Pulmonary Function Tests and Lung Cancer Surgery

Shortness of breath, aka difficulty breathing, is caused by a myriad of reasons; the list of such reasons does not end. The primary tool to evaluate a person’s shortness of breath is the Pulmonary Function Test (PFT). PFTs consist of three key elements whose results are compared to a generalized standard based on sex, age and height.

Spirometry

Spirometry evaluates airflow in and out of the lungs.  An individual should exhale at least 70% of their total breath in the first one second. The amount of air a person exhales in the first one second is called the FEV1 (Forced Expiratory Volume, 1 second). The total amount of air they can exhale is called the FVC (Forced Vital Capacity). If a person requires more time to breathe out 70% of their total breath (designated FEV1/FVC < 70%), then they have airflow obstruction. Airflow obstruction is one of the defining characteristics of asthma and chronic obstructive pulmonary disease (COPD). These diseases are classically treated with inhaled medicines that improve airflow by enlarging the airways or decreasing airway inflammation that obstructs air flow.

Lung Volumes

The second part of the PFT, called lung volumes, measures the size of a patient’s lung. This includes specialized measurements of not just the entire volume (TLC for Total Lung Capacity), but volumes of air that we store in reserve for times like exercise when we maximize lung use. Loss of lung volume is a classic finding for people with lung fibrosis or those who lost lung infections, acute respiratory distress syndrome (ARDS) or lung surgery. Thankfully, the lung has redundancy and people with only one good lung are often without symptoms.

Diffusing Capacity

The third aspect of the PFT is called the diffusing capacity called the DLCO. This measures the capability of the lung to absorb oxygen and expel the waste gas, carbon dioxide. COPD, lung fibrosis and long standing, undertreated asthma can lead to a reduction in the efficiency of the lung to bring in critical oxygen. In advanced stage disease, a patient’s use of oxygen is required to overcome significant deficiencies in this aspect of their lung.

Uses for PFTs

Clinicians use PFTs to diagnose disease but also to monitor disease or look for medication side effects that can affect the lung. Surgeons also use them to determine if a patient has appropriate lung function to support the intended surgery. PFTs are an important test to evaluate a person’s breathing symptoms and usually take about one hour to complete.

PFTs in Lung Cancer Surgery Evaluation

 For lung cancer surgery, data show removal of part of the lung is safer if the predicted amount of lung left after surgery will be at least 40% of normal for the FEV1 and DLCO (see above for explanation of these acronyms). To predict the amount of lung left after surgery, we estimate the how much lung will be lost with the surgery and make the appropriate calculations.

One way to do this is by a simplistic generalization of the lung. The lung can be divided into five lobes; Left Upper Lobe, Left Lower Lobe, Right Upper Lobe, Right Middle Lobe and the Right Lower Lobe. To remove the lung cancer, surgeons can take one of these lobes. Simplistically, if each lobe represents 20% of the lung, 80% of the lung and its resultant function will be left after surgery. Multiplying 0.80 by the pre-operative FEV1 and DLCO will determine if 40% will be present after surgery.

However, in lung disease, the disease does not always affect the entire lung equally. If the previous calculation is close or God forbid below the 40% cut off, a better way to predicted post-operative lung function uses a special scan. Called a perfusion scan, this scan determines how much blood flows to the top, middle and bottom of the lung. Because blood flow typically matches airflow, this can better estimate lung function. By comparing the blood flow in the part of the lung that will be removed, clinicians can come up with a more accurate prediction for the post-operative lung function.

Finally, there is another test called a cardiopulmonary exercise test with exhaled gas analysis (CPET) that evaluates a person’s functional status.  You may have heard of the primary measurement of this test called the V dot O2 or VO2. This value has been popularized in the exercise community as a way of measuring physical fitness. For lung cancer, achieving a VO2 > 15 ml/kg/min allows the person to get surgery.

Historically, surgeons used to see if their patients could walk up three flights of stairs without stopping. If they could, then they knew their patients was good for surgery. Analysis of patients able to walk three flights of stairs found their VO2 roughly correlates with at least a CPET VO2 of 15.

And although this is the current clinical paradigm, improvements in surgery with robotic thoracic surgery will improve the ability of more patients to qualify for surgery to treat their lung cancer.

These improvements will help cure more patients ‘Til No One Dies of Lung Cancer.