Chronic Alveolar Hypoventilation

*BASICS OF VENTILATION PHYSIOLOGY:
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! Tidal volume (VT) = Vdead space (Vd) +V alveolar (VA)
! Minute ventilation (MV) = TV x RR
! VA = Vco2/Paco2 x K (constant)


*DEFINITION:
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Alveolar hypoventilation is defined as insufficient ventilation leading to an increase in PaCO2 (ie, hypercapnia). Alveolar hypoventilation is caused by several disorders that are collectively referred as hypoventilation syndromes. Alveolar hypoventilation also is a cause of hypoxemia. Thus, patients who hypoventilate may develop clinically significant hypoxemia. The presence of hypoxemia along with hypercapnia aggravates the clinical manifestations seen with hypoventilation syndromes.
Alveolar hypoventilation may be acute or chronic and may be caused by several mechanisms.


*ETIOLOGIES Of ALVEOLAR HYPOVENTILATION:
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1) Chemo sensitivities/sensors:
o Appropriate response to primary metabolic alkalosis (1mEQ HCO3 corresponds to 0.6
mmHg rise of PaCO2)
o Impaired peripheral chemo receptor (carotid bodies) response to hypoxia-s/p CEA
o Impaired central chemo receptor (midbrain and medulla) response to acidosis

2) Brain stem dysfunction (medulla central pattern generator cells):
o Stroke-thrombosis, embolic, bleed
o infiltration-neoplasm, sarcoidosis
o demyelinating disorders-MS
o Drug/toxins-narcotics, BDZ, ETOH
o Infection-encephalitis, bulbar poliomyelitis, basilar meningitis
o Primary alveolar hypoventilation

3) Spinal cord, peripheral nerves and respiratory muscles dysfunction
o Motor neuron disease-ALS, polio
o Peripheral neuropathy-phrenic nerve compression, resection, GBS,
o Neuromuscular junction-MG, ELS,
o myopathies-drugs, dystrophies, hypothyroidism, DM/PM

4) Chest wall dysfunction:  stiff chest wall imposes large elastic load on the lung, increase work of
breathing, increased dead space ventilation (tachypneic with small TV breaths)
o obesity hypoventilation (Pickwinian syndrome)
o kyphoscoliosis
o fibrothorax
o post-thoracoplasty
o ankylosing spondylitis

5) Lung and airways dysfunction
o COPD-increased dead space ventilation
o Upper airway obstruction
o Cystic fibrosis
o Obstructive sleep apnea


*REASONALBLE DIAGNOSITIC APPROACH TO A PATIENT:
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- Check a TSH
- ABG: elevated A-a gradient is seen in disorders of chest wall, lung or airways; normal A-a
gradient with central or neurological disorders
- PFT’s: restrictive pattern is seen with chest wall disorders, obstructive with airway disease, PI/PE
max are decreased with neuromuscular disorders
- Sleep studies: central apneas seen with central disorders, obstructive apneas with OSA, note all
disorders will worsen with sleep
- If above studies are normal, tests of respiratory control can be performed. Pt is stimulated by
hypoxia and hypercapnea and ventilatory responses are recorded.
- Consider: diaphragmatic EMG to evaluate phrenic nerve, MRI head for brainstem mass


*COMPLICATIONS OF CHRONIC ALVEOLAR HYPOVENTILATION:
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- Decreased alveolar ventilation results hypercapnea, and hypoxemia.
- Sleep accentuates these abnormalities because of decreased respiratory drive
- Physiologic consequences include increased erythropoesis, metabolic alkalosis, pulmonary
vasoconstriction, cerebral vasodilatation, impaired sleep
- Clinically, patients present with polycythemia, pulmonary hypertension, cor pulmonale, morning
headache, fatigue, daytime somnolence and poor sleep