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 SevereAcid 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

• A ASA

• M Methanol, Metformin

• U Uremia

• D DKA

• P Paraldehyde, Phenformin

• I Isoniazid, Iron

• L Lactate

• E Ethylene glycol

• C CO, CN

• A AKA, alcohol

• T Toluene, Theophylline

• O Other– 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………