Heart and mediastinum

· two thirds of the cardiac shadow lies to the left of the midline and one-third to the right

· the cardiac shadow is normally approximately one third of the chest cavity

· serial X-rays should be checked for signs of a widening mediastinum (sign of possible tamponade) and check for signs of mediastinal shift.

Lung fields

· lung markings of normal lungs are vascular shadows, which are visualised as branching grey linear shadows extending bilaterally to the edge of the chest wall.

· check that the lungs have re-expanded, and that there is no pneumothorax ( black area with no lung markings frequently visible in the apex) or haemothorax (white area with a defining line usually visible in the bases)

· increased opacities may denote pulmonary oedema, infection, collapse of lung or pleural fluid

Pleural space

A potential space only, so should not be visualised. Check for fluid or air build up in this space.

Chest tubes

Ensure no kinking of tubes

Diaphragm and stomach

Both hemidiaphragms are rounded, with the right hemidiaphragm about 2cm higher than the left.

Abnormalities

· the hemidiaphragm shadows may be obliterated indicating the adjacent tissue has become non air containing (pulmonary oedema)

· diaphragm displaced upwards (pulmonary collapse)

· diaphragm displaced downwards (pneumothorax or hyper-inflated lung)

Soft tissue

Observe for any swelling or signs of surgical emphysema

Chest X-ray interpretation

Chest X-ray interpretation

X-rays are a form of radiant energy, whose wavelength can penetrate substances that are opaque to light rays. When passing through an object they are variably absorbed, depending on the radio density of the object.

Daily portable chest x-rays are used to monitor the status of postoperative cardio- thoracic surgical patients. Additional X-rays are also required following invasive line insertion and post intubation. Second or third daily X-rays may be indicated for longer-term patients.

· identification and prompt rectification of pulmonary and cardiac conditions

· ensures correct placement of invasive lines and devices.

· adjunct to ongoing respiratory therapy

Positioning of patient

Positioning can affect the quality of the X-ray. Any change in how the patient, X-ray, tube or film is positioned alters the angle and distorts the appearance of the chest organs on the film.

A portable Anterior - Posterior chest X-ray shows the patient’s heart larger .It is not as sharply defined as it appears on a Posterior- Anterior film taken in the X-ray department, because the heart is further away from the film plate and appears magnified in the portable A/P film.

Cardiac patients

X-rays are taken in the supine position while intubated then in the upright position once extubated and stable.

Thoracic patients

X-rays are taken in the upright position once the patient is fully conscious post-operatively.

Technical interpretation

Once the portable X-ray is returned to the unit

· check the name and date is correct

· place the left side of the X-ray to your right on the viewing box

· assess the degree of penetration. The vague outline of the thoracic vertebra should be just visible behind the heart shadow

· make sure the patient is not rotated on the film. Rotation distorts the mediastinal border and makes the right and left lung fields appear different

· check whether there has been adequate inspiration. Good inspiration exposes the 6^th rib anteriorly at the middle of the R) hemi-diaphragm

· compare with the patient’s old films

Guidelines for Chest X-ray interpretation

Devices

· Endotrachael tube - should have their tip 2cm above the carina. Too far down will cause a (R) main bronchus intubation and possible (L) lung collapse

· Tracheostomy tube - should end well above the carina, and above the mid clavicular junction

· CVP Catheter – the tip should be positioned in the Superior Vena Cava, approximately to the right of the middle of the aortic arch

· Swan Ganz Catheter - will be visible through the right atrium and the right ventricle with the tip of the catheter placed about 5-6 cm beyond the bifurcation of the main pulmonary artery. It should not be seen beyond the mediastinal shadow

· IABP catheter - the tip should be below the level of the aortic notch and at approximately the 4^th intercostal rib

· Naso-gastric Tube - visible in the stomach

· Pleural tubes - visible in the pleural cavity

· Temporary pacemaker wires - A thin lead visible at its attachment to the outside of the atrium or ventricle.

· Permanent pacemaker - if placed transvenously (often via subclavian vein) then it is positioned in the right ventricle with the tip seen at the apex of the heart in the AP view. Wires are thicker for permanent pacemakers and easily visible in proximity to the heart.

CT scan & MRI of heart

Trans-oesophageal echocardiography

Particularly useful for evaluating mitral valve function. Usually performed in theatre to evaluate mitral valve function pre and post mitral valve repair.

Computed tomography

CT scanning is frequently used to exclude post operative pulmonary emboli (spiral CT of the chest), determine presence of thoracic aortic aneurysms, and confirm or exclude neurological sequalae of cardiac surgical procedures (embolic, haemorhagic, hypoperfusion brain injury). Refer to chapter five (Transport of the critically ill) for safe management of patients requiring this procedure. CT uses thin, fan shaped X-ray beam that pass through the patient. The number of X-rays deleted from the beam is directly proportional to the density of the tissue in its path. Contrast media CT scanning can also be used although special precautions must be followed as contrast media is used (renal impairment, allergy).

Cardiac catheterisation

Cardiac catheterisation involves passing a radiopaque catheter via an artery or vein into the chambers of the heart to obtain chamber pressures, ventricular function and wall motion. Coronary angiography examines abnormalities in coronary artery blood flow via injection of contrast medium directly into the vessels.

Right heart catheter is performed to evaluate:

· valvular disease

· congenital heart disease

· pericardial tamponade

· electrophysiological studies

Left heart catheter

· evaluation of aortic and mitral valve disease

· measurement of left ventricular function

Coronary angiography

· unstable angina

· atypical chest pain

· primary PTCA within first hour of AMI

· to check patency of coronary artery grafts in selected patients with threatened AMI post operatively. Refer to chapter five (Transport of the critically ill) for safe management of patients requiring this procedure.

Magnetic resonance imaging

A non-invasive test that provides high resolution tomographic images. Indications include abnormalities of the aorta, gross patency of myocardial revascularisation conduits and pericardial disease. MRIs generate strong magnetic fields and therefore patients must remove all metal jewellery and appliances before scanning. Very small amounts of metal such as surgical haemoclips and staples are not adversely affected. Larger permanent devices such as pacemakers, physiological support equipment such as ventilators and infusion pumps are affected. The cardiothoracic unit does not have appropriate equipment to transport ventilator dependent patients to the MRI department and the procedure should be co-ordinated with the general ICU.