a) Give the differential diagnosis for hypercapnic respiratory failure.
 
b) Outline features from the clinical examination that assists in making a diagnosis/diagnoses.

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College Answer

Differential Diagnosis:
1. CNS:
1. Drugs (prescription, illicit, deliberate ingestion/OD)
2. Brain stem lesion
3. Any intra cranial lesion with mass effect (haemorrhagic stroke or traumatic brain injury),
4. Central Sleep Apnoea
2. Spinal Cord:
1. Cervical spinal cord injury/tumour
3. Peripheral Neuro-muscular:
1. Polio, MND, Guillan Barre, Myopathies, Myasthaenia Gravis
4. Chest wall:
1. Kyphoscoliosis, ankylosing spondylitis
2. Obesity-hypoventilation syndrome
5. Respiratory:
1. Asthma/COPD
2. Obstructive sleep apnoea
3. Re-breathing / increased dead space
6. Cardiovascular:
1. Acute severe left heart failure

Clinical examination
1. Neurological:
a. Cranial nerves
b. UMN and LMN signs.
2. Chest wall and rib cage mechanics:
a. Evaluation of thoracic cage component
b. Effect of obesity on ventilation
3. Respiratory:
a. Signs of acute on chronic bronchospasm, chronic lung disease
4. Cardiovascular:
a. Signs of Cor Pulmonale
b. Signs of Left heart failure (dilated cardiomyopathy/valvular heart disease)

Discussion

Causes of Hypercapnia
Decreased minute ventilation

Central nervous system

  • Drugs affecting respiratory drive, eg. opiates
  • Brainstem or cortical lesion affecting consciousness
  • Central sleep apnoea
  • Spinal cord injury

Neuromuscular

  • Neuropathy, eg. Guillain-Barre
  • NMJ disorder, eg. myasthenia gravis
  • Myopathy

Respiratory

  • Decreased lung compliance, eg. pulmonary oedema
  • Decreased chest wall compliance, eg. kyphosis or obesity
  • Increased airway resistance, eg. COPD or asthma

Metabolic, endocrine and environmental

  • Metabolic alkalosis
  • Hypothyroidism
  • Hypothermia
Increased dead space

Increased anatomical dead space

  • Unusually long ventilator circuit (eg. while down in MRI)

Increased alveolar dead space (i.e. ventilated but not perfused)

  • Bullous emphysema, COPD
  • Interstitial pulmonary fibrosis
  • Large pulmonary embolism
Increased CO2 production

Increased metabolic rate

  • Hyperthermia (including malignant hyperthermia)
  • Hyperalimentation
  • Hyperthyroidism
  • Seizures, status epilepticus
   

Features of clinical examination that assist in making a diagnosis:

Observation:

  • Obesity
  • Short fat neck of OSA
  • Cushingoid appearance (OSA, but also suspicious of long term steroids for some sort of autoimmune condition, or COPD)
  • Wasting and cachexia of severe CCF, end-stage COPD or cancer
  • Abnormal breathig pattern (eg. the abdominal breathing of a C-spine quad)

Start with the hands.

  • Clubbing (suggestive of chronicity)
  • Cyanosis
  • Unilateral small muscle wasting (lung mass invading brachial plexus)
  • Pulse (collapsing pulse of AR?)

Axillae and neck

  • Lymph nodes
  • JVP (cardiac causes of ventilation failure)
  • Dissection scars from lymph node clearance; radiotherapy tattoos

Face and cranial nerves

  • Plethoric "mitral facies"
  • Droop, cranial nerve signs of stroke
  • Horner's syndrome (malignancy or stroke)
  • Temporalis wasting (malnutrition)

Chest

  • Abnormal chest wall movement (eg. flail segment or unilateral phrenic nerve paralysis)
  • Subcutaneous emphysema on palpation, suggestive of pneumothorax
  • Percussion findings (eg. dullness of an effusion)
  • Auscultation findings of wheeze or creps (spasm or APO)

Abdomen

  • Recent abdominal wounds (is pain or infection preventing diaphragm excursion?)
  • Distension (Gas? Poop? Ascites?)

Lower limbs

  • Oedema of CCF or prolonged bed stay
  • Muscle wasting of quads (another feature of malnutrition)