Hi
If your FVC (forced vital capacity) is 77% of predicted and hasn't significantly changed in two years then it would indicate that your respiratory muscle status is stable at this time and therefore you should have not a reason for concern. In contrast, a trend of decreasing FVC over time would mirror the decline of respiratory muscle strength in ALS. I wouldn't be concerned about total volume changes since they are not a good marker for respiratory muscle strength and are mostly affected by lung disease, obesity and many other factors that are not relevant to ALS.
Your FVC value will decline if your muscles involved in expiration or inspiration significantly weaken, although pulmonary disease such as asthma, COPD or interstitial pulmonary fibrosis would also cause a decrease in FVC.
The maximum inspiratory pressure MIP (PImax) and SNIP values primarily reflect diaphragm muscle strength in ALS and are the most sensitive tests for early detection of weakening of the diaphragm. Your MIP and MEP values are essentially normal, ignore the parentheses that they are outside of the normal range, but they do not represent numerically negative values
Your PEF (PEFR) of 115% of predicted as well as your normal MVV (96%) support that your respiratory muscles are functioning adequately or you would not be able to achieve such values. In addition, if you have no shortness of breath at rest, lying down, and during moderate exertion this would support that you do not have a significant decline in your respiratory muscle strength and therefore you need not be worried about declining respiratory function at this point.
The accuracy of pulmonary function results is dependent upon proper calibration of the equipment, the pulmonary function technician's ability to get the greatest effort possible from the patient, the patient's ability to maintain a good seal around the mouthpiece, and finally patient’s cooperation. Any problem with the foregoing will affect the validity of the results.
If you plan to do pulmonary function testing on yourself I suggest that you also purchase calibration equipment and practice your maneuvers until they are consistent however, I do think that performing too much testing at home could lead to a lot of anxiety and that you would be better off having your pulmonary function tests performed in a professional environment. If you still have concerns about this matter I suggest you go to a different pulmonary lab, preferably one at a university hospital, and get a second opinion.
I am curious about the Brand of equipment you plan to use at home and if it meets ATS standards? I have used a simple and inexpensive peak flow meter and oximeter to keep objective track of my respiratory function at home since they will able to help me to objectively keep track of what is going on should I sense that my breathing has deteriorated.
Best wishes
Eliot