Control of Breathing & High Altitude

Chemoreceptor control of ventilation

Rhythm and drive. Medullary neurons generate the rhythm (dorsal respiratory group: inspiration; ventral group: active expiration), and pontine centers shape it (pneumotaxic center shortens inspiration; apneustic center prolongs it). Chemoreceptors adjust ventilation to hold PaCO₂, pH, and PaO₂ in range.

Central chemoreceptors. Located near the ventral surface of the medulla, they respond to the pH of the CSF. CO₂ diffuses freely across the blood-brain barrier, is hydrated to carbonic acid, and releases the H⁺ that stimulates the receptors; plasma H⁺ and HCO₃⁻ cross poorly, so they act only slowly. CSF has little protein to buffer H⁺, so a small rise in PaCO₂ causes a large fall in CSF pH. This makes the central receptors the dominant minute-to-minute driver of ventilation, but they do NOT sense O₂.

Peripheral chemoreceptors. The carotid bodies (carotid bifurcation; afferents in CN IX, glossopharyngeal) and aortic bodies (aortic arch; afferents in CN X, vagus) are the only ventilatory sensors of hypoxemia. They respond to PaO₂ < 60 mmHg, plus a rising PaCO₂ and falling pH. Firing climbs steeply below 60 mmHg, where the O₂–hemoglobin curve leaves its plateau and saturation begins to drop quickly. The carotid bodies also drive the hyperventilation of metabolic acidosis (e.g., Kussmaul breathing in DKA), because arterial H⁺ reaches the central receptors poorly.

Chronic CO₂ retainers. In chronic hypercapnia (e.g., severe COPD), CSF bicarbonate rises over days and returns CSF pH toward normal, so the central chemoreceptors reset and respond less to the high PaCO₂. Ventilation then leans on the peripheral hypoxic drive, and excess supplemental O₂ can blunt that drive and worsen hypercapnia (somnolence, CO₂ narcosis). High-flow O₂ also worsens V/Q matching by releasing hypoxic vasoconstriction in poorly ventilated units, and the Haldane effect (oxygenated hemoglobin holds less CO₂) adds CO₂ to plasma.

Never withhold O₂ from a hypoxemic CO₂ retainer; titrate to a modest target (SpO₂ about 88–92%) and follow the PaCO₂.

High altitude: hypoxemia and acclimatization

Why inspired O₂ falls. Barometric pressure drops with elevation while FiO₂ stays 21%, so inspired PO₂ falls: PiO₂ = FiO₂ × (Patm − 47 mmHg water vapor), about 150 mmHg at sea level. Alveolar and arterial PO₂ fall with it, giving hypoxemia with a normal A-a gradient, the same pattern as pure hypoventilation.

Acute response. Hypoxemia stimulates the peripheral chemoreceptors, causing hyperventilation. Blowing off CO₂ produces an initial respiratory alkalosis and, by the alveolar gas equation (PAO₂ = PiO₂ − PaCO₂/0.8), partly restores alveolar PO₂. The lungs respond at once, but the kidney takes days to excrete bicarbonate, so acclimatization is gradual.

Chronic acclimatization. Over days to weeks:

• Kidneys excrete bicarbonate, compensating the respiratory alkalosis.
• 2,3-BPG rises, right-shifting the O₂–Hb curve to aid tissue unloading.
• Erythropoietin (from renal peritubular interstitial cells) rises, causing polycythemia and more O₂-carrying capacity.
• Mitochondrial density increases.

Acetazolamide. This carbonic anhydrase inhibitor blocks proximal bicarbonate reabsorption, causing a bicarbonate diuresis and a mild non-anion gap metabolic acidosis. The acidosis offsets the respiratory alkalosis and stimulates ventilation, in effect pre-acclimatizing the climber, so it is used to prevent acute mountain sickness (headache, nausea, fatigue, poor sleep). Descent remains the definitive treatment for severe altitude illness.

Chronic hypoxia and the right heart. Hypoxic pulmonary vasoconstriction normally diverts blood away from poorly ventilated alveoli; the pulmonary circulation is unique in constricting to hypoxia, whereas systemic arterioles dilate. At altitude the alveolar hypoxia is global, so vasoconstriction occurs throughout both lungs and pulmonary artery pressure rises. Sustained, this causes pulmonary hypertension and right ventricular hypertrophy. Chronic mountain sickness (Monge disease) combines excessive polycythemia, hypoxemia, and right heart strain.

Sleep apnea: obstructive vs central

Obstructive sleep apnea (OSA). Repetitive upper-airway collapse during sleep despite continued respiratory effort: the chest and abdomen keep moving, but no air flows. Classic patient: obese, thick neck, loud snoring, witnessed apneas, and daytime somnolence. Polysomnography confirms it: an apnea-hypopnea index (AHI) ≥5 events/hour with symptoms (or ≥15 regardless of symptoms). CPAP, which splints the airway open, is first-line; supplemental O₂ alone does not relieve the obstruction.

Central sleep apnea (CSA). Airflow stops WITHOUT respiratory effort because CNS drive is lost. Causes include heart failure (Cheyne-Stokes respiration: crescendo-decrescendo breathing alternating with apneas, as slow circulation delays chemoreceptor feedback), CNS lesions, opioids, and high altitude.

Consequences. Chronic nocturnal hypoxemia and repeated sympathetic surges cause systemic hypertension, pulmonary hypertension progressing to cor pulmonale, atrial fibrillation and bradyarrhythmias, and secondary (EPO-driven) polycythemia. Obesity hypoventilation syndrome (BMI ≥30 with awake PaCO₂ >45 mmHg) often coexists with OSA.

Reading O₂ numbers: PaO₂, SaO₂, and content

Three different quantities. PaO₂ is the tension of O₂ dissolved in plasma; it reflects alveolar PO₂ and gas exchange, not hemoglobin. SaO₂ is the percentage of hemoglobin binding sites carrying O₂. O₂ content is the total O₂ carried and depends on Hb, SaO₂, and PaO₂: CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂), normally about 20 mL O₂/dL, almost all bound to hemoglobin. Because each reflects a different step, they can move independently.

Anemia. Fewer carriers: PaO₂ and SaO₂ are normal, but content is low.

Carbon monoxide poisoning. CO binds hemoglobin with roughly 200-fold greater affinity than O₂, occupying binding sites (low true SaO₂ and content) and left-shifting the curve so the remaining O₂ unloads poorly. PaO₂ stays normal because dissolved O₂ is unaffected. Standard pulse oximetry cannot tell carboxyhemoglobin from oxyhemoglobin, so it reads falsely normal; co-oximetry measures carboxyhemoglobin directly. Treatment is 100% O₂ (hyperbaric in severe cases).

Neither anemia nor CO lowers PaO₂, and a normal pulse oximeter reading after smoke inhalation does not exclude CO poisoning; send co-oximetry.