Diffusion

Objectives

• To detect diffusion defect.
• Judge severity of defect.
• Variables causing diffusion defect.
• To be able to detect reason for diffusion defect?
• To validate appropriate performance of test.

68 year old female with dyspnea at rest for 5 years.

Diffusion defect is considered present when the % predicted diffusion is less than 75%.

The 2 std deviation for normal range is 25%.

Our patient has diffusion of %, hence has a diffusion defect.

Severity of diffusion defect:

• Mild: 60%-75%
• Moderate: 40%-60%
• Severe: Under 40%

Our patient's diffusion is % of predicted. The diffusion defcet is .

Variables causing diffusion defect: Thickening of alveolar capillary membrane Low hemoglobin Dicreased alveolar surface area Dicreased pulmonary capillary bed V/Q mismatch

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The primary object of performing the test is to identify alveolar capillary membrane thickness. We have to find ways to make corrections for other variables.

• Corrected diffusion: Correction made for Hemoglobin
• DL/VA: Correction made for alveolar surface area.
• V/Q mismatch: If patient is obstructed we can assume there is a mismatch
• Capillary bed: We dont routinely evaluate this. If the lung volumes and flow indices are normal one should suspect a low diffusion as being secondary to reduced pulmonary vascular bed.

  In our patient the low diffusion is not corrected by Hemoglobin, there is no obstruction to consider V/Q mismatch and diffusion is low even when expressed by DL/VA. Hence the low diffusion in this patient is due to alveolar capillary membrane thickness.

 

 

 

 

 

 

 

 

 

 

 

 

In performing DLCO we ask the patient to inhale an air mixture containing Carbon monoxide and Helium. Helium is to obtain the alveolar surface area for that breath (single breath TLC). Patient is expected to hold the breath for 10 seconds and exhale. Technician collects the terminal portion of exhalation (alceolar air) and measures the carbon monoxideconcentration. A complex formula is then used tio calculate DLCO and VA BTPS. You find out the current Hemoglobin to make the necessary correction. In evaluating the graph note the depth of inhalation, duartion of breath holding and the timing of collection of alveolar air.

Our patient has performed the test satsfactorily.