Monthly Archives: October 2012

Peak VO2 and low body weight

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The PFT Lab I am associated with performs cardiopulmonary exercise tests for pre-op cardiothoracic surgery patients with lung cancer. Surgeons have to make a decision to operate or not based at least in part on the amount of function they think will remain after a lung resection and this depends on which and how many lobes are involved. This calculation is done by taking a percentage based on the amount of lung tissue that will be lost which is weighted using ventilation-perfusion scan results off the baseline DLCO and vital capacity. When the result is inconclusive or there are other medical factors that can affect a patient’s prognosis the results from a CPET can help with the surgeon’s decision.

The criteria that has been most often cited as an indicator of acceptable surgical risk is a peak VO2 greater than 15 ml/kg/min. About ten years ago we encountered a patient with a significantly low body weight. His max VO2 was about 16 ml/kg/min which appeared to make him a good candidate for surgery but his actual maximum oxygen consumption in LPM was 45% of predicted. This discrepancy was due to the fact that his BMI was about 14. We re-calculated what his peak VO2 would have been if his body weight had been normal and that was less than 10 ml/kg/min. For this reason our recommendation at that time was that the patient was a poor candidate for surgery. Continue reading →

Poor quality test results. When do you accept them?

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The ATS/ERS have published standards for most Pulmonary Function tests. These standards include the criteria that are used for assessing test quality. What these standards don’t address however, is how poor a test’s quality can be yet still be acceptable for reporting.

We’ve all had patients that aren’t able to perform tests correctly. There can be many reasons why this happens. Lack of comprehension due to cultural or language barriers. Dementia. Extreme shortness of breath. Fatigue. Neuromuscular disease. Whatever the reason, you’ve done the tests with a patient and gotten the best you think you can get, how bad do they have to be before you don’t report them? Continue reading →

What do you say to your patients and how do you say it?

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Pulmonary Function testing is almost unique among medical tests in that it requires maximum effort and cooperation from the patient for quality results. The better you are able to communicate with a patient the more likely you are to be able to get good quality test results. This is one of the more interesting and the more difficult parts of being a technician.

Performing tests means being a cheerleader to some extent and over the years I’ve seen a number of different approaches. At one extreme, which thankfully I haven’t seen for a while, is what I call the “Nazi” approach which is mostly yelling and telling the patient as condescendingly as possible that they aren’t doing the test right and to do it again. At the other extreme is the “Flower Child” who is chatty, sympathetic, cheerful and can never, ever bring themselves to tell the patient they are doing anything wrong.

Neither extreme can consistently get good quality tests out of patients because neither makes an effort to explain what the tests are for, how they need to be performed and most importantly, to lead and correct the patient in a way that makes sense to the patient. The three most important rules in patient communication is first to explain everything, second to keep explaining and finally to explain some more. I’ve always found that every bit of time I spent explaining what, how and why always saved double that when it came to performing the tests.

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DL/VA is really K in disguise

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One misconception about DLCO results that I’ve heard over and over is that DLCO/VA is DLCO “adjusted” for lung volume and that using it is a way of “normalizing” DLCO for lung volume. This is not true and the information that DL/VA can give you about a patient’s lung without also considering VA as well is very limited and can be misleading.

WARNING, MATH AHEAD!

The equation for calculating DLCO is: 

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Evaluating spirometry predicted equations for the elderly

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I’ve mentioned previously that the PFT Lab I am associated with recently went through a major hardware and software update. As part of this update we decided to change spirometry predicted equations to NHANESIII. The lab has used the Morris equations for at least the last 25 years and this has caused us to revisit a number of issues associated with interpretation of results, one of which is age.

The software update included the NHANESIII equation set but when we selected it we found that the software would not calculate predicted values for patients over the age of 80. The manufacturer said that this was because that was as far as the age range went in the original NHANESIII study and for this reason they could not extend it. Furthermore, their recommendation was to use the Crapo or Knudsen equations for ages above 80 because they were “more linear”. Continue reading →

Is the MVV clinically relevant?

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The respiratory system is in part a mechanical pump or bellows. The Maximum Voluntary Ventilation test (MVV, aka Maximum Breathing Capacity, MBC) is intended to measure the maximum ventilation a patient is capable of. As such the results are dependent on a patient’s lung volume, respiratory muscle strength and endurance, airway resistance and overall inertia of the thoracic cage.

When I started doing PFT’s in the early 1970’s the MVV was a standard part of a complete workup. This has long since changed and I have not performed the MVV test routinely in over 25 years but I’ve always wondered what the MVV test is actually supposed be measuring in a clinical sense.

The ATS/ERS statement on spirometry recommends that the MVV test be 12 seconds long and that for optimum results the patient’s tidal volume should be approximately 50% of their VC at a respiratory rate of 90 breaths per minute. Tidal volume is accumulated during exhalation and at the end of the test the accumulated volume is then multiplied by 5. The ATS/ERS statement also suggests that MVV values that are less than FEV1 (L) x 40 indicate suboptimal patient effort.

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Supine spirometry

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If you are not already performing supine spirometry you should consider adding it to your arsenal of tests. Other than an exam table no new equipment is needed to perform it and it is a simple technique that can provide useful information towards diagnosing and monitoring diaphragmatic dysfunction. It is non-invasive when compared to a transdiaphragmatic pressure test (which requires an esophageal balloon), does not require ionizing radiation (fluoroscopy) and is likely more accurate and better tolerated than MIP and MEP tests. Candidates for this test include patients with neuromuscular diseases, suspected or known diaphragmatic paralysis or any patient complaining of dyspnea that cannot be explained by other routine testing.

Vital capacity is dependent on a number of factors, an important one being the range of motion of the diaphragm. The initial position of the diaphragm and the distance it can move is determined by effect of gravity on the abdomen and its contents. For this reason vital capacity is greatest when performed in the upright position and lower when performed in the supine position.

Persons with normal lung function usually see a decrease in FVC from upright to supine of about 3% to 8%. In individuals with diaphragmatic dysfunction this decrease is usually over 10%. Patients with unilateral diaphragmatic paralysis tend to have a decrease of at least 15% whereas those with bilateral diaphragmatic paralysis tend to have decreases greater than 25%.

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Lung volume measurements and software smarts.

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Many years ago I used to think that lung volume measurements were the easy part of PFTs. As time has gone on I’ve seen that getting accurate lung volume measurements is actually more difficult than getting accurate spirometry and DLCO results mostly because the errors tend to be subtle.

The errors that occur in lung volume measurements tend to cause an overestimation of lung volumes. This often means that restrictive diseases can be unrecognized or that hyperinflation and gas trapping can be diagnosed where it does not exist.

I see questionable lung volume test results more often than I’d like even from experienced technicians. When I find what went wrong I try to use these as “teachable moments” for all the the lab staff. Despite this the number of questionable test results never seems to drop below a certain level. I’d much prefer the error level was zero but since this is a situation that involves humans making measurements on humans I am likely being overly optimistic. A more realistic goal is to ask that the testing systems be smarter.

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