1303 The Tidal Volume Gap: Recognizing Bilevel Failure in Progressive Motor Neuron Disease

Abstract Introduction Progressive muscular atrophy (PMA) often leads to ventilatory failure that may not be fully reflected by routine pulmonary function testing. Because respiratory muscle decline is progressive, fixed-pressure bilevel ventilation may fail to provide adequate support as disease severity evolves. Report of case(s) A 61-year-old woman presented with imbalance and distal weakness. Lower motor neuron signs on electromyography supported a diagnosis of PMA. She endorsed dyspnea both at rest and while supine, and pulmonary function testing (PFT) revealed normal spirometry, mildly reduced vital capacity (74% predicted), normal maximal inspiratory pressure, and a reduced maximal expiratory pressure (47% predicted). She also had a persistently elevated serum bicarbonate above 28 mEq/L. She began nocturnal bilevel positive airway pressure (BiPAP), but gradually developed worsening dyspnea, eventually requiring near-continuous BiPAP use. She was transitioned to Average Volume-Assured Pressure Support (AVAPS) ventilation, using a nasal mask during the day and a full-face mask at night. Following this change, she reported improved dyspnea and mask tolerance. Conclusion The rate of decline of respiratory muscle strength can vary in disorders of muscle atrophy. While many patients with respiratory insufficiency are started on fixed-pressure BiPAP, this mode of ventilation may become insufficient as respiratory muscle weakness progresses, even when pulmonary function testing appears relatively preserved. Specifically, the same pressure support can yield progressively smaller tidal volumes, resulting in hypercapnia, sleep fragmentation, and worsening dyspnea. This patient’s normal spirometry and preserved maximal inspiratory pressure illustrate how standard PFTs may underestimate functional ventilatory impairment. In contrast, elevated bicarbonate and inadequate measured tidal volumes may be more sensitive indicators of chronic hypoventilation. Volume-assured pressure support modes such as AVAPS may be beneficial in these settings. These modes have been shown to improve ventilation stability, increase average tidal volume, and reduce nocturnal hypercapnia in certain neuromuscular and chronic hypoventilation syndromes. Additionally, optimization of non-invasive ventilation (NiV) in motor neuron disease can improve sleep continuity, sleep quality, and overall quality of life, even as the underlying neuromuscular disease continues to progress. Therefore, NiV mode selection should be continuously revisited, and volume-targeted strategies should be considered when fixed-pressure support no longer meets physiologic needs. Support (if any)