Measurement of the vascular pedicle width in a portable supine chest X-ray (Figure 1, page 943):
(1) Draw a vertical at the point where the left subclavian artery exits from the aortic arch (in the diagram, this is shown to be along the lateral aspect of the artery).
(2) Determine the point where the superior vena cava crosses the right mainstem bronchus.
(3) Draw a line at right angles from line in #1 to the point in #2.
(4) The length of the line in #3 is the vascular pedicle width.
A person with a vascular pedicle width > 70 mm AND a cardiothoracic ratio > 0.55 is more than 3 times likely of having a pulmonary artery occlusion pressure > 18 mm Hg than if these findings are not present.
where:
• The cardiothoracic ratio (from Chapter 6, Cardiovascular) is:
cardiothoracic ratio =
= (transverse diameter of the heart on AP view) / (transverse diameter of the bony thoracic cage)
Using the line data from Figure 3, page 945, an estimate of the total blood volume can calculated from the vascular pedicle width:
total blood volume in mL =
= (220.8 * (vascular pedicle width in mm)) - 6600
An algorithm based on the X-ray findings can be used to evaluate the patient.
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Parenchymal Infiltrate on Chest X-ray
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VPW
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Interpretation
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no infiltrate
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narrow (<= 70 mm) or decreasing
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normal vs diuresis vs bleeding
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no infiltrate
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wide (> 70 mm) or increasing
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possibly fluid overload (check input and output)
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infiltrate present
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narrow (<= 70 mm) or decreasing
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ARDS, pneumonia, hemorrhage (permeability)
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infiltrate present
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wide (> 70 mm) or increasing
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heart or kidney failure (hydrostatic)
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from Figure 8, page 948
Limitations:
• Unless care is taken to standardize the measurements, variability can occur depending on where and how the lines are drawn.
