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Effects of breath holding technique on DLCO results.

Of all the tests in the pulmonary function laboratory, the single breath diffusing capacity using carbon monoxide, DLCO, is the test most prone to inter and intra laboratory error and variation due to both faulty technologist technique and to general difficulties and complexities in measurement. Having had our diffusing capacity tested in a large number of pulmonary function laboratories, it is very alarming that we found that inter-laboratory DLCO results varied by up to 60% through out our State of California, in hospitals as well as in private physician offices. In this paper we would like to describe factors, excluding gross technologist error, which may help explain this inter laboratory variation. Fifteen years ago the California Thoracic Society, CTS, initiated and carried out a pilot voluntary certification program for California pulmonary function laboratories which involved individual teams, consisting of a pulmonologist and pulmonary technologist, traveling together on site visits to pulmonary function laboratories, which had volunteered, and checking staff qualifications as well as all calibration and quality assurance procedures. If standards had been found to be met, then the laboratory was certified as having met minimal CTS defined laboratory standards for pulmonary function testing. I would like to refer to these particular standards here in a succeeding paper. This was a short term pilot study and the CTS decided to not continue or expand the pilot certification program for pulmonary function laboratories due to financial considerations. This pilot study had been initiated in response of the inter-laboratory variance in DLCO results found in laboratory controlled subjects as described above. The American Thoracic Society, ATS, has discussed establishing a similar pulmonary function laboratory credentialing project on the national level but has not acted. It is clear that the need for such a project has not dissipated.

As all of you know, performing accurate DLCO measurements is an art and probably the most difficult and demanding test we perform in the pulmonary function laboratory. There are many sources of error inherent in testing a patient's DLCO, including variation in breath holding time, low inspiratory capacity and variable inspiratory flow rates, insufficient time between maneuvers, improper hemoglobin and carboxyhomoglobin corrections, choosing inappropriate equations or nomagrams for predicted values, patients cardiac output status, predicted equations and calculations, altitude considerations, test gas quality and tank pressure, demand valve resistance, gas analyzer accuracy, valve integrity and function, among others,

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There are many other factors that influence DLCO. Age, gender, and height are independent factors that influence DLCO and of course must be entered accurately. Height should always be measured, with the patient in bare feet, by the technologist as patient reported heights are invariably inaccurate. Obesity is not a predictor variable until the weight-to-height ratio (kilometers per centimeter) exceeds 1.0. Body position also affects DLCO. It has been reported that changing from a standing to sitting position produces a 10% to 15% increase in DLCO, and moving from a sitting to a supine position increases DLCO by 15% to 20%. The reported variation in DLCO from morning to evening appears to be due to the minor decrease in the morning hemoglobin (Hb) concentration and the rise or back pressure of carboxyhemoglobin (COHb) often becomes a factor only when testing is repeated with more than four attempts. Having very recently exercised leads to a rapid increase in DLCO by increasing pulmonary capillary blood volume and cardiac output. There is also a temporary increase in DLCO during the first trimester of pregnancy, presumably due to increased blood flow. Hemoglobin concentration will affect DLCO: polycythemia causes an increase, anemia causes a decrease and in both instances, final DLCO values must corrected to normal. Elevations in COHb decrease DLCO by creating CO back-pressure in venous blood and by reducing available Hb binding sites (due to the high affinity of CO to Hb). Because PACO changes as a function of altitude, an adjustment in the measurement and calculation of DLCO is recommended for laboratories at significant altitudes above sea level.

It is clear to us that there is an additional, but mostly ignored, consideration, which has been previously reported by Smith and Rankin in the Journal of Applied Physiology, vol. 7, no. 6, Dec, 1969, and which can have a significant effect on a patient's DLCO results, I am referring to the manner in which the patient breath hold is maintained during the DLCO testing maneuver. On most pulmonary function testing machines, after the patient inspires to total lung capacity with the test gas, a manual or automatic valve can be selected to close so that the patient's held breath rests tightly against a valve or obstruction. Another technique is to instruct the patient to maintain the breath hold by resting against a closed glottis. Some patients can be seen actually bearing down during the breath hold, as in a valsalva maneuver, which has the effect of compromising the capillary blood flow, and inhibiting increased gas exchange, decreasing DLCO. A third technique is to maintain the breath hold with no help at all. In other words, asking the patient to hold the inspired volume in place solely through their own inspiratory muscle strength. It has been found by this laboratory, that maintaining the breath hold with no valve or by not resting against a closed glottis can easily result in consistently increased DLCO results of at least five percent. With some testing devices in an open glottis breath hold, occasionally a patient may inhale slightly during the breath hold which may also affect the outcome by adding ambient air to the mix but we have found this not to be a factor.

There a few theories which might explain the higher DLCO results with the patient's physical maintenance of a ten second breath hold. The resultant increased lung inflation, helped by the patient maintaining the breath hold, may be a partial cause of the increased DLCO values since the maneuver tends to result in expanded lung volume and alveolar filling. Resting against the valve or the patient's epiglottis has the effect of decreasing blood flow during the breath hold as in a type of valsalva maneuver. With less pulmonary capillary blood flow, the DLCO will be significantly decreased. By having the patient maintain an inspired breath hold there may be an increased capillary blood flow and an increased recruitment of alveoli resulting in a higher DLCO.

High negative inspiratory pressure as produced by the patient's effort, regardless of breath holding method., seems to increase DLCO measurement results. This increase might be caused be increased capillary recruitment during a strong inspiratory pressure maneuver just as mild exercise state can produce the same effect. This has been reported in a study done by D.J. Cotton and al, in Respiration Physiology, vol. 54, Oct., 1 983, and might be explained by the pressure-volume relationship of the normal pulmonary capillary bed. On the other hand significant mechanical resistance against the patient's inspiratory effort has been shown to decrease DLCO results. This effect is similar to the, so called, Muller's maneuver which is a strong inspiratory effort against a closed airway or glottis. The effort decreases intrapulmonary and intrathoracic pressures and expands pulmonary gas. It is used during fluoroscopic examination to help visualize esophageal varices because it also causes engorgement of intrathoracic vascular structures. The overall rate or speed of the inspiratory effort is also an important factor in producing accurate DLCO results. Within the ten second breath hold, if there is a slow inspiratory rate, less time is available for alveolar volume diffusion. This may be especially significant since the alveolar rate of diffusion is greatest in the first half of the ten second breath hold. The expiratory flow rate is also a factor in that severe airway obstruction also results in less alveolar diffusion time.

With the possible significant effect of breath holding technique on a patient's DLCO results, it is important that the entire DLCO maneuver be done in a consistent fashion from test to test and patient to patient. Perhaps recommendations might be made in the future as to the best and most accurate overall method of breath holding. And of course, predicted values should be constructed using only one single breath hold technique. It is clear that there are many other factors which affect DLCO measurements and the extremely high inter-laboratory variability might in the future be addressed by some program of pulmonary function laboratory testing or certification through a national professional organization, as apposed to the example of Clinical Laboratories which are regulated by both state and national standards. It's the example of: "If we don't do it, they will do it for us."

Jim Harvey MS, RPFT, RCP works in the Pulmonary Function Laboratory at Stanford Hospital and Clinics in Palo Alto, and teaches. Pulmonary Function at Skyline College in San Bruno, California.

by Jim Harvey MS, RPFT, RCP & Sue Siegel RPFT, RRT, RCP
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Title Annotation:PULMONARY FUNCTION TESTING
Author:Harvey, Jim; Siegel, Sue
Publication:FOCUS: Journal for Respiratory Care & Sleep Medicine
Geographic Code:1USA
Date:Sep 1, 2010
Words:1479
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