© Springer International Publishing Switzerland 2016
Antonio M. Esquinas (ed.)Noninvasive Mechanical Ventilation10.1007/978-3-319-21653-9_55. ICU Ventilators Versus BiPAP Ventilators in Noninvasive Ventilation
(1)
Critical Care Medicine, Cairo University Hospitals, Giza, Egypt
5.1 Introduction
In contrast to the closed-circuit ventilation of invasive ventilation, noninvasive ventilation (NIV) is an open-circuit ventilation where leaks are inherent and, paradoxically, essential to its success. The success of NIV, whether in the acute setting, weaning, or long-term therapy is dependent on all three aspects for its use, appropriate patient selection, suitably fitting interface, and a specifically designed machine. The choice of a ventilator may be crucial for the success of NIV in the acute setting, because intolerance and excessive air leaks are significantly correlated with NIV failure [1].
5.2 Leaks and Ventilator Performance
In contrast to unintentional leaks creating difficulties with ventilation, intentional leaks are “venting leaks.” They should be created in the system in two instances. The first is to prevent the accumulation of CO2 and rebreathing due to the dead space present in the interface, which may reach up to 800 ml in total face masks [2]. This accumulated air should be vented via exhalation ports in the interface. In the second instance, single-limb circuits should contain either a port for continuous intentional leaks or an exhalation valve. The intentional leaks are constant and controllable. Other sites of leaks, including that between the interface and the patient’s face and mouth leaks with nasal masks, are sudden, variable, and unpredictable.
Leaks are less marked during expiration than during inspiration, because upper airway pressure decreases markedly when mechanical insufflation switches off to permit expiration. However, positive end-expiratory pressure (PEEP) may still promote expiratory leaks, where its level is proportional to the occurrence of leaks. Such leaks interfere with proper ventilation by affecting the trigger, pressurization during insufflations, and cycling off to exhalation. This ultimately leads to poor ventilation, lack of patient compliance, and prolongation or failure of NIV. Expiratory leaks can mimic an inspiratory effort for the ventilator, leading to auto-triggering, and inspiratory leaks can mimic a sustained inspiration, leading to delayed cycling [3].
If leak flow reaches the trigger threshold, auto-triggering occurs. Because of this, the frequency of auto-triggering does not depend on the magnitude of the increase in leak. On the other hand, if the leak is large enough, the ventilator may not detect respiratory efforts, leading to miss-triggering [3]. Vignaux et al. [4] demonstrated that auto-triggering was present in 13 % of patients, and delayed cycling in 23 % of patients during NIV. Auto-triggering per se may also induce miss-triggering if inspiratory time is prolonged, because of auto-triggering overlapping the patient’s next inspiratory effort. In other words, cycle asynchrony can produce trigger asynchrony. Additionally, leaks can lead to aerophagia, odynophagia, dry mouth, eye irritation, and nasal symptoms, and noise may result, all of which reduce therapeutic compliance [5].
Ventilators used in NIV must be capable of detecting and properly estimating leaks and compensating for such leaks. Indeed, the response of these ventilators will vary according to the degree of leak, their capability to compensate, ventilation target (pressure vs volume targeted ventilation), and the type of intrinsic lung disease (obstructive vs restrictive pattern).
5.2.1 Leak Estimation and Compensation
The leak volume, as estimated from the difference in inspiratory and expiratory volumes, occurs during both inspiration and expiration. In the past, tidal volume has been estimated from the expiratory volume. However, given the observation that volume is also lost during expiration, tidal volume (Vt) can be underestimated from expiratory volume, and, consequently, crucial inspiratory leakage might be overestimated [6, 7]. Conceivably, the expired-volume method for measuring Vt might underestimate the Vt if leaks occur during expiration and therefore may induce overcompensation.
The simplest way to estimate the patient’s Vt during leaks is to measure expiratory Vt and to consider that Vt is underestimated in case of expiratory leakage [7]. Ventilators with an expiratory valve have no expiratory circuit and no pneumotachograph connected to the patient interface. Consequently, these ventilators cannot measure expiratory Vt and, therefore, the patient’s real Vt during leaks [7]. By measuring pressure and flow inside the ventilator, while taking into account the ventilator turbine speed throughout the entire respiratory cycle and detecting the beginning and end of inspiration, the ventilators with single-limb circuits with intentional leak are able to rebuild the patient’s flow pattern and to establish a “baseline” breathing pattern corresponding to the patient’s zero flow [7].
Khairani et al. [7] evaluating the ability of home ventilators to maintain the minimum Vt in volume-targeted pressure support ventilation (VT-PSV) in seven different NIV ventilators using different circuits. They concluded that ventilators that can be used with a single-limb circuit with intentional leak outperform devices that use double circuits or expiratory valves, where the latter could even paradoxically exacerbate the Vt drop during unintentional leak when used in VT-PSV mode. All but one of the studied ventilators with a double-limb circuit and all studied ventilators with an expiratory valve misinterpreted leaks as an increase in Vt and therefore decreased their inspiratory pressure to the minimal preset level, thereby paradoxically exaggerating the fall in Vt.
5.3 Comparison Between Ventilators
Three major types of ventilators have been commonly used for NIV over the past two decades: regular intensive care unit (ICU) ventilator (with no NIV capabilities or algorithm), ICU ventilator with NIV algorithm, and dedicated NIV ventilators. In general, in ICU ventilators without algorithms for leak compensation, a minimal amount of leak can be attained because the ventilator can only minimally compensate for the decline in pressure. If leaks are greater, the ventilator leak alarm will be activated, and the leaks will abort the breath due to disconnection. The failure to operate alarm is activated at higher levels. In the latter case, the system alarm for disconnection may be modified to a higher level, however, this still cannot be compensated for. ICU ventilators are more powerful and have more adjustable features (trigger type and sensitivity, slope of pressurization, cycling criteria) and monitoring capabilities. Their downside is cost, size, and the knowledge required for their safe use.
NIV ventilators, on the other hand, are portable devices with a turbine-type blower capable of delivering a high inspiratory flow rate (>100 l/min), are easier to use, and are less costly [8]. Most of the first generation bi-level ventilators, however, had important technical limitations, including limited pressure-generation ability, poor performance if respiratory-system load increased, risk of CO2 rebreathing, and lack of ventilatory monitoring, alarms, or battery [1]. Although there have been many updates, NIV ventilators still cannot administer high inspired O2 concentrations, nor can they reliably provide high (>20 cmH2O) levels of pressure support. These two factors could prove to be a limitation in patients with hypoxemic respiratory failure, in whom high levels of FiO2 and PEEP are required. In addition, CO2 rebreathing can occur with some circuits, and they often lack monitoring capability [8].
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