Fig. 78.1
Functioning of a bi-level ventilator in the spontaneous mode (S-mode). Notice that patient ventilation is augmented while no support is given during phases of apnea
Fig. 78.2
Functioning of a bi-level ventilator in the spontaneous-timed mode (ST-mode). Notice that ventilator support is given during breathing efforts as well as during apneas
Fig. 78.3
Functioning of a servo-ventilator. Notice that ventilator support is given disproportionately to breathing efforts
On the basis of this pathophysiological background, it is not surprising that the usual bi-level noninvasive positive-pressure ventilation (NPPV) might even worsen breathing patterns dominated by central apneas [13]. Proportional-assist ventilation, on the other hand, which applies pressure in proportion to the patient’s effort, promotes periodic breathing [14]. If ventilation is given in disproportion to patient effort, it is likely to improve central apneic breathing patterns. Servo-ventilation has such a disproportional approach, and several studies have shown the superiority of different servo-ventilators in this instance [15–17].
Our research group showed that the effect of ventilation on the human breathing pattern can change over time [18]. In this randomized controlled trial in patients with persistent CPAP-induced central apneas, known as complex sleep apnea, bi-level ventilation improved central breathing pattern in the short term, but resulted in reemergence of central apneas after 6 weeks. Servo-ventilation, on the other hand, compensated apneas permanently (Fig. 78.4). It is therefore recommended to choose the type of noninvasive pressure support wisely and reevaluate treatment effect over time.
Fig. 78.4
Servo-ventilation compared with bi-level ventilation (IPPV group) in patients with complex sleep apnea. Note that only servo-ventilation compensates central events after 6 weeks follow-up [18]
In terms of outcome, servo-ventilation has been shown to effectively reduce the apnea-hypopnea index [15–19] and improve sleep quality [17], quality of life [20], and left ventricular ejection fraction in chronic heart failure patients [20, 21]. A recent multicenter trial however showed, that servo-ventilation might increase mortality in patients with an ejection fraction below 45 % [22].
Newer-generation servo-ventilators share the capability to regulate expiratory pressure according to the grade of upper airway obstruction using techniques that are also used in automatic CPAP devices (APAP).
It is important to know that function differs between servo-ventilators. Due to patent law, different companies have developed different algorithms. The main goal of servo-ventilators is to keep ventilation constant. Whereas some devices measure the actual flow and are programmed to keep flow constant, others detect relative minute ventilation in shifting timeframes. Algorithms providing automatic back-up rates as well as autotitration of expiratory pressure are also different between servo-ventilators. For this reason, these devices are not arbitrarily interchangeable but must be titrated individually to each patient [23, 24].
More sophisticated feedback control systems have been introduced for the treatment of central and mixed sleep apnea, but they appear to have a low tolerance [25]. Studies comparing this approach to conventional servo-ventilation are missing to date.
78.3 Conclusion
For obstructive sleep apnea, conventional CPAP therapy remains the gold standard. Central apneas, however, require a more targeted approach. Central apneas are usually caused by variations in CO2 regulation. To achieve CO2 homeostasis, ventilation must compensate for apneas without augmentation of uncompromised breathing efforts. Servo-ventilators deliver pressure disproportional to the patient’s own breathing effort and stabilize the human CO2 feedback-control system.
References
