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Noninvasive Mechanical Ventilation 

Noninvasive Mechanical Ventilation
Chapter:
Noninvasive Mechanical Ventilation
Author(s):

John W. Kreit

DOI:
10.1093/med/9780190670085.003.0016
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date: 12 July 2020

Up until now, every chapter of this book has focused on mechanical ventilation delivered through an endotracheal tube. Although so-called invasive ventilation can be life-saving, it can also cause significant morbidity. For example, bypassing the upper airway greatly increases the risk of ventilator-associated pneumonia. Furthermore, the sedation that is almost invariably required prolongs the duration of mechanical ventilation, increases the risk of delirium and long-term cognitive impairment, and contributes to the generalized weakness and debilitation that are so common following critical illness.

It has long been recognized that positive-pressure ventilation can also be delivered “non-invasively” to critically ill patients through several different types of “interfaces” (usually a tight-fitting face mask). In theory, at least, this should reduce the need for intubation and decrease the risks associated with invasive mechanical ventilation.

During the past 30 years, studies have shown that noninvasive ventilation (NIV) is beneficial when used to treat selected patients with respiratory failure, and its use has become increasingly common. Today, NIV is an essential part of ICU care, but it’s also used in many other venues, including the pre-hospital setting, emergency department, post-operative care unit, and medical and surgical wards.

This chapter describes the machines and circuits used to deliver NIV, reviews its indications and contraindications, and explains how to initiate and adjust NIV.

Noninvasive Mechanical Ventilators

Bi-level Ventilators

Noninvasive ventilation is a lot more complicated than simply substituting a face mask for an endotracheal tube. In fact, until fairly recently, ICU ventilators were unable to deliver NIV. That’s why specialized ventilators were developed specifically for this purpose. These machines are usually referred to as “bi-level” ventilators, because, as shown in Figure 16.1, they generate two different, clinician-set levels of airway pressure (PAW).

Figure 16.1 Plot of airway pressure (PAW) versus time produced by a noninvasive, bi-level ventilator.IPAP = inspiratory positive airway pressure; EPAP = expiratory positive airway pressure; IPG = inspiratory pressure gradient

Figure 16.1 Plot of airway pressure (PAW) versus time produced by a noninvasive, bi-level ventilator.

IPAP = inspiratory positive airway pressure; EPAP = expiratory positive airway pressure; IPG = inspiratory pressure gradient

Most bi-level ventilators have their own set of terms and abbreviations. The pressure during inspiration is usually referred to as the inspiratory positive airway pressure, or IPAP, and the pressure throughout expiration is the expiratory positive airway pressure, or EPAP. As you can see from Figure 16.1, EPAP is simply another name for PEEP. The difference between IPAP and EPAP is the inspiratory pressure gradient, which drives gas into the lungs during each mechanical breath. Bi-level ventilators should not be confused with CPAP (continuous positive airway pressure) machines, which maintain a constant pressure level throughout the entire respiratory cycle. Since there is no gradient between inspiration and expiration, CPAP machines provide no inspiratory assistance.

There are several important differences between bi-level and conventional ICU ventilators. We’ll start with the ventilator circuit (Figure 16.2). Remember from Chapter 4 that ICU ventilators have a circuit with separate inspiratory and expiratory limbs and an integrated expiratory valve. Bi-level ventilators use a single-limb circuit, which, instead of an expiratory valve, has an exhalation port that is adjacent to the mask. The exhalation port is always open, so there is a constant gas leak from the circuit. In NIV jargon, this is referred to as an “intentional” leak to distinguish it from the “unintentional” leaks that usually occur around the mask.

Figure 16.2 (A) ICU ventilators use a double-limb circuit with a demand valve and an expiratory valve. Equipment dead space equals the volume of the endotracheal (ET) tube and any tubing between the patient and the junction of the inspiratory and expiratory limbs.(B) Bi-level ventilators use a single-limb circuit. Gas flows from the ventilator throughout the respiratory cycle, and a portion exits through the exhalation port. Exhaled gas also leaves through the exhalation port. Equipment dead space equals the volume of the mask and the circuit between the patient and the exhalation port. In both figures, the arrows show the direction and magnitude of gas flow.

Figure 16.2 (A) ICU ventilators use a double-limb circuit with a demand valve and an expiratory valve. Equipment dead space equals the volume of the endotracheal (ET) tube and any tubing between the patient and the junction of the inspiratory and expiratory limbs.

(B) Bi-level ventilators use a single-limb circuit. Gas flows from the ventilator throughout the respiratory cycle, and a portion exits through the exhalation port. Exhaled gas also leaves through the exhalation port. Equipment dead space equals the volume of the mask and the circuit between the patient and the exhalation port. In both figures, the arrows show the direction and magnitude of gas flow.

ICU ventilators provide gas flow only during inspiration while the expiratory valve is closed. Since the circuit is closed, all the gas from the ventilator enters the patient’s lungs. When inspiration ends, flow stops, the expiratory valve opens to allow exhalation, and then closes again to maintain a clinician-set level of PEEP. In contrast, bi-level machines provide gas flow throughout both inspiration and expiration. Since the circuit is open (i.e., it has a continuous intentional gas leak), the pressure in the ventilator circuit is determined by the net flow—that is, the difference between the total flow provided by the ventilator and the flow passing through the exhalation port. During inspiration, flow increases to achieve the set inspiratory pressure. When inspiration ends, flow falls until the set expiratory pressure is reached. Tidal volume is directly proportional to the pressure (and flow) gradient between inspiration and expiration.

The circuits used by ICU and bi-level ventilators also differ in the amount of dead space they create. You can think of this as the volume of exhaled gas that’s inhaled or “rebreathed” at the beginning of each breath. Since endotracheal and tracheostomy tubes contain less volume than the normal upper airway, dead space is actually reduced with invasive ventilation. During NIV, though, potential dead space is increased because exhaled gas enters the mask and the ventilator circuit proximal to the exhalation port (Figure 16.2B). This would ordinarily cause the patient to rebreathe exhaled CO2, reduce the efficiency of CO2 excretion, and increase the minute ventilation needed to maintain a normal arterial PCO2. This doesn’t happen, though, because of the constant gas flow that’s maintained throughout expiration. This flow not only generates EPAP, it also washes exhaled gas from the mask and circuit and prevents CO2 rebreathing. On most machines, an EPAP of at least 4 cmH2O is recommended to ensure adequate CO2 wash-out.

The most important distinguishing feature of bi-level ventilators is that they are “leak-tolerant.” The magnitude of both the intentional and unintentional leaks depends on many factors, including the set airway pressure, the duration of inspiration, the type of mask, and patient characteristics that can prevent a tight fit, such as a beard, or the absence of teeth or dentures. Furthermore, the volume lost from the system often varies from breath to breath. Bi-level ventilators compensate for this variable leakage by continuously monitoring inspiratory and expiratory flow, inspired and expired volume, and circuit pressure both within the machine and at the exhalation port. Proprietary algorithms then rapidly adjust flow to maintain the set levels of inspiratory and expiratory pressure.

Modes and Breath Types

Modern bi-level ventilators allow the clinician to select from several different modes and breath types. As with conventional ICU ventilators, terminology varies among manufacturers. Most bi-level machines used in the ICU provide the following options (as defined in Chapter 5):

  • CMV-PC—Patients receive a clinician-set number of mandatory pressure control breaths but may trigger any number of additional pressure control breaths (Figure 16.3A).

  • SIMV-PC—Patients receive a clinician-set number of mandatory pressure control breaths but may trigger any number of additional pressure support breaths (Figure 16.3B).

Figure 16.3 Plots of airway pressure (PAW) versus time illustrating mandatory and spontaneous breaths during (A) CMV-PC and (B) SIMV-PC.

Figure 16.3 Plots of airway pressure (PAW) versus time illustrating mandatory and spontaneous breaths during (A) CMV-PC and (B) SIMV-PC.

ICU Ventilators and Noninvasive Ventilation

I told you before that noninvasive ventilation is more complicated than just substituting a mask for an endotracheal tube. In fact, if we were to do that with a standard ICU ventilator and double-limb circuit, the unintentional gas leak around the mask could cause significant problems during every phase of the respiratory cycle.

  • Triggering—ICU ventilators provide a mechanical breath when inspiratory effort lowers either PAW or the base flow below a clinician-set level. By allowing room air to enter the mask during inspiration, a large leak would reduce the drop in PAW or flow produced by patient effort. This would effectively reduce ventilator sensitivity and lead to ineffective triggering and increased patient work of breathing. On the other hand, a leak during expiration could cause breaths to be “auto-triggered” by lowering PAW or the base flow.

  • Inspiration—The volume entering the patient’s lungs would be reduced by the volume that leaks around the mask.

  • Transition from inspiration to expiration—Since pressure support breaths are flow-cycled, inspiration ends and expiration begins only when flow falls below a minimum, set level. When a leak is present, there will always be a pressure gradient driving flow between the ventilator, the mask, and the outside air, and this could prevent flow from reaching the level needed to cycle a conventional ICU ventilator.

  • Expiration—ICU ventilators maintain a set level of PEEP by closing the expiratory valve before the respiratory system returns to its resting or equilibrium volume. If there’s a leak around the mask, gas will flow out of the lungs even after the expiratory valve closes, and PEEP will progressively fall toward zero (atmospheric pressure).

Within the past few years, manufacturers of ICU ventilators have added software with algorithms that have made these machines much more leak-tolerant. Consequently, the inherent problems with triggering, cycling, inadequate volume delivery, and maintenance of PEEP have been largely eliminated, and these ventilators can now be used for both invasive and noninvasive ventilation. Since ICU ventilators use a circuit with separate inspiratory and expiratory limbs and an active exhalation valve, there is no need for an exhalation port with its intentional gas leak.

Modes and Breath Types

ICU ventilators vary with respect to the modes and breath types available during NIV, so it’s important to know the options on your machine. In general, ventilators provide some or all of the mode-breath type combinations previously listed for bi-level machines. Some machines also allow:

  • CMV-VC—Patients receive a clinician-set number of mandatory volume control breaths but may trigger any number of additional volume control breaths.

  • SIMV-VC—Patients receive a clinician-set number of mandatory volume control breaths but may trigger any number of additional pressure support breaths.

Patient Selection

Box 16.1 lists the general indications for NIV. Noninvasive ventilation is predominantly used to treat or prevent hypercapnia and respiratory acidosis in patients with established or impending ventilation or oxygenation-ventilation failure. It is used less often to improve arterial PO2 and SaO2 in patients with oxygenation failure. Generally accepted contraindications to NIV are listed in Box 16.2.

Summary of Available Evidence

When compared with standard medical therapy, randomized controlled trials have shown that NIV decreases the need for intubation and reduces mortality in patients with:

  • Hypercapnic acidosis caused by an exacerbation of COPD

  • Cardiogenic pulmonary edema

The effectiveness of NIV in patients with established or impending hypercapnic acidosis from other causes, such as asthma, neuromuscular diseases, restrictive chest wall diseases, and obesity-hypoventilation syndrome, is less clear. That’s because, depending on the disease, there are either insufficient data or studies have reported conflicting results. Despite this lack of evidence, it’s clear that, for many patients with these disorders, NIV can improve dyspnea, tachypnea, and gas exchange and may eliminate the need for intubation. In the absence of contraindications, a trial of NIV is warranted in most patients.

The role of NIV in patients with oxygenation failure is more controversial. Some randomized controlled trials and observational studies have shown improved outcomes and reduced need for intubation in patients with pneumonia and even mild ARDS, whereas others have failed to demonstrate these benefits. What’s concerning, though, is that failure to improve with NIV is associated with significantly increased mortality. This has suggested to some investigators that NIV causes an unsafe delay in intubating patients with progressive, hypoxemic respiratory failure. If NIV does, in fact, reduce the need for intubation in some patients, it’s likely that this benefit occurs within a relatively narrow window during the course of the disease. Since this “sweet spot” has not been identified, NIV should be used with caution in this patient population. A brief, closely monitored trial may be worthwhile, but lack of significant, objective improvement requires immediate intubation and invasive mechanical ventilation.

How to Initiate, Monitor, and Adjust Noninvasive Ventilation

The Patient–Ventilator Interface

Either a full (covers nose and mouth) or a total (covers nose, mouth, and eyes) face mask should be used. It’s important to select the type and size that optimize patient comfort while minimizing gas leaks.

Ventilator Settings

Bi-level Ventilators

In patients with established or impending hypercapnic acidosis, the ventilator should ideally be set to provide an adequate number of guaranteed breaths, each with a tidal volume of 400–500 ml. This can be done using any of the mode–breath type combinations that were previously described. Suggested initial settings are shown in Table 16.1. Note that you must set the IPAP and EPAP and that the tidal volume is proportional to the difference between these pressures (the inspiratory pressure gradient). On SIMV-PC, the pressure support level must also be specified. It’s important to assess the effect of these initial settings within 10–15 minutes and make changes, as needed.

Table 16.1 Initial Settings on a Bi-Level Ventilator

Mode

CMV-PC

SIMV-PC

Mandatory rate (bpm)

14

14

FIO2

1.0

1.0

IPAP (cmH2O)

12

12

EPAP (cmH2O)

5

5

PS level (cmH2O)

12

IPAP = inspiratory positive airway pressure; EPAP = expiratory positive airway pressure; PS = pressure support

If the patient requires additional ventilation because of persistent or worsening dyspnea, tachypnea, or respiratory acidosis, increase the IPAP in increments of 3–5 cmH2O. Tidal volume and minute ventilation will usually increase in proportion to the inspiratory pressure gradient. IPAP should be increased as often as every 10 minutes and titrated to clinical effect.

If oxygenation is inadequate, incrementally increase the EPAP by 3–5 cmH2O. This will augment alveolar recruitment, reduce shunt fraction, and improve the PaO2 and SaO2. It’s important to remember that when EPAP is changed, most bi-level ventilators do not automatically adjust the IPAP to keep the inspiratory pressure gradient constant. This means that you must always change the EPAP and IPAP by the same amount if you want to maintain the same level of inspiratory support.

In general, keep inspiratory airway pressure below 25–30 cmH2O. Higher pressures are usually poorly tolerated, often produce an unacceptably high leak, and may lead to gastric inflation.

ICU Ventilators

If an ICU ventilator is used, you must first make sure that it’s capable of and set to deliver NIV. Initial settings and subsequent adjustments are the same as when using bi-level ventilators. Unfortunately, the terminology usually differs, so it’s important to become familiar with the options provided by your machine. Initial recommended settings are listed in Table 16.2.

Table 16.2 Initial Settings on ICU Ventilators (NIV-capable)

Mode

CMV-PC

SIMV-PC

Mandatory rate (bpm)

14

14

FIO2

1.0

1.0

Driving pressure (cmH2O)

7

7

PEEP (cmH2O)

5

5

PS level (cmH2O)

12

PEEP = positive end-expiratory pressure; PS = pressure support

Instead of setting total inspiratory pressure (IPAP), most ICU ventilators require that you specify the inspiratory pressure gradient. This is the driving pressure of pressure control breaths and the pressure support level of pressure support breaths. You must also set a PEEP level. Total inspiratory pressure is then the sum of the set inspiratory pressure gradient and PEEP. This allows you to adjust inspiratory support simply by changing the driving pressure or pressure support level, and changes in PEEP do not affect the inspiratory pressure gradient.

Monitoring

When used to treat acute or acute-on-chronic respiratory failure, NIV should always be initiated in the ICU. Patients must be closely monitored to assess their response to NIV and to determine appropriate adjustments in ventilator settings. Cardiac monitoring, continuous pulse oximetry, and airway-suction devices are essential, and equipment and personnel must be immediately available for endotracheal intubation, should the need arise. In patients with hypercapnic acidosis, arterial blood gases should be monitored closely for the first few hours to follow PaCO2 and arterial pH, and an indwelling arterial catheter is usually needed.

Converting to Invasive Mechanical Ventilation

Although this decision must be made on a case-by-case basis, there are two important guiding principles. First, most patients should be intubated if they fail to improve or worsen during the first few hours of NIV, or if they develop additional contraindications to its use (e.g., depressed mental status or agitation, excessive secretions, hypotension). Second, always err on the side of performing early, elective intubation rather than waiting until rapid deterioration leads to a potentially life-threatening airway emergency.

Additional Reading

Cabrini L, Giovanni L, Oriani A, et al. Noninvasive ventilation and survival in acute care settings: A comprehensive systematic review and meta-analysis of randomized controlled trials. Crit Care Med. 2015;43:880–888.Find this resource:

Gregoretti C, Pisani L, Cortegiani A, Ranieri VM. Noninvasive ventilation in critically ill patients. Crit Care Clin. 2015;31:435–457.Find this resource:

Hess DR. Noninvasive ventilation for acute respiratory failure. Respir Care. 2013;58: 950–969.Find this resource:

Scala R, Naldi M. Ventilators for noninvasive ventilation to treat acute respiratory failure. Respir Care. 2008;53:1054–1080.Find this resource: