Airway mucosal blood flow in humans. Response to adrenergic agonists.

We measured the uptake of the soluble inert gas dimethyl ether (DME) from a segment of the conducting airways to estimate mucosal blood flow (Qaw) noninvasively. The subjects inhaled, from the functional residual capacity position, a 300-ml gas mixture containing 35% DME, 8% helium, 35% oxygen, and the balance nitrogen; they held their breath for 5 s and then exhaled into a spirometer. During exhalation, the instantaneous concentrations of DME and helium were recorded together with expired gas volume. The maneuver was repeated with breathhold times of 5, 10, 15, and 20 s. We calculated Qaw using the time-dependent decrease in DME concentration in relation to the helium concentration in an expired volume fraction between 80 and 130 ml (representing an anatomic dead-space segment distal to the glottis) and the mean DME concentration. In 10 healthy nonsmokers, mean (+/- SE) Qaw was 8.0 +/- 1.3 ml/min, or 8 +/- 2 microliters/min/cm2 mucosal surface. We obtained a value of 12 +/- 3 microliters/min/cm2 in a validation experiment in sheep. Inhaled methoxamine (nebulized dose 10 mg) caused a 65 +/- 19% decrease (p < 0.05), and albuterol (nebulized dose 2.5 mg) a 92 +/- 17% increase (p < 0.05), in mean Qaw in seven subjects, with the maximum changes occurring immediately or 15 min postinhalation. We conclude that the DME uptake method is an acceptable noninvasive means of estimating airway mucosal blood flow in humans and its modification by vasoactive substances.

The second generation of nasal continuous positive airway pressure devices. Are they created equal?

Nasal continuous positive airway pressure (NCPAP) is considered the preferred medical treatment of obstructive sleep apnea. Because NCPAP exerts its beneficial effects by maintaining positive airway pressure, we proposed to test the ability of the newer "second generation" NCPAP machines to maintain a constant airway pressure during simulated breathing on a lung model. Each of the seven new NCPAP devices tested were examined under conditions of changing inspiratory flow, end expiratory pressure (EEP) and resistance added to the patient end of the NCPAP hose. During inspiration airway pressure fell and in expiration it increased relative to the EEP in all machines. Using the standard NCPAP hose and a breathing pattern consistent with normal breathing during sleep, the maximum decline in pressure during simulated inspiration and excess pressure during simulated expiration was -0.5 cm H2O and 0.6 cm H2O, respectively. Adding resistance, increasing inspiratory flow but not EEP exaggerated this effect. All of the machines behaved similarly in this regard. Further, the performance of the NCPAP devices did not deteriorate after 6 hours of uninterrupted operation. We conclude that the second generation NCPAP machines may be interchanged without another laboratory trial to readjust the EEP.

A comparison of nine nasal continuous positive airway pressure machines in maintaining mask pressure during simulated inspiration.

Nasal continuous positive airway pressure (CPAP) "splints" the airway and prevents inspiratory collapse of the upper airway in patients with obstructive sleep apnea. Nine nasal CPAP machines were compared for their ability to maintain airway pressure at various simulated inspiratory flows. Each machine was connected to a vacuum system at 20, 40, and 60 L/min flow after it was initially set at test pressures of 5, 10, or 15 cm H2O and the system or "mask" pressures were measured. In all machines, mask pressure fell during simulated inspiration and the declines in mask pressure were as high as 5 cm H2O. Because machines varied in their ability to maintain a test pressure, it is recommended that the nasal CPAP machine used in the home be the same as that which was tested in the sleep laboratory. If a different machine is used, it may require adjustment to assure efficacy.