Lung Function Measurement after Lung Transplantation


By Ramsey Hachem, M.D.

Washington University School of Medicine Barnes-Jewish Hospital
Division of Pulmonary and Critical Care

January, 2014

A frequent question that comes up after lung transplantation is what should my lung function be and will it ever be 100%. There are many variables that affect lung function and the percent predicted values, especially after transplantation. Spirometry is a simple and non-invasive assessment of lung function. Importantly, measurements are relatively easy to obtain and inexpensive allowing serial measurements over time. Spirometry measures the forced vital capacity (FVC), the forced expiratory volume in 1 second (FEV1), and the forced expiratory flow between 25% and 75% of forced vital capacity (FEF25%-75%). After taking as large a breath as possible, the patient exhales forcefully and completely into the spirometer. The FVC is the volume of air that the patient can forcefully exhale. The FEV1 is the volume of air that the patient can exhale in the first second of a forced exhalation. Lastly, the FEF25%-75% is the flow rate of air between 25% and 75% of the FVC. The FVC and FEV1 are measured in liters while the FEF25%-75% is measured in liters per second. There are sophisticated equations that convert the absolute values into percent of predicted or percent of normal values based on gender, age, height, and race. Decrements in FEV1 and FEF25%-75% without a concomitant decrement in FVC are characteristic of obstructive ventilatory defects as would typically be seen in asthma, emphysema, and bronchiolitis obliterans syndrome (BOS) after lungs transplantation. On the other hand, decrements in FVC are suggestive of restrictive ventilatory defects as would be seen in pulmonary fibrosis.

In general, lung transplant clinicians rely on the absolute values of FVC, FEV1, and FEF25%-75% rather than the percent predicted values. The primary determinants of lung function (FVC and FEV1) are gender, age, height, and race. In general, men have higher lung function than women, and lung function peaks between 20 and 30 years of age. In healthy, nonsmoking adults, lung function decreases slowly with age; on average, there is a 30 milliliter per year decrement in FEV1 even in the absence of environmental pollution. This is due to slowly progressive subclinical age-related emphysema. Height is another major determinant of lung function and taller individuals have higher FVC and FEV1 measurements. Race is the last variable that affects lung function as individuals of different races may have different trunk to height ratios.

After lung transplantation, there are numerous factors that influence peak lung function. Obviously, bilateral lung recipients will have higher lung function than single lung recipients. But, there are important donor-related factors that Influence lung function. Donor height is the most influential variable; recipients of lungs from taller donors will generally have higher peak lung function. Donor height is an important factor in donor selection for an individual recipient. Obviously, recipient height will influence this selection. In addition, the recipient’s underlying lung disease will determine his or her chest cavity size. For example, emphysema, cystic fibrosis, and obstructive lung diseases in general are associated with hyperinflation of the lungs as a result of trapped gas at the end of exhalation. Over time, this results in hyperinflation of the chest cavity size. In contrast, pulmonary fibrosis progressively reduces lung volumes and there is a concomitant decrease in chest cavity size over time. As a result, patients with obstructive lung diseases can generally accept lungs from donors who are taller than they are while those with restrictive lung diseases such as pulmonary fibrosis generally have to have donors who are shorter than they are, and this will influence peak lung function after transplant. For example, some patients with emphysema may have peak FVC and FEV1 measurements over 100%, and sometimes over 130%, of their predicted values after transplantation, while some patients with pulmonary fibrosis may have peak FVC and FEV1 measurements 70-75% of their predicted values. Nonetheless, although their lung function is lower than predicted, these patients usually have no functional or clinically significant limitations.

In addition, complications early after transplantation may affect peak lung function. Recipients who develop severe primary graft dysfunction (PGD) tend to have lower maximal lung function after transplantation because of resultant scarring. Similarly, recipients who develop severe pneumonia in the early period after transplantation typically have reduce peak lung function. For these reasons, lung transplant clinicians rely primarily on the absolute values of FVC and FEV1 rather than the percent predicted values, and track these longitudinally. The best lung function is defined as the average of the two highest measurements after transplantation, and subsequent changes in lung function are compared to this average. In summary, there are multiple determinants of maximal lung function after transplantation, and different patients will likely have different peak lung function because of donor-related factors and early post-operative complications.