Overview
This lecture covers the mechanisms controlling alveolar ventilation and details the differential diagnosis and pathophysiology of respiratory acidosis.
Control of Ventilation
- Ventilation is regulated by the medullary respiratory center integrating brain and peripheral chemoreceptor input.
- Central chemoreceptors (mainly in the medulla) detect pH changes and adjust ventilation to regulate arterial pH.
- The cerebral cortex can increase ventilatory drive (pain, anxiety) and provides tonic inhibition.
- Pontine respiratory centers help fine-tune respiratory rate and tidal volume.
- Progesterone receptors in the CNS, when stimulated, increase ventilation.
- Peripheral chemoreceptors (carotid and aortic bodies) respond to changes in pCO₂, pH, and significant drops in pO₂.
- Other receptors (J receptors, stretch receptors, irritant receptors) further modulate respiratory drive.
Alveolar Ventilation Equation
- Alveolar ventilation is calculated as respiratory rate × (tidal volume − dead space).
- Partial pressure of arterial CO₂ (pCO₂) = (CO₂ production rate × inspired air pressure) ÷ alveolar ventilation.
- Main causes of increased pCO₂: low respiratory rate and low tidal volume; less commonly, high dead space or increased CO₂ production.
Causes of Respiratory Acidosis
- Obstructive lung diseases: COPD (most common), asthma (only in severe attacks).
- Central hypoventilation: drug overdose (alcohol, narcotics, benzodiazepines, others), brainstem lesions, obesity hypoventilation syndrome, central hypoventilation syndrome (Ondine's curse).
- Neuromuscular disorders: diseases affecting spinal cord, peripheral nerves, neuromuscular junction, or muscles.
- Poor compliance of the respiratory system: interstitial lung disease, ARDS, pulmonary edema, chest wall deformities (e.g., kyphoscoliosis).
- Hypermetabolic states: rare conditions like extreme fever, seizures, or overfeeding.
Common Clinical Entities
- COPD: airway inflammation and parenchymal destruction cause airway obstruction, air trapping, hyperinflation, and decreased alveolar ventilation.
- Drug overdose: respiratory depressants first lower respiratory rate; risk increases dramatically when multiple drug classes are combined.
- Obesity hypoventilation syndrome (OHS): characterized by obesity and awake hypoventilation; distinct from but often overlapping with obstructive sleep apnea; complications include hypoxia and right heart failure.
Rare Conditions
- Central hypoventilation syndrome (Ondine's curse): failure of autonomic ventilation control, typically normal voluntary breathing, may be congenital or acquired.
Key Terms & Definitions
- Respiratory acidosis — acid-base disorder caused by insufficient ventilation leading to CO₂ retention.
- Alveolar ventilation — volume of air reaching perfused alveoli per minute.
- Tidal volume — air volume inhaled or exhaled in a normal breath.
- Dead space — lung regions where gas exchange does not occur.
- Obesity hypoventilation syndrome (OHS) — chronic hypoventilation during wakefulness in obesity without other causes.
- Central hypoventilation syndrome (Ondine's curse) — impaired automatic respiratory drive, especially during sleep.
Action Items / Next Steps
- Review the causes of respiratory alkalosis in the next lecture.
- Be familiar with key causes and clinical features of respiratory acidosis for exams.