Measurement of ventilation
The variables
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Minute ventilation (VE) = tidal volume (VT) x respiratory rate (RR)
Alveolar ventilation (VA) = (VT - dead space) x RR
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- Only alveolar ventilation participates in gas exchange
- PaCO2 is the definitive measure of alveolar ventilation
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PaCO2 proportional to CO2 production / alveolar ventilation
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- -> a normal or rising PaCO2 in a tachypnoeic, distressed patient means ventilatory failure, not improvement
Dead space
| Type | |
|---|---|
| Anatomical | Conducting airways, ~2 mL/kg (~150 mL) - measured by Fowler's method |
| Alveolar | Ventilated but not perfused alveoli |
| Physiological | Anatomical + alveolar - Bohr/Enghoff equation: Vd/Vt = (PaCO2 - PeCO2)/PaCO2 |
- Normal Vd/Vt ~0.2-0.35, rising with age, PE, ARDS and emphysema
- Rapid shallow breathing is inefficient: at a fixed VE, halving VT and doubling RR wastes a far greater proportion on dead space
Clinical context
- Respiratory muscle weakness is the cause of death in most neuromuscular disease - motor neurone disease, Duchenne muscular dystrophy, myasthenic crisis, Guillain-Barre
- FVC <50% predicted is a common trigger for NIV assessment in neuromuscular disease
- Obesity hypoventilation syndrome affects ~10-20% of people with obesity and OSA; commonly diagnosed only after an admission with hypercapnic failure
- Sleep-disordered breathing is present in most advanced neuromuscular disease before daytime symptoms appear
Control of breathing
- Central chemoreceptors (medulla) - respond to CSF H+ derived from CO2 -> the dominant minute-to-minute driver
- Peripheral chemoreceptors (carotid + aortic bodies) - respond to PaO2 (below ~60 mmHg), H+ and PaCO2
- Cortical, pontine (pneumotaxic/apneustic), vagal stretch (Hering-Breuer)
Mechanisms of hypoventilation
| Level | Example |
|---|---|
| Won't breathe (central drive) | Opioids, sedatives, brainstem stroke, congenital central hypoventilation, chronic CO2 retention |
| Can't breathe (neuromuscular) | MND, GBS, myasthenia, myopathy, phrenic nerve palsy, high cervical cord injury |
| Can't breathe (chest wall/load) | Kyphoscoliosis, obesity, ankylosing spondylitis, massive ascites, flail chest |
| Airway/lung load | Severe COPD, asthma, upper airway obstruction |
- Diaphragm: C3, 4, 5 - a lesion above C3 abolishes spontaneous ventilation
- In neuromuscular disease the diaphragm fails before the accessory muscles, and supine is worse than upright - hence orthopnoea
Bedside
- Respiratory rate is the most predictive and most poorly recorded vital sign
- Accessory muscle use, abdominal paradox (inward abdominal movement on inspiration = diaphragm fatigue/paralysis), tracheal tug
- Count-to-30 on one breath, single-breath count, cough strength
- Cyanosis and the pulse oximeter tell you about oxygenation, not ventilation - SpO2 can be 100% in a patient who is dying of CO2 retention on oxygen**
Blood gases
- ABG/VBG is the reference for alveolar ventilation
- VBG PCO2 correlates adequately for screening (~+3-8 mmHg above arterial); use an ABG when precision matters
- A-a gradient separates hypoventilation (normal gradient) from V/Q mismatch, shunt or diffusion limitation (raised)
- Bicarbonate identifies chronicity - a raised HCO3 with hypercapnia means it has been present for days
- Transcutaneous CO2 monitoring - continuous, non-invasive; used in sleep studies and NIV titration
- Capnography (end-tidal CO2) - confirms tube placement, monitors trends; ETCO2 underestimates PaCO2 by the dead space gradient
Lung function in suspected ventilatory failure
- FVC, and FVC supine vs erect
- *A fall of >=20% from erect to supine indicates significant diaphragm weakness* - a highly useful bedside-adjacent test
- Maximal inspiratory and expiratory pressures (MIP/MEP)
- MIP (normal roughly >80 cmH2O men, >60 women); MEP <40-45 cmH2O -> ineffective cough
- Effort-dependent; a low value with good technique is meaningful, a low value with poor effort is not
- SNIP (sniff nasal inspiratory pressure) - a natural manoeuvre; useful when facial or bulbar weakness prevents a good mouthpiece seal (MND, myasthenia)
- Peak cough flow - <270 L/min indicates impaired secretion clearance; <160 L/min means an ineffective cough -> assisted cough/cough-assist device
- Sniff nasal or oesophageal/transdiaphragmatic pressure - the reference standard, specialist centres
- Phrenic nerve conduction studies, diaphragm ultrasound (thickening fraction), fluoroscopic sniff test
When to test respiratory muscle strength
When to test respiratory muscle strength
- Known or suspected neuromuscular disease
- Weak cough, recurrent chest infection
- Unexplained dyspnoea, especially orthopnoea
- Reduced vital capacity or unexplained change in DLCO on routine lung function
- Isolated respiratory muscle weakness: dec VC, normal or raised KCO with reduced VA**
- To monitor known weakness - improving, stable or worsening
- Pre-operative assessment in neuromuscular disease
Acute
- In neuromuscular respiratory failure (GBS, myasthenic crisis), intubate on the trend, not the gas
- The "20/30/40 rule": FVC <20 mL/kg, MIP worse than -30 cmH2O, MEP <40 cmH2O -> impending failure
- Hypercapnia is a late and preterminal finding - waiting for it is the classic error
- NIV is not a substitute for intubation in bulbar weakness or rapidly progressive GBS
- Serial FVC (4-6 hourly) is the monitoring tool, not serial ABGs
- Reverse the cause: naloxone, flumazenil (cautiously), neostigmine/IVIg/plasma exchange, decompression
Chronic - domiciliary NIV
Chronic - domiciliary NIV
- Indications: symptomatic nocturnal hypoventilation, daytime PaCO2 >45 mmHg, FVC <50% predicted or MIP worse than -60 cmH2O in neuromuscular disease; obesity hypoventilation; chest wall disease; selected stable hypercapnic COPD
- In motor neurone disease, NIV improves survival and quality of life (Bourke trial) - except in severe bulbar disease, where only quality of life improves
- Cough augmentation: manual assisted cough, breath-stacking, mechanical insufflation-exsufflation when peak cough flow is low
- Vaccination, weight management, treat coexisting OSA
- Advance care planning while the patient can still communicate - tracheostomy ventilation, ceilings of care
Oxygen caution
- *Oxygen alone does not treat hypoventilation and may worsen it* (V/Q change, Haldane effect, reduced hypoxic drive)
- Target SpO2 88-92% in anyone at risk of hypercapnia - and provide ventilation, not more oxygen
Associated conditions
- Motor neurone disease, Duchenne and other muscular dystrophies, myotonic dystrophy (disproportionate ventilatory failure and central hypoventilation)
- Myasthenia gravis, Lambert-Eaton, Guillain-Barre, critical illness neuromyopathy
- Acid maltase deficiency (Pompe) - classically presents with diaphragm weakness before limb weakness
- Phrenic nerve injury - cardiac surgery, neuralgic amyotrophy, tumour
- Cervical spinal cord injury; poliomyelitis and post-polio syndrome
- Obesity hypoventilation, kyphoscoliosis, ankylosing spondylitis, thoracoplasty
- Opioid and sedative-induced respiratory depression
- Hypothyroidism, hypokalaemia, hypophosphataemia, hypomagnesaemia (reversible muscle weakness)
- Corticosteroid myopathy
Sequence of decompensation
- Vital capacity is the best single longitudinal marker in neuromuscular disease - its rate of fall predicts survival and times NIV initiation
- Sequence of decompensation: nocturnal hypoventilation in REM -> nocturnal hypoventilation throughout sleep -> daytime hypercapnia -> acute-on-chronic failure
- -> symptoms are morning headache, unrefreshing sleep, daytime somnolence, orthopnoea - ask about these before the gas is abnormal
- In MND, NIV prolongs median survival by ~7 months and improves quality of life
- Diaphragm paralysis from neuralgic amyotrophy or cardiac surgery often recovers over 6-24 months; phrenic nerve injury from tumour does not
- Complications: atelectasis, recurrent aspiration and pneumonia, cor pulmonale, sleep fragmentation, acute-on-chronic hypercapnic failure
- A weak cough kills more slowly and more certainly than a weak diaphragm
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