO3 increase acute- 1 mEq/L for 10 mm Hg rise in PaCO2- max. 38 chronic- 4 for 10- max. 45

Respiratory alkalosis- CO2 decrease- HCO3 decrease acute- 2 for 10- max. 18 chronic- 5 for 10- max. 15

Metabolic acidosis- HCO3 decrease- CO2 decrease last 2 digits of pH or one and a half+8- max. 15

Metabolic alkalosis- HCO3 increase- CO2 increase point 7+20- max. 55

Anion gap (AG) Na + unmeasured cations (UC) =

Cl + HCO3 + unmeasured anions (UA) UC- Ca, Mg, K, IgG UA- albumin, PO4, SO4, organic acids AG = UA-UC = Na-(Cl+HCO3) Normal AG- 9 ± 3 mEq/L Helps classify metabolic acidosis

UrinaryAG = UA-UC = (Na+K)-ClTo differentiate hyperchloremic/normal AG metabolic acidosis

Acid-Base disorders

Simple- one defect with appropriate compensation

Mixed- more than one defect

Respiratory acidosis Hypoventi lation CNS- depression, trauma,

Pickwickian syndrome, polio, tetanus Nerve/muscle- GBS, myasthenia,

myopathies, snake venom Lung- ARDS, aspiration, edema,

pneumothorax, restrictive lung disease, COPD

Airway- asthma, broncho/laryngospasm Inadequate mechanical ventilation

Respiratory alkalosis Hyperventilation CNS- head injury, CVA Psychogenic Pain, fever, stress Lung- PE, pneumonia, asthma, edema High altitude Progesterone in pregnancy, cytokines in sepsis,

toxins in CLD Iatrogenic

Metabolic acidosis High anion gap- added acids Ketoacidosis- DM, alcoholic, starvation Lactic acidosis- shock, extreme exertion ARF/CRF Toxins- ethylene glycol, methanol, salicylates Normal anion gap- hyperchloremic-

bicarbonate loss Renal tubular acidosis Severe diarrhea, GI fistulas, drainage of biliary/pancreatic

secretions

Metabolic alkalosis Chloride depletion- U Cl<10 mmol/L-

saline responsive Vomiting Diuretics K depletion- U Cl>20 mmol/L-

saline resistant Hyperaldosteronism Cushing’s syndrome Bartter’s syndrome Other Laxative abuse Severe hypoalbuminemia

Example 1 Poorly controlled diabetic presents with polyuria Na-136, K-4.8, Cl-101, Cr-0.09 Urine- 2+ ketones, 4+ glucose ABG- pH-7.26, pCO2-16, HCO3-7.1

pH- acidosis Low HCO3- metabolic acidosis Compensation- 1.5x7.1+8=18.5 mm Hg- appropriate Anion gap- 136-(101+7)= 28- increased

Final A-B Dx- high anion gap metabolic acidosis

Example 2 Middle aged man with diarrhoea for 2 days Na-134, K-2.9, Cl-113, Cr-0.3 ABG- pH-7.31, pCO2-33, HCO3-16

pH- acidosis Low HCO3- metabolic acidosis Anion gap- 134-(113+16) = 5- normal Compensation- 1.5x16+8 = 32- appropriate

Final A-B Dx- normal anion gap metabolic acidosis

Example 3 Young female, on postop. Morphine & oxygen,

noted drowsy the next day ABG- pH-7.16, pCO2-61.9, HCO3-21.2

pH- acidosis Increased pCO2 & decreased HCO3- mixed acidosis-

predominant respiratory acidosis Compensation- 24+22/10 = 26- mild metabolic acidosis

Final A-B Dx- mixed acute respiratory & mild metabolic acidosis

Electrolytes, glucose, RFT, lactate would have helped

Example 4 Elderly woman with weakness & areflexia with poor oral intake Na-145, K-1.9, Cl-86 ABG- pH-7.58, pCO2-49, HCO3-44.4

pH- alkalosis High HCO3- metabolic alkalosis Compensation- 0.7x44.4+20 = 51- appropriate

Final A-B Dx- severe metabolic alkalosis Emergency management of low K

Example 5 Elderly woman with CHF on diuretics, admitted with lobar

pneumonia ABG- pH-7.64, pCO2-32, HCO3-33

pH- alkalosis Low pCO2 & high HCO3- mixed alkalosis Compensation- For low CO2- HCO3 = 20- inappropriateFor high HCO3- pCO2 = 43- inappropriate

Final A-B Dx- mixed metabolic & respiratory alkalosis

Example 6 Elderly man had cardiac arrest resuscitated ABG- pH-6.85, pCO2-82, HCO3-14

pH- acidosis High pCO2- respiratory acidosis Low HCO3- metabolic acidosis

Final A-B Dx- mixed respiratory & metabolic acidosis

Electrolytes, RFT, lactate would have helped

Example 7 Elderly man with CHF, admitted with vomiting for 5 days, was

hyperventilating Na-127, K-5.2, Cl-79, BUN-55, Cr-0.38, glucose-98 ABG- pH-7.58, pCO2-21, HCO3-19

pH- alkalosis Low pCO2- respiratory alkalosis Anion gap- 29- high anion gap metabolic acidosis Compensation- 24-10=14- actual 19- met. Alkalosis

Final A-B Dx- triple disorder- R.al & M.al & high AG M.ac