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respiratory acidosis; hypercapnic acidosis

Inadequate pulmonary excretion of CO2. Etiology: 1) CNS depression (inhibition of medullary respiratory center) a) CNS depressants - anesthesia, sedatives, opiates b) infection c) brain injury - head trauma - stroke - brain tumor d) central hypoventilation syndrome e) obesity-hypoventilation syndrome f) central sleep apnea g) hypercapnia h) myxedema coma 2) neuromuscular disorder (disorders of respiratory muscles) a) myopathy - polymyositis b) Guillain-Barre syndrome c) myasthenic crisis d) amyotrophic lateral sclerosis (ALS) e) spinal cord injury f) periodic paralysis g) hypokalemia h) hypophosphatemia 3) musculoskeletal disorders a) morbid obesity b) kyphoscoliosis 3) pulmonary diseases (disorders of gas exchange across alveolar capillaries) a) chronic obstructive pulmonary disease b) obstructive sleep apnea c) asthma d) interstitial lung disease - pulmonary fibrosis e) pneumothorax, hemothorax f) pulmonary edema g) pulmonary embolism h) pneumonia i) ARDS 4) increased carbon dioxide production a) extreme exercise b) seizures c) heart failure (severe) 5) increased wotk of breathing 6) mechanical ventilation - hyperalimentation 7) upper airway obstruction - tonsillar hypertrophy - vocal cord paralysis - foreign body aspiration - laryngospasm - tracheal stenosis - angioedema Clinical manifestations: 1) acute a) confusion b) asterixis c) papilledema d) headache e) somnolence f) seizures 2) chronic a) memory loss b) inattention c) irritability, agitation 3) cardiovascular a) hypertension b) tachycardia c) heart failure d) cardiac arrhythmias [2] Laboratory: 1) arterial blood gas (ABG) a) decreased pH b) increased pCO2 2) serum bicarbonate: a) compensatory metabolic alkalosis generally does not result in serum HCO3- > 35 meq/L b) 1 meq/L acute increase in serum HCO3- for every 10 mm Hg increase in pCO2 c) 3.5 meq/L chronic increase in serum HCO3- for every 10 mm Hg increase in pCO2 d) 3-5 days for compensatory metabolic alkalosis * Rules for predicting serum HCO3- compensation for pure respiratory acidosis 1) acute: 1 meq increase in serum HCO3- for each 10 mm Hg increase in pCO2 (PaCO2, arterial) 2) chronic: 3.5 meq increase in serum HCO3- for each 10 mm Hg increase in pCO2 3) failure of serum HCO3- to increase by the expected value suggests complicating metabolic acidosis 4) excessive increase in serum HCO3- suggests complicating metabolic alkalosis [2] 5) in calculating the anticipated HCO3- compensation use the midpoint of the reference interval 25-26 meq/L as the baseline Complications: - acute respiratory acid associated with risk of hypoxemia rather than acidemia or hypercapnia - uncompensated respiratory acidosis is associated with increased mortality in mechanically-ventilated patients with traumatic brain injury [3] - compensated respiratory acidosis is not [3] Management: 1) therapy directed towards improving ventilation a) correct hypoxia - oxygen as needed b) excessive oxygen may blunt respiratory drive & worsen hypoventilation in patients with chronic respiratory acidosis c) airway injury may necessitate endotracheal intubation to protect airway [2] 2) administration of bicarbonate a) may be harmful since low pH is a stimulus for respiration b) may be beneficial in patients with asthma on a ventilator 1] may improve response to beta-adrenergic agonists 2] may allow less vigorous ventilation reducing risk of barotrauma c) may be indicated to maintain plasma pH above 7.1 3) post-hypercapneic metabolic alkalosis can usually be corrected with saline & discontinuation of loop diuretics [2]

General

acidosis

References

  1. Manual of Medical Therapeutics, 28th ed, Ewald & McKenzie (eds), Little, Brown & Co, Boston, 1995, pg 63
  2. Medical Knowledge Self Assessment Program (MKSAP) 11, 14, 15, 16, 17, 18, 19. American College of Physicians, Philadelphia 1998, 2006, 2009, 2012, 2015, 2018, 2021. - Medical Knowledge Self Assessment Program (MKSAP) 19 Board Basics. An Enhancement to MKSAP19. American College of Physicians, Philadelphia 2022
  3. Tiruvoipati R, Pilcher D, Botha J et al Association of Hypercapnia and Hypercapnic Acidosis With Clinical Outcomes in Mechanically Ventilated Patients With Cerebral Injury. JAMA Neurol. Published online March 19, 2018 PMID: 29554187 https://jamanetwork.com/journals/jamaneurology/fullarticle/2675293 - Hemphill JC 3rd. Arterial Partial Pressure of Carbon Dioxide and Secondary Brain Injury - 6 Degrees of Separation? JAMA Neurol. Published online March 19, 2018. PMID: 29554181 https://jamanetwork.com/journals/jamaneurology/fullarticle/2675290