Case Studies

Case 4

Two 60-year-old patients are being evaluated for dyspnoea. On examination, both patients have an oxygen saturation of 88%, small lung volumes to percussion and normal cardiac examinations. Patient A has diffuse bilateral inspiratory crackles and digital clubbing. Patient B has clear lungs and difficulty in rising from his chair and raising his hands over his head.

• 1. What patterns of abnormalities do these patients exhibit?

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  Correct answer:
  Both patients have restrictive ventilatory defects based on the reduced TLC. FEV1 and FVC are reduced proportionally, so the FEV1/FVC is normal; therefore, there is no obstructive ventilatory defect. Patient A has reduced D_LCO, signifying a gas transfer defect.

• 2. Based on the lung function results, what is the most likely pathophysiology explaining each patient’s symptoms?

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  Correct answer:
  Restrictive ventilatory defects may be due to stiff lungs, stiff chest wall or weak respiratory muscles. Diseases causing stiff lungs will reduce all lung volumes/capacities simultaneously, including TLC, FRC and RV. Most parenchymal lung diseases will also cause a reduced D_LCO, whereas chest wall disease and respiratory muscle disease will not. An increased RV is also not compatible with stiff lungs. Thus, Patient A seems to have a problem with stiff lungs. The relatively normal FRC and D_LCO in Patient B suggest that the lungs and chest wall are normal. Either a stiff chest wall or weak muscles may cause an increased RV. Patient B’s lung function and difficulty in rising out of a chair and raising his arms suggest a muscle disease. Diseases causing weak respiratory muscles will reduce TLC, because the patient cannot inspire deeply.

• 3. What is the likely explanation for the differences in FRC and RV between the two patients?

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  Correct answer:
  The FRC is determined by the balance between the inward pull of the lung elastic recoil pressure and the outward pull of the chest wall. Therefore, FRC will be reduced either if the net lung recoil pressure increases (due to stiff, low-compliance lungs) or if the net outward pull of the chest wall decreases (e.g. when scarring of the chest wall produces an added inward recoiling force). Since FRC is determined by the balance between two opposing static forces, respiratory muscle weakness should not influence FRC. However, in clinical practice, patients with respiratory muscle weakness often have a slightly reduced FRC. The mechanism for this finding is probably related to the lack of deep breaths or sighs causing microatelectasis that will increase lung recoil and decrease compliance.RV is the amount of gas remaining in the lung at the end of maximal expiration. In adults, RV is determined by airway collapse at low lung volumes. However, this presupposes adequate expiratory muscle strength to actively lower lung volume below FRC (which can be reached from TLC passively). Patient A has normal muscle strength and airways that resist collapse due to the parenchymal lung disease, resulting in a reduced RV. Patient B has weak expiratory muscles, resulting in an elevated RV.

• 4. What is the differential diagnosis for Patient A?

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  Correct answer:
  The differential diagnosis is long and includes the disorders discussed in Chapter 30. Briefly, these would include occupational/environmental disorders, connective tissue/autoimmune diseases, drug/treatment-induced diseases, primary lung disorders or idiopathic disorders. Idiopathic pulmonary fibrosis or cryptogenic fibrosing alveolitis is likely in a 60-year-old with lung crackles, clubbing, no significant past history, no signs or symptoms of extrapulmonary disease and the lung function shown for Patient A.

• 5. What is the differential diagnosis for Patient B?

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  Correct answer:
  Respiratory muscle weakness may be due to a variety of neuromuscular diseases that can involve the spinal cord, motor nerves, neuromuscular junction or skeletal muscles:

  • Spinal cord: tumour, syringomyelia, polio, amyotrophic lateral sclerosis, tetanus;
  • Motor nerves: brachial/phrenic nerve neuritis, trauma;
  • Neuromuscular junction: myasthenia gravis, botulism, organophosphate poisoning;
  • Skeletal muscle: muscular dystrophy, myositis, mitochondrial disease, myopathy (nutritional, drug, metabolic, inherited).

• 6. Both patients have hypoxaemia. Which patient is more likely to have hypercapnia?

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  Correct answer:
  Hypoxaemia in interstitial lung diseases is usually due to ventilation–perfusion mismatching. Patient A likely has hypocapnia because patients with interstitial lung disease tend to hyperventilate in response to stiff lungs and hypoxia. This will lower arterial carbon dioxide tension. In contrast, Patient B likely has hypoxaemia due to alveolar hypoventilation, and is therefore hypercapnic. Patient B is also more likely to develop acute respiratory failure with the limited ventilatory reserve due to the muscle weakness.