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CHAPTER 44: Noninvasive Ventilation 379

NIPPV failure has been shown to be independently associated with death turbine or piston NIPPV ventilators due to the exclusive use of dry
in a observational study. 20 gases. In this case gas humidification is mandatory. Two types of
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■ EQUIPMENT humidification systems can be used to overcome the problem: heated
humidifiers (HH) or heated and moisture exchanger filters (HME).
Several types of ventilators can be used to deliver NIPPV to patients Firm recommendations cannot be made between the two systems, but
31
with ARF. Turbine ventilators specifically designed for NIPPV have been the humidification ability of HME is reduced in the presence of leaks
specifically developed to work with leaks, but standard ICU ventilators and their internal volume imposes an additional workload on the patient
and transport ventilators can be used as well especially when they have by generating CO 2 rebreathing. In patients with hypercapnic respiratory
dedicated software. Turbine ventilators designed for NIPPV deliver two failure, this can diminish the effectiveness of NIPPV in reducing blood
levels of positive airway pressure synchronized on patient’s effort (flow CO 2 levels and correcting respiratory acidosis. 32,33 Leaks, however, may
or pressure trigger) or time triggered, reproducing pressure support markedly reduce the importance of this problem by washing the circuit
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ventilation or pressure control ventilation plus PEEP. 21 from CO 2 contaminated gas. A similar problem of CO 2 rebreathing
Α distinguishing feature of turbine ventilators is the compensation for air occurs when turbine ventilators (using ambient room air), equipped
leakage, which is the main cause of patient-ventilator asynchrony during with a one-line circuit, are used with the minimal level of PEEP allowed
34,35
NIPPV. Indeed, as much as 43% of patients receiving NIPPV with venti- on these ventilators. The expiratory flow generated to create the
22
lators without NIV modes manifest patient-ventilator asynchrony, mainly PEEP level is indeed used to flush the exhaled CO 2 from the circuit.
attributed to leakage. Expiratory leaks can be falsely interpreted as patient With low PEEP levels, high minute ventilation, and/or a high respiratory
23
efforts leading to autocycling. Inspiratory leaks, by hampering flow rate rate, this can have adverse clinical effects that may require addition of a
deceleration, prevent the ventilator from recognizing the end of patient’s nonrebreathing valve to the circuit.
effort. This can cause prolonged insufflation, triggering delay and, if exces- The interface used to connect the patient to the ventilator is usually
sive, ineffective efforts. Notably, the level of support during NIPPV tends to a full face mask covering both the nose and the mouth. Although nasal
exacerbate the incidence and severity of leakage-related asynchrony. interfaces are available, their use in ICU patients frequently results in
ICU ventilators initially manufactured to function without leaks were major leakage through the mouth that diminishes the effectiveness of
subsequently equipped with a special adaptive algorithm, as well as some NIPPV and promotes patient-ventilator asynchrony and discomfort. 36,37
modern transport ventilators, referred to as the NIV mode, designed Full face masks could be either oronasal or total face masks, both
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to mitigate the impact of leaks. Bench comparisons assessing in vitro appearing similar in terms of efficacy and patient tolerance. Full face
ventilator performance suggest that both turbine ventilators specially masks are responsible for unwanted effects including skin breakdown
designed for NIPPV and late-generation standard ICU ventilators over the nose, conjunctivitis related to leakage of air directed toward the
13,39
equipped with NIV modes are satisfactory for delivering NIPPV to ICU eyes, rebreathing, claustrophobia, and overall discomfort.
patients with severe dyspnea. 21,22,24-26 In a clinical study, the activation of These problems prompted efforts to design improved interfaces. The
the NIPPV algorithm in modern ICU ventilators significantly reduced first improvement consisted in varying the pressure sites on the face to
the incidence of all types of asynchronies associated with leaks and this achieve better tolerance during prolonged use, and subsequently much
40,41
effect was more pronounced at higher levels of pressure support. ICU larger masks enclosing the entire face or head were developed. Use of
27
ventilators, transport ventilators, and NIPPV-dedicated turbine venti- a helmet has been suggested, primarily for patients with acute hypoxemic
42-44
lators have been compared in terms of patient-ventilator asynchrony respiratory failure. Because helmets may induce more rebreathing
in the presence of leaks. Dedicated NIPPV ventilators ensured better than other masks, they may be less suitable for patients with hypercap-
patient-ventilator synchronization than ICU and transport ventilators, nic respiratory failure. The helmet probably improves patient comfort
even after NIV algorithm activation. The algorithm ameliorated trig- and tolerance, at the price, however, of decreased effectiveness in CO 2
45
gering and cycling synchronization albeit in a heterogeneous manner clearance and possibly respiratory muscles unloading. It was shown,
among ventilators. Turbine ventilators were, in general, much better however, that the helmet required higher pressures than conventional
28
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at avoiding autocycling. No outcome data, however, like NIV success masks to reproduce the same efficacy. A good clinical tolerance is crucial
or failure, have been shown to be associated with specific asynchronies. to successful NIPPV. In their large observational survey, Carlucci and
Therefore, if an ICU ventilator is used it seems of good common sense colleagues identified two independent predictors of failure: the severity
to use the dedicated NIV algorithm. score (as assessed by the Simplified Acute Physiology Score (SAPS II)
17
Adequate patient monitoring may be essential to assess patient- and clinical tolerance. Interestingly, clinical physiologic studies with
ventilator interaction, to detect leaks, and to fine-tune pressure levels. integral masks compared to standard full face masks seem to indicate
Careful observation of the airway pressure and flow-time curves on the comparable efficacy in terms of respiratory muscle unloading, suggesting
ventilator screen can detect patient-ventilator asynchronies which, if that the theoretical risk of rebreathing associated with the large internal
47,48
normalization and accelerate volume may be small or nonexistent in clinical practice.
adequately corrected, might fasten Pa CO 2
patient’s adaptation. Whether this also ensures higher NIPPV success ■ ACUTE EXACERBATION OF CHRONIC RESPIRATORY FAILURE
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rates remain to be determined.
Airway gas conditioning, that is, the warming and humidification Numerous studies concerning NIPPV have been performed in patients
of the inspired gas, constitutes a physiological procedure performed by with obstructive pulmonary disease, and the prevalence of COPD is high
the human airway during normal breathing. When the upper airway is and increasing. NIPPV has demonstrated over the years important posi-
bypassed, as during invasive MV, it is indispensable to artificially heat tive clinical results for treatment of acute exacerbations of COPD. 13,49-53
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and humidify gas prior to delivery. During NIPPV, gas is transferred A Cochrane database review showed a decrease in mortality, a reduced
to the alveoli through the mouth and nose, but the normal airway gas need for intubation, less treatment failure, faster clinical improvement,
conditioning mechanisms can be defeated in case of high flow of the as well as reduction in treatment complications and length of hospital
inspired air, high inspiratory airway pressure settings, high inspired stay associated with NIPPV in this indication.
oxygen fraction for turbines. All these factors contribute to the necessity Pathophysiology: Exacerbation of COPD is a common cause of admis-
of artificial heating and humidification during NIPPV. Other associated sion to the hospital and ICU. In addition to worsening of dyspnea
effects including structural and functional damage to the nasal mucosa, and acute bronchitis symptoms, rapid and shallow breathing with
high nasal airway resistance, increased work of breathing, poor patient hypoxemia and hypercapnia are usually present and can lead to the
tolerance, and difficult intubation in case of NIPPV failure reinforce development of right ventricular failure and encephalopathy. The
these requirements regarding gas delivery during NIPPV sessions. 30,31 pathophysiologic pathway involves an inability of the respiratory sys-
ICU ventilators provide much lower level of humidity compared to tem to maintain adequate alveolar ventilation in the presence of major

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