Step-by-Step ABG Approach to Respiratory Acidosis and Alkalosis 🔥| Nephrology Board Review

🚨 Can you interpret respiratory acid-base disorders like a nephrology expert? Master the ABG approach, compensation rules, and mixed disorders! In this high-yield nephrology board review, we break down how to diagnose respiratory acidosis and respiratory alkalosis, calculate expected compensation, identify acute versus chronic disorders, and recognize mixed acid-base abnormalities. 🧠 1. START WITH THE ABG pH below 7.35: Acidemia pH above 7.45: Alkalemia PaCO₂ reflects the respiratory component. HCO₃⁻ reflects the metabolic component. Determine whether the primary process is respiratory or metabolic. 🔥 2. RESPIRATORY ACIDOSIS Primary problem: PaCO₂ rises due to inadequate alveolar ventilation. Common causes: COPD or severe asthma Opioids and sedatives Neuromuscular weakness Obesity hypoventilation syndrome CNS depression or severe airway/lung disease ⚡ Expected HCO₃⁻ compensation: Acute: HCO₃⁻ increases by approximately 1 mEq/L for every 10 mmHg rise in PaCO₂ above 40. Chronic: HCO₃⁻ increases by approximately 3.5–4 mEq/L for every 10 mmHg rise in PaCO₂ above 40. 💡 If HCO₃⁻ is higher or lower than expected, suspect an additional metabolic alkalosis or metabolic acidosis, respectively. 🔥 3. RESPIRATORY ALKALOSIS Primary problem: PaCO₂ falls due to excessive alveolar ventilation. Common causes: Hypoxemia Pulmonary embolism Sepsis Pregnancy Anxiety or pain Liver disease Early salicylate toxicity ⚡ Expected HCO₃⁻ compensation: Acute: HCO₃⁻ decreases by approximately 2 mEq/L for every 10 mmHg fall in PaCO₂ below 40. Chronic: HCO₃⁻ decreases by approximately 4–5 mEq/L for every 10 mmHg fall in PaCO₂ below 40. 💡 A bicarbonate change outside the expected range suggests a mixed disorder. 🩺 4. ACUTE vs CHRONIC: WHY IT MATTERS Acute respiratory disorders initially produce limited renal compensation. Over several days, the kidneys adapt by changing bicarbonate reabsorption and acid excretion. Chronic respiratory acidosis produces greater bicarbonate retention. Chronic respiratory alkalosis produces greater bicarbonate excretion. 🎯 5. HIGH-YIELD BOARD PEARLS Do not assume respiratory alkalosis is caused by anxiety until serious causes, such as pulmonary embolism or sepsis, are considered. In COPD, a high PaCO₂ with elevated HCO₃⁻ may reflect chronic respiratory acidosis; compare with prior ABGs when available. In salicylate toxicity, respiratory alkalosis plus anion gap metabolic acidosis is a classic mixed disorder. Compensation does not usually normalize the pH completely or overshoot it. An unexpected response should prompt evaluation for a second primary disorder. 📚 Practice tip: Always interpret the pH, PaCO₂, and HCO₃⁻ together, calculate expected compensation, and assess oxygenation and the clinical context. #Nephrology #ABG #AcidBase #RespiratoryAcidosis #RespiratoryAlkalosis #AcidBaseDisorders #NephrologyBoardReview #MedicalEducation #CriticalCare