Seeing shouldn’t always be believing

Although the numerical results are of course important, visual inspection of the volume-time and flow-volume loop graphs from a spirometry test are a critical part of interpretation. Spirometry quality and performance issues that don’t show up in the numbers are often highly evident in the graphs. Choices we make in creating and configuring reports however, can hide important visual details and have the potential to decrease interpretation quality.

Recently I was inspecting the results for a spirometry test. There wasn’t anything particularly unusual about the numbers or the graphics on the report, I just like to make spot-checks on spirometry quality and wanted to make sure the best results had been selected. When I pulled up the raw test date on my computer screen I noticed an unusual wavering pattern in the volume-time curve. I don’t remember seeing a volume-time curve like this before and when I checked all of the patient’s efforts were similar and all showed similar oscillations.


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Single-breath DLCO Breath-holding time (BHT)

The single-breath DLCO maneuver can rightly be criticized as being an artificial maneuver that bears little resemblance to normal breathing. It is only by standardizing the maneuver that clinically relevant and reproducible results can be obtained. One important aspect of this standardization is the breath-holding period.

The single-breath DLCO maneuver begins with a subject exhaling to RV, followed by an inhalation of the test gas mixture to TLC and then a 10-second breath-holding period, ending with an exhalation during which a sample of alveolar air is collected. The initial choice of a 10-second breath-hold period was largely arbitrary and was selected in order to strike a balance between being a short enough period that for most patients to hold their breath, long enough to minimize the inspiratory and expiratory phases and long enough to allow for a sufficiently measurable amount of carbon monoxide to be taken up.

During the inspiratory phase of the DLCO maneuver, carbon monoxide uptake does not begin until the inhaled gas has passed both the test system’s and the subject’s anatomic dead space and reached the first functional alveolar-capillary unit. The full rate of carbon monoxide uptake will not occur until the diffusing gas mixture has reached all available alveolar-capillary units and these units have reached their maximum surface area. The rate of carbon monoxide uptake therefore increases throughout inhalation and reaches a maximum near TLC.

During the exhalation phase, carbon monoxide uptake continues even as the alveolar sample is being taken. For this reason the concentration of carbon monoxide at the beginning of the sampling period tends to be higher than at the end of the sampling period. The size of the washout volume and the alveolar sample volume, which to some extent determines how long a patient has to exhale before the acquisition of an alveolar sample is complete, will also have an effect on exhaled gas concentrations.

Because the point at which carbon monoxide uptake starts and the point at which it ends are to some degree indeterminate, several methods for standardizing the measurement of the single-breath DLCO breath-hold period have been developed. Of these, the Ogilvie method starts measuring the breath-hold period at the very beginning of inhalation and stops at the beginning of the alveolar sampling period. The Epidemiology Standardization Project (ESP) method, on the other hand, also stops at the beginning of the alveolar sampling period but instead starts measuring at 50 percent of the inhaled volume. Finally, the Jones-Meade method starts measuring at 30 percent of the inspiratory time and stops in the middle of the alveolar sampling period.


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Assessing change over a long period of time

Because our lab database goes back 24 years, we’ve started to see a certain number of patients who had last been seen ten or even twenty years ago fairly often. For this reason I’ve been thinking about what is a clinically significant change over that long a time period. The guidelines my lab uses for interpreting change in test results came about from a consensus among the department’s pulmonary physicians close to twenty years ago. As usual there are some discrepancies between our guidelines and those the ATS-ERS have published.

Test: %Change Minimum Change:
FVC >=10% >= 200 ml
FEV1 >=10% >= 200 ml
TLC >=10% ?
DLCO >=10% >= 2 ml/min/mmHg

Our criteria came primarily from the standards for repeatability in test results. The ATS-ERS guidelines for interpretation takes repeatability into consideration but also what appears to the minimum statistically significant clinical change. For year to year changes these are:

Test: %Change
FVC >=15%
FEV1 >=15%
DLCO >=10%

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DLO2/Qc, SaO2 and CPETs

There are a number of simple observations that can be made during a cardio-pulmonary exercise test (CPET) that can point you immediately in a specific diagnostic direction. Recently I was reminded of this while reviewing the CPET results on patient with a complicated medical history whose test had been requested as part of a pre-operative assessment.

Most patients that are candidates for cardio-thoracic surgery do not need to have a CPET and that’s because it is usually straightforward to determine who is high risk and who is low risk from other routine tests. When risk is hard to determine or equivocal, the cardio-thoracic surgeons will order a CPET. They are primarily interested in the VO2 max and Ve-VCO2 slope since there are a number of widely accepted pre-op assessment algorithms that use these values. Even if the CPET results indicate the patient is high risk, the test details can help determine whether there is anything that can be done to improve the patient’s odds.

The patient whose report I was reviewing had moderately severe airway obstruction (FEV1 57% of predicted), mild restriction (TLC 77% of predicted) and a moderate gas exchange defect (DLCO 51% of predicted). This would normally pre-dispose me to look for a pulmonary vascular or pulmonary mechanical exercise limitation but there was a single test value that told me the limitation was going to be cardiovascular instead. That test value was the SaO2 at peak exercise which was 99%.

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