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R6 Respirology
Approach to the Respiratory Patient
Toronto Notes 2019
Table 8. Expected Compensation for Specific Acid-Base Disorders
Factors that Shift the Oxygen-Hb Dissociation Curve to the Right
“CADET, face right!” CO2
Acid
2,3-DPG
Exercise
Temperature (increased)
Note: 2,3-DPG (2,3-diphosphoglycerate) is now called 2,3-BPG (2,3-biphosphoglycerate)
Acidosis ←→ Hyperkalemia Alkalosis ←→ Hypokalemia
Note: Mixed acid-base disturbances can still have a “normal” pH
Osmolar Gap = measured osmolarity
– calculated osmolarity; for calculated osmolarity think “2 salts and a sticky BUN” (2Na + glucose + urea)
Anion Gap Metabolic Acidosis
MUDPILESCAT
Methanol
Uremia
Diabetic ketoacidosis/starvation ketoacidosis Phenformin/Paraldehyde
Isoniazid, Iron, Ibuprofen Lactic acidosis
Ethylene glycol Salicylates
Cyanide, Carbon dioxide Alcoholic ketoacidosis Toluene, Theophylline
Disturbance
Respiratory Acidosis
Acute Chronic
Respiratory Alkalosis
Acute Chronic
Metabolic Acidosis Metabolic Alkalosis
PaCO2 (mmHg) (normal ~40)
 10  10
 10  10
1 5-7
HCO3– (mmHg) (normal ~24)
1 3
2 5
1 10
4. if there is metabolic acidosis, what is the anion gap and osmolar gap?
■ anion gap = [Na+]–([Cl–]+[HCO3–]); normal 5-14 mmol/L
■ osmolar gap = measured osmolarity – calculated osmolarity = measured – (2[Na+] + glucose +
urea); normal ≤10 mmol/L
◆ abnormal osmolar gap indicates the presence of alcohols
5. if anion gap is increased, is the change in bicarbonate the same as the change in anion gap? ■ if not, consider a mixed metabolic picture
Table 9. Differential Diagnosis of Respiratory Acidosis
Increased PaCO2 secondary to hypoventilation
Respiratory Centre Depression (Decreased RR)
Drugs (anesthesia, sedatives, narcotics)
Trauma
Encephalitis
Stroke
Central apnea
Supplemental O2 in
chronic CO2 retainers (e.g. COPD)
Neuromuscular Disorders (Decreased Vital Capacity) Myasthenia gravis Guillain-Barré syndrome Botulism
Poliomyelitis
Muscular dystrophies
ALS
Myopathies
Chest wall disease (obesity, kyphoscoliosis)
Lung Disease
COPD
Asthma Pulmonary edema Pneumothorax Pneumonia
ILD (late stage) ARDS
Mechanical Hypoventilation (Inadequate Mechanical Ventilation)
Table 10. Differential Diagnosis of Respiratory Alkalosis
Decreased PaCO2 secondary to hyperventilation
Pulmonary disease (pneumonia, edema, PE, interstitial fibrosis)
Severe anemia
Heart failure
High altitude
Respiratory Centre Stimulation
Drugs (ASA, progesterone, theophylline, catecholamines, psychotropics, nicotine, salicylates)
Chest wall disease (obesity, kyphoscoliosis) Hepatic failure
Gram-negative sepsis Pregnancy
Anxiety
Pain
Mechanical Hyperventilation (Excessive Mechanical Ventilation)
• seeNephrology,NP16fordifferentialdiagnosisofmetabolicacidosisandalkalosis