Volume-cycled ventilators

Piper et al. (157) used domiciliary NIV for up to 18 months in four CF patients with chronic ventilatory failure who had failed to respond to optimal conventional measures. Using a volume-cycled ventilator, their Pac02 fell, sleep quality improved, and RMS increased. This may suggest at least a stabilizing effect. 

Bi-level NIV may be used as a first-line treatment, with supplemental oxygen (27). Expiratory airway pressure is titrated to control h5q)opneas and apneas, and inspiratory airway pressure is added to control Paco2. If bi-level NIV fails, nasal volume ventilation may be used (29). In many patients with OHS and predominant OSA, once hypercapnia has improved (which may take several weeks) nCPAP may be used (29). Thirteen obese patients (n = 13) with a BMI > 35, aged 28-69 years with severe OSAS and hypercapnia (8.2 0.3 kPa) and failing to respond to initial CPAP therapy, were treated via a nasal nocturnal volume-cycled ventilator, which was tolerated by all patients. Significant improvements in daytime arterial blood gas levels were obtained after 7 to 18 days of nasal intermittent positive pressure ventilation (29) in 10 of the 13 patients three months later, 12 of the 13 patients could be converted to nCPAP therapy and one patient remained on NIV. In another study (37), the same results were observed after three months of home nocturnal bi-level NIV in seven patients, three of whom had severe obesity. 

Figure 1 The girdle of the lAPV with its air sac connected to the tubing of a volume-cycled ventilator. This 45-year-oId DMD patient, continuously ventilator dependent for 24 years and having no measurable vital capacity, used the lAPV for daytime ventilatory support for 15 years. Abbreviations lAPV, intermittent abdominal pressure ventilator DMD, Duchenne muscular dystrophy.

Ventilatory support can be provided with NIPPV from volume-cycled ventilators via an angled mouthpiece, lipseal, nasal, or oral-nasal interface. Simple 15- or 22-mm-angled mouthpieces are most convenient for daytime ventilatory support (Fig. 3). To use mouthpiece IPPV, adequate neck rotation and oral motor function are necessary to prevent leakage from the mouth or nose. In addition, the patient must open the glottis and vocal cords, dilate the hypopharynx, and maintain airway patency to receive the air. 

Opioids are potent respiratory depressants, causing a dose-dependent decrease in respiratory frequency, tidal volume and minute ventilation and increased arterial partial pressure of carbon dioxide (PaC02) (Carvey 1998). Opioids depress chemosensors in the brainstem, decreasing the ventilatory response to carbon dioxide. Opioids also depress rhythmicity in the dorsal respiratory group in the nucleus tractus solitarius, attenuating the respiratory cycle. Opioids, however, do not diminish hypoxic ventilatory drive. Significant elevations in Paco2 can result in increased ICP after opioid administration. 

In a resting state, the respiration rate is normally 12 to 15 breaths/min. For an average-sized adult with a tidal volume (the amount of air exchanged per breath cycle) of about 0.5 L, 6 to 8 L of air is moved per minute in either direction. Physical activity increases ventilation (respiratory rate X tidal volume), i.e., the amount of air exchanged per minute. Voluntary efforts can increase the rate of ventilation 20 to 30 times over the resting concentration, but only briefly. Invol-... 

Figure 1 Airway pressure, flow and volume tracings, over time, depicting the five basic breaths available in most modem ventilators. Breaths are classified by their trigger, target/limit, and cycle variables. Abbreviation fl, inspiratory time. Source From Ref- 74.

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