Article Figures & Data
Figures
- Fig. 1.
Representative tracings of flow, tidal volume, and airway pressure (Paw) during administration of continuous positive airway pressure (CPAP) and bi-level PAP. In the left panel note that there are small undulations in the CPAP level that are generated by the patient's inspiratory and expiratory effort, and the consequent displacement of inspiratory and expiratory tidal volume. Such inflections are usually negligible in a responsive CPAP device. In this instance the CPAP is set at 14 cm H2O. In the right panel there are large decrements in the pressure during exhalation (expiratory positive airway pressure [EPAP], which is set at 4 cm H2O), whereas during inspiration the inspiratory positive airway pressure (IPAP) is set at 14 cm H2O, which would conceivably provide the same level of airway splinting as a CPAP of 14 cm H2O. Note the larger tidal volumes and flow patterns consequent to the pressure assist provided by the bi-level PAP device. In this instance, a pressure support or assist level of 10 cm H2O (IPAP minus EPAP) is being administered, with consequently greater tidal volume and inspiratory flow.
- Fig. 2.
Physiological effects of positive airway pressure (PAP) therapy. PAP therapy splints the upper airway (black crosses and arrows), achieves positive intrathoracic pressure (white crosses), decreases venous return, increases lung volume, decreases after-load, and can increase cardiac output. The bidirectional vertical arrows signify the traction on the upper airways affected by the increase in end-expiratory lung volume. Such a traction effect can assist in the splinting open of the upper airway.
- Fig. 3.
Expiratory pressure contour modification (right) and conventional continuous positive airway pressure (CPAP) (left). In the right panel, during CPAP with expiratory pressure modification, note the decrease in airway pressure that is greatest during the inspiratory-to-expiratory transition, with rapid return of airway pressure to the set level of 14 cm H2O. Such a “physiological” decrease in expiratory pressure level is in contrast to the step-decrease in pressure level during bi-level PAP therapy (see Fig. 1).
- Fig. 4.
Bench-study testing of an auto-titrating positive airway pressure (auto-PAP) device that was subjected to snoring mimicked by a built-in loudspeaker (saw-tooth like oscillations in pressure [shorter arrow]). The auto-PAP device detects the snoring and responds by an increase in airway pressure, measured by 2 pressure transducers located distal and proximal to the auto-PAP device (Pdistal and Pproximal). The longer arrow depicts the point in time when the auto-PAP begins to respond, whereas the complete pressure response to the one snoring event (a 2 cm H2O increase in airway pressure) is better delineated by the dashed line.
- Fig. 5.
Principles of operation of servo ventilation. The air flow tracing depicts a classical crescendo (orange arrow) and decrescendo (red arrow) pattern of Cheyne-Stokes respiration, followed by an ensuing central apnea. The servo-controlled automatic adjustment of the inspiratory positive airway pressure [IPAP] level is inversely related to the changes in peak flow over a moving time window. Specifically, during the crescendo pattern of peak flow rates (orange arrow) the pressure assist (or IPAP) level decreases in order to dampen the rise in inspiratory peak flow rate (or tidal volume). Conversely, during the decrescendo pattern of peak flow rates (red arrow) the pressure assist (or IPAP) level increases in order to dampen the fall in inspiratory peak flow rate (or tidal volume). Therefore, the servo system dampens the inherent oscillatory behavior of the patient's breathing pattern and smoothes respiration. During a central apnea, however, the device backup rate kicks in and ventilates the patient (right side of the figure). The maximum and minimum IPAP (IPAPmax and IPAPmin) are set at 17 cm H2O and 9 cm H2O (dashed blue lines). The expiratory positive airway pressure (EPAP) is set at 7 cm H2O. During any given breath the pressure assist or pressure support is equal to the IPAP minus the EPAP.
- Fig. 6.
Settings of the servo ventilator. The expiratory positive airway pressure (EPAP) is set at a level that can treat obstructive apneas and obstructive hypopneas and before central hypopneas or apneas manifest. Some investigators believe that the EPAP should not exceed 15 cm H2O in patients with heart failure and central apneas, because of concerns surrounding decreased venous return and consequent hypotension in a preload-sensitive cardiac condition. The inspiratory positive airway pressure (IPAP) is determined by the device algorithm, within a pre-specified range (between IPAPmin and IPAPmax) prescribed by the physician, so as to be able to provide variable pressure support (viz, pressure support = IPAP minus EPAP). The IPAPmin is generally assigned a value equal to the EPAP or 2 cm H2O above the EPAP, so that the minimum pressure support can be 0–2 cm H2O. In some devices, however, the difference between IPAPmin and EPAP is preset and the physician does not have to choose the minimum pressure support level. The maximum IPAP is not to exceed 30 cm H2O. The backup rate can be set at automatic or can be set manually. In general, the backup respiratory rate is set 2 breaths below the patient's spontaneous respiratory rate during calm wakefulness breathing, and titrated upwards if the patient manifests persistent central apneas during titration. Many studies have employed a backup rate of 15 breaths/min.39,59 Paw = airway pressure.
- Fig. 7.
Principles of operation of volume-assured pressure support or assist. Flow, tidal volume (VT), and airway pressure (Paw) tracings are shown. Note that in this instance the VT and flow decrease progressively between the 2 shorter arrows. The device detects such a VT drop and responds by increasing the inspiratory positive airway pressure (IPAP) (longer arrow) and restores the VT to near the target. The new, yet higher, IPAP is better shown by the difference between the dashed line and the pre-existing IPAP prior to the increment. Conversely, the IPAP could decrease if the measured VT were to exceed the target VT prescribed by the provider.
