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