Ventilatory muscle endurance training (VMET) involves increasing minute ventilation ( ) against a low flow resistance at rest to simulate the hyperpnea of exercise. Ideally, VMET must maintain normocapnia over a wide range of  . This can be achieved by providing a constant fresh gas flow to a sequential rebreathing circuit. The challenge to make VMET suitable for home use is to provide a source of constant fresh gas flow to the circuit without resorting to compressed gas.

Our VMET circuit was based on a commercial sequential gas delivery breathing circuit (Pulmanex Hi-Ox, Viasys Healthcare, Yorba Linda, CA USA). Airflow was provided either by a small battery-driven aquarium air pump or by the entrainment of air down a pressure gradient created by the recoil of a hanging bellows that was charged during each inhalation. In each case, fresh gas flow was adjusted to be just less than resting  . Eight subjects then breathed from the circuit for three 10min periods consisting of relaxed breathing, breathing at 20 and then at 40L/min. We monitored  , end-tidal PCO₂ (PetCO₂) and hemoglobin O₂ saturation (SpO₂).

During hyperpnea at 20 and 40L/min, PetCO₂ did not differ significantly from resting levels with either method of supplying fresh gas. SpO₂ remained greater than 96% during all tests.

Isocapnic VMET can be reliably accomplished with a simple self-regulating, sequential rebreathing circuit without the use of compressed gas.