ACID/BASE DISORDERS Resident Rounds Rob Hall PGY3 April 24, 2003

Objectives Approach to A/B disorders Clinical examples of each disorder Differential dx of each disorder Combined disorders

Should we even do ABGs? MANY studies showing that venous gases have similar pH and pC02 to ABGs MANY studies show that ABGs rarely change management

How to interpret an ABG What is the pH? Is there an acidemia or alkalemia? Is it respiratory or metabolic? Is there any compensation? Is the compensation appropriate? What is the anion gap?

Took some pills ABG –pH 7.25 –PC02 22 –HC03 15 Interpretation? Is there a second acid base disorder? Metabolic acidosis + respiratory alkalosis –Think ASA!!

Compensation: the clue to mixed disorders ACIDOSIS –Respiratory Acute 1:10 Chronic 1:3 –Metabolic 1:1 ALKALOSIS –Respiratory Acute 1:10 Chronic 1:2 –Metabolic 0.6:1

80 female with suspected ischemic gut…… pH 6.9, PC02 35, HCO3 8 Why is the acidemia important?

Consequences of Severe Acid Base Disorders Severe Acidemia –Negative ionotropy –Arrythmias –Reduced response to catecholamines –Hyperkalemia –Muscle weakness –Altered LOC and seizures –Poor enzyme function Severe Alkalemia –Reduced coronary blood flow –Arrythmias –Hypokalemia –Altered LOC and seizures –Poor enzyme function

Case 75 yo female Altered LOC Fever Sinus tachycardia Tachypnea ABG: pH 7.50, pC02 30, HC03 23 Interpretation? Diagnosis? Differential dx of the acid/base disorder?

Respiratory Alkalosis Pain Anxiety Pregnancy Pulmonary disease/hypoxia CNS disorder Thyrotoxicosis ASA

Cases 70yo smoker since birth COPD exacerbation pH 7.15, pC02 60, HC03 26 –Is he a chronic CO2 retainer? pH 7.35, pC02 60, HC03 32 –Interpretation? pH 7.05, pC02 100, HC03 32 –What is his “normal” pC02?

Chronic Respiratory Acidosis You know that the HC03 increases in a 1:3 ratio to the increase in pC02 If the HC03 is up by 7, the pC02 is chronically up by about 20 What is the differential dx of respiratory acidosis?

Respiratory Acidosis HYPOVENTILATION –Brain stem –Spinal Cord –Motor neuron –Peripheral nerve –NMJ –Muscle –Chest wall –Obesity hypoventilation IMPAIRED GAS EXCHANGE –Airway obstruction –Bronchospasm –Pneumonia –Pulmonary edema –PE –Aspiration –COPD

ANION GAP What is the anion gap? What is the formula? What is a “normal” anion gap? What could cause a LOW anion gap?

ANION GAP Na+ K+ Ca++ Mg++ Cl- HCO3- P04- S04- Albumin Organic acids

Low Anion Gap Hypoalbuminemia Increased Ca, Mg, K Lithium intoxication Multiple myeloma

What is the Delta Gap? Delta Gap –Change in AG – change in HC03 –(AG – 12) – (24 – HC03) –Essentially looks for similar changes in anion and drop in bicarb as a marker for additional acid base disorders –Questionable validity

Case 55yo male, street person, found lying in snow by CPS, confused, no history, denies ingestions, no PMHx or meds Temp 33, HR 72, BP 120/60, RR 28, sats 98%, GCS 13 Exam unremarkable except shivering ABG: pH 7.26, pC02 13, HC03 5 Na 129, K 4.7, Cl 88, C02 7 What is the A/B disorder? What other labs do you want?

Case BUN 15, Cr 136 ASA –ve Lactate 1.2 CarboxyHb 0.8% EtOH –ve Toxic alcohols –ve Glucose 2 Urine ketone +ve What is the dx? What is the ddx of an increased AGMA?

Increased AGMA: AMUDPILECATO AASA MMethanol, Metformin UUremia DDKA PParaldehyde, Phenformin IIsoniazid, Iron LLactate EEthylene glycol CCO, CN AAKA, alcohol TToluene, Theophylline OOther –H2S –Any toxin that leads to lactic acidosis (essentially all severe overdoses with hypotension, seizures)

How to narrow the ddx with an increased AGMA Normal glucose rules out DKA BUN, Creatinine ASA level ABG for carboxyHb, lactate Toxic alcohol level

Which toxins cause an increased AGMA independent of lactate? Methanol Ethylene glycol ASA

10yo girl, DKA, pH is 6.9 Would you give bicarb? What is the theoretical reason to give bicarb for acidemia? What are the complications? What are indications for bicarb? Is there any evidence for or against bicarb?

Metabolic Acidosis and bicarbonate therapy: Complications –Paradoxical CSF acidosis –Hypokalemia –Hypocalcemia –Hypernatremia –Volume overload –Overshoot alkalosis Indications for Bicarb –pH < 7.10 –ASA –Methanol –Ethylene glycol –NOT DKA (increased rates of cerebral edema): Glaver NEJM 2001

Ddx of Normal AGMA Gain acid –Acid ingestion –Obstructive uropathy –Pyelonephritis –Distal renal tubular acidosis Bicarb loss –GI Diarrhea Bowel fistual Pancreatic, biliary, or intestinal drains Ureteroenterostomy –Renal Proximal RTA Acetazolamide

Ddx of Metabolic Alkalosis Chloride Responsive –Vomiting –NG drainage –Diuretics –Vilous adenoma Chloride Resistant –Primary hyperaldosteronism –Cushing’s –Steroids –Ectopic ACTH –Barter’s syndrome

A mud pile cat! SSSSSuffering ssssssucatash: look at the size of those………