Competition between gas exchange and speech production in ventilated subjects

Competition between airflow requirements for speaking and gas exchange occurs in ventilator-dependent tracheotomized subjects who can 'steal' air from alveolar ventilation during the ventilator's inflation phase to produce sound. We wondered whether these subjects adopted strategies to minimize hypoventilation when speaking, particularly when ventilatory drive and respiratory discomfort are increased by hypercapnia. We recorded speech and ventilatory and speaking volumes in five ventilated subjects during reading and extemporaneous speech. All subjects spoke during the ventilator's inflation (and expiratory) phase, losing approximately 15% of their inspired tidal volume. During induced hypercapnia (15 mmHg increase in PetCO2) which caused shortness of breath, all subjects could still speak adequately. Two subjects 'adapted' to hypercapnia by reducing the air used for speaking during inflation. In contrast, one subject reacted, as normal subjects do, by increasing the airflow per syllable (a mal-adaptive strategy in ventilated subjects). These changes were modest despite the strong hypercapnic stimulus.