Respiratory system

Published on 06/02/2015 by admin

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TOPIC 2 Respiratory system

Topic Contents

Imaging – Plain radiography 8

Test: The chest x-ray 8
Other imaging 14

Test: Computed tomography (CT) scan 14
Test: Ventilation-perfusion scan (VQ scan) 16
Test: Positron emission tomography (PET) scan 17
Thoracic imaging modality in the trauma patient 18
Pulmonary function tests 18

Test: Functional residual capacity 18
Test: Volume/time curve 19
Test: Flow/volume curve (dynamic) 21
Test: Transfer factor/diffusing capacity 25
Intraoperative respiratory monitoring 26

Test: Pulse oximetry (SpO2) 26
Test: Capnography 26
Test: Shunt fraction 30
Pressure-volume (P/V) curve analysis 31

Test: Static airway compliance 31
Blood gas analysis 33

Test: Arterial blood gas analysis 33

Imaging – Plain radiography

Test: The chest x-ray

Indications

A ‘routine’ preoperative chest x-ray (CXR) is not necessary unless there are specific indications (see NICE Guidelines1). Management is changed as a result of less than 1% of preoperative films, increasing as the American Society of Anaesthesiologists (ASA) score rises. Acute change in cardiac/respiratory signs or symptoms is the only undisputed indication in elective nonthoracic surgery. Similarly there seems to be no clear advantage to a ‘routine’ chest x-ray in the intensive care unit (ICU).

The chest x-ray is an essential part of the trauma series and is considered an important adjunct in the diagnosis of chest wall fractures, pneumothorax, haemothorax, and injuries to the heart and great vessels.

How it is done

The standard view is the posteroanterior (PA) film, with the patient facing the cassette and the x-ray tube 2 m away. An anteroposterior (AP) film is taken when the patient is unable to stand, with the cassette behind the patient. In the AP chest x-ray the heart will be magnified, and it is inappropriate to comment on cardiomegaly (unless it is gross) on this basis. A PA view shows the scapulae clear of the lungs whilst in AP they always overlap.

Interpretation

The chest x-ray (Fig. 2.1) is a two-dimensional representation of a three-dimensional structure. An understanding of chest anatomy is essential to interpretation of plain chest x-ray abnormalities.

image image

Fig. 2.1 Posteroanterior chest x-ray (A) with diagrammatic annotation (B). The hilum is higher on the left but the hemidiaphragm is lower. The carina is at T4 on expiration and T6 on inspiration.

Owing to the routine use of CT scanning, the lateral chest x-ray and decubitus film are now rarely seen. A lateral x-ray may differentiate structures that are unclear on PA chest x-ray. A decubitus film is taken with the patient on his or her side to assess:

(1) The volume and mobility of a pleural effusion on the dependent side
(2) A suspected pneumothorax in the nondependent side in a patient who cannot be examined erect.

Abnormalities of the lung

Consolidation – infiltration of the alveolar space by inflammatory tissue. The classic example is pneumonia – airspace disease and consolidation, which is not usually associated with volume loss. There may be an associated (parapneumonic) effusion.

The silhouette sign – an interface between isodense structures in contact with each other. The radiographic distinction between anatomical borders of lung and soft tissue can be ‘lost’ by abnormalities of the lung, which increase its density.

Air bronchogram – when the air spaces fill with pus or fluid, the alveoli fill first with the bronchi being relatively spared, therefore the bronchi stand out. This is called an air bronchogram and is a sign of airspace disease such as consolidation, pulmonary oedema or atelectasis.

Atelectasis: a linear increased density and volume loss on chest x-ray. Some indirect signs of volume loss include vascular crowding or mediastinal shift towards the collapse. There may be compensatory hyperinflation of adjacent lobes, or hilar elevation (upper lobe collapse) or depression (lower lobe collapse).

Interstitial disease

The interstitial space surrounds bronchi, vessels and groups of alveoli. Disease in the interstitium manifests itself by reticulonodular shadowing (criss cross lines or tiny nodules or both). The main two processes affecting the interstitium are accumulation of fluid (pulmonary oedema) and inflammation leading to fibrosis (Fig. 2.2 and Box 2.1).

image

Fig. 2.2 Pulmonary oedema: diffuse hazy shadowing in a bat-wing distribution and upper lobe blood diversion.

Box 2.1 The most common causes of interstitial fibrosis are:

• Idiopathic (IPF, >50% of cases)
• Collagen vascular disease
• Cytotoxic agents and nitrofurantoin
• Pneumoconioses
• Radiation
• Sarcoidosis

Pulmonary oedema may be cardiogenic or noncardiogenic. In congestive heart failure, the pulmonary capillary wedge pressure (PCWP) rises and the upper zone veins dilate – this is called upper zone blood diversion. With increasing PCWP, interstitial oedema occurs with the appearance of Kerley B lines and prominence of the interlobar fissures. Increased PCWP above this level causes alveolar oedema, often in a classic perihilar ‘bat wing’ pattern. Pleural effusions also occur. Unusual patterns may be found in patients with chronic obstructive pulmonary disease (COPD) who have predominant upper lobe emphysema.

A helpful mnemonic for noncardiogenic pulmonary oedema is NOT CARDIAC: near-drowning, oxygen therapy, transfusion or trauma, CNS disorder, ARDS, aspiration, or altitude sickness, renal disorder, drugs, inhaled toxins, allergic alveolitis, contrast or contusion.

COPD is often seen on CXR as diffuse hyperinflation with flattening of diaphragms and enlargement of pulmonary arteries and right ventricle (cor pulmonale). In smokers the upper lung zones are commonly diseased.

Abnormalities of the mediastinum

On the PA film, the heart takes up to half of the total thoracic measurement in adults (more in children).

In mediastinal deviation, tension pneumothorax and pleural effusion push the mediastinum away; lung collapse pulls it towards the affected side.

Findings for pneumomediastinum include streaky lucencies over the mediastinum that extend into the neck and elevation of the parietal pleura along the mediastinal borders.

Causes of pneumomediastinum include:

• Asthma
• Surgery (postoperative complication)
• Traumatic tracheobronchial rupture
• Abrupt changes in intrathoracic pressure (vomiting, coughing, exercise, parturition)
• Ruptured oesophagus
• Barotrauma
• Crack cocaine.

Pericardial effusion causes a globular enlarged heart shadow. More than 400 mL of fluid must be in the pericardium to lead to a detectable change on plain x-ray. If it is chronic then there may be little functional impairment. An echocardiogram is indicated to detect impairment in right ventricular diastolic filling and to guide drainage.

Abnormalities of the hila and pulmonary vessels

Enlarged hila could be due to an abnormality in any of the three structures that lie there:

• The pulmonary artery (e.g. pulmonary hypertension or pulmonary embolus)
• The main bronchus (e.g. carcinoma)
• Enlarged lymph nodes (e.g. sarcoidosis).

Pulmonary embolism (PE)

Most chest x-rays in patients with a PE are normal (Box 2.2). The primary purpose of a chest film in suspected PE is therefore to rule out other diagnoses as a cause of dysponea or hypoxia. Further imaging is indicated, such as V/Q scan, pulmonary arteriogram and CT pulmonary angiogram (CTPA).

Box 2.2 Chest x-ray signs of pulmonary embolus may include:

• Westermark’s sign (oligemia in affected area)
• Enlarged hilum (caused by thrombus impaction)
• Atelectasis with elevation of hemidiaphragm
• Pleural effusion
• Consolidation

Pleural abnormalities

Pleural effusion

On an upright film, an effusion will cause blunting of the costophrenic angle. Sometimes a depression of the involved diaphragm will occur. A large effusion can lead to a mediastinal shift. Approximately 200 mL of fluid is needed to detect an effusion in the frontal film – a lateral chest x-ray (though now rarely done) is more sensitive.

In the supine film, an effusion will appear as a graded haze that is denser at the base. To differentiate it from lung disease, vascular shadows can usually be seen through the effusion.

Common causes for a pleural effusion include:

• Congestive heart failure
• Infection (parapneumonic)
• Trauma
• PE
• Tumour
• Autoimmune disease.

Pneumothorax

A pneumothorax is air inside the thoracic cavity but outside the lung. It appears as air without lung markings in the least dependent part of the chest. It is best demonstrated by an expiration film. It can be difficult to see when the patient is in a supine position – air rises to the medial aspect of the lung becoming a lucency along the mediastinum. It may also collect in the inferior sulci causing a deep sulcus sign (Fig. 2.3).

image

Fig. 2.3 Left-sided pneumothorax with a deep sulcus sign.

Causes of spontaneous pneumothorax include:

• Idiopathic
• Asthma
• COPD
• Pulmonary infection
• Neoplasm
• Marfan syndrome.

A hydropneumothorax is both air and fluid in the pleural space. It is characterized by an air–fluid level on an upright or decubitus film in a patient with a pneumothorax.

Further investigations

Computed tomography (CT) scan, ventilation/perfusion (VQ) scan, etc (see below).

Limitations and complications

• To be adequate, a chest x-ray must be:

– Taken on inspiration (unless suspecting a pneumothorax) – on good inspiration the diaphragm should be at the 8th to 9th posterior rib or the 5th to 6th anterior rib
– Adequately penetrated – thoracic disc spaces should be visible through the heart but bony details should not
– Not rotated – medial heads of the clavicles equidistant from vertebral bodies.

• Ionizing radiation is relatively contraindicated for women who might be pregnant. See below for radiation doses.

Other imaging

Test: Computed tomography (CT) scan (Fig. 2.4)

Indications

• Identification of structural thoracic pathologies, including abnormalities of the pulmonary vasculature, lung parenchyma and mediastinum.
• CT pulmonary angiogram (CTPA) is the investigation of choice for diagnosis of nonmassive pulmonary embolus.
image

Fig. 2.4 A CT scan consistent with acute lung injury.

How it is done

• A beam of x-rays is passed through the chest and picked up on iodide crystals within a detector. These iodide crystals emit photons when struck by x-rays, which are detected by a photomultiplier.
• The x-ray tube passes around the patient, allowing multiple data to be collected for each section of the chest being viewed. This data is converted by a computer into two- or three-dimensional images.
• Computed tomography can be given with intravenous contrast to highlight the pulmonary arterial tree. This is a CTPA. It can only show central emboli and gives a large dose of radiation to the patient.

Data presented as

• CT scans are viewed ‘from the feet up’ with the patient on their back.
• As with a conventional x-ray, denser tissues are paler.

Limitations and complications

• CT scans give large doses of radiation to the patient (Box 2.3).
• The CT scanner is often in a distant part of the hospital and considerable logistic difficulties arise when transporting a critically ill patient there.
• Medical staff may receive a radiation dose if a patient cannot be left alone in the CT scanner.
• Visualization of the pulmonary vasculature requires intravenous contrast, which is contraindicated in patients with iodine allergy and may worsen renal impairment, especially in patients taking metformin.
• Lesions less than 1 cm in size may be missed.
• CT scans without contrast cannot differentiate between structures of very similar density.

Box 2.3 Radiation dose (millisieverts)

Annual background 2.5–3
Chest x-ray 0.1
Abdominal x-ray 2
CT head 2
CT chest 8
CT abdomen 10

Test: Ventilation-perfusion scan (VQ scan) (Fig. 2.5)

Indications

• Aims to detect mismatches in lung ventilation and perfusion, such as PE.
• Although CTPA is the tool of choice for detection of PE, there is a lower radiation dose for VQ scans than for CTPA, making it the investigation of choice for the pregnant patient with suspected pulmonary embolus.
image

Fig. 2.5 A ventilation/perfusion scan showing multiple perfusion abnormalities. This is suggestive of several pulmonary emboli.

How it is done

• The pulmonary arterial circulation is viewed using intravenous technetium-99, which fixes in the distal pulmonary capillaries showing where blood is flowing. At the same time, the patient breathes a radioactive gas, usually krypton-81, which shows ventilation in the lungs.

Interpretation

Normal range

• A normal scan shows the same appearance in both the ventilation and perfusion images, showing no mismatch.

Abnormalities

• A PE obstructs distal blood flow and so there are defects in perfusion. This is seen on a ventilation-perfusion scan as a perfusion mismatch, there being no obstruction to ventilation.

Further investigations

• A CTPA may help with diagnosis if the VQ scan is equivocal.

Limitations and complications

• A VQ scan may suffer from poor specificity: any abnormality in lung tissue will affect ventilation and perfusion to some extent. A VQ scan will therefore only detect PE in otherwise fairly normal lungs.
• This investigation exposes the patient to a dose of ionizing radiation, albeit small.
• This investigation cannot usually be done out of hours.

Test: Positron emission tomography (PET) scan

Indications

• Detection of tumours and bony deposits.

How it is done

• PET uses deoxyglucose, a glucose analogue labelled with fluorine-18, which is taken up avidly by malignant cells. Fluorine-18 emits positrons, which combine with electrons to produce photons.

Further investigations

• Staging investigations and tissue diagnosis directs subsequent management.

Limitations and complications

• The (albeit small) dose of radiation given makes this examination relatively contraindicated in pregnant women.
• Because of its reliance on normal glucose metabolism, a PET scan can be unreliable in diabetic patients or those with insulin abnormalities.

Thoracic imaging modality in the trauma patient

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