Supporting oxygenation and ventilation are two of the most common interventions in intensive care; in 2007–2008, approximately 41% of patients in Australian and New Zealand ICUs received invasive mechanical ventilation and 8% received non-invasive ventilation (NIV).¹ The technology available for supporting oxygenation and ventilation is complex, ranging from simple interventions, such as nasal cannulae through to invasive mechanical ventilation and extracorporeal support. Additionally, the meaning of ventilator terminology is often unclear and terms may be used interchangeably. Critical care nurses must have a strong knowledge of the underlying principles of oxygenation and ventilation that will facilitate an understanding of respiratory support devices, associated monitoring priorities and risks.

Oxygen Therapy

Oxygen is required for aerobic cellular metabolism and ultimately for human survival, with some cells, such as those in the brain, being more sensitive to hypoxia than others. Refer to Chapter 13 for discussion of oxygen delivery and consumption, the oxyhaemoglobin dissociation curve, hypoxaemia and tissue hypoxia; this material provides rationales for clinical decisions regarding the administration of oxygen therapy or ventilation strategies. Oxygen therapy should be considered for patients with a significant reduction in arterial oxygen levels, irrespective of diagnosis and especially if the patient is drowsy or unconscious.

Indications

Indications for oxygen therapy include:

• cardiac and respiratory arrest

• type I respiratory failure

• type II respiratory failure

• chest pain, cardiac failure, myocardial infarction

• low blood pressure, cardiac output

• increased metabolic demands

• carbon monoxide poisoning

Complications

Administration of oxygen, regardless of the delivery device, has potential adverse effects. High concentrations of oxygen cause nitrogen washout, resulting in absorption atelectasis.

Hypoventilation and CO₂ Narcosis

High-dose oxygen therapy may lead to hypoventilation, worsening hypercapnia and CO₂ narcosis due to inhibition of the hypoxic drive in a small proportion of patients with chronic obstructive pulmonary disease (COPD). These patients require close monitoring of PaCO₂ levels when oxygen therapy is instituted or increased. Although COPD patients frequently may have a lower baseline SpO₂ (88–94% compared to 96–100% in patients with no lung pathology), treatment of hypoxia is still essential, and oxygen should not be withheld or withdrawn while hypoxia remains, even if hypercapnia worsens.^2,³

Practice tip

Oxygen should not be withheld or withdrawn while hypoxia remains, even if hypercapnia worsens.

Oxygen Toxicity

Administration of high concentrations of oxygen may lead to oxygen toxicity; symptoms include non-productive cough, substernal pain, reduced lung compliance, interstitial oedema, and pulmonary capillary haemorrhage. These symptoms may be mistakenly attributed to the underlying illness, especially in a sedated and ventilated patient. Many of the symptoms abate once the percentage or fraction of inspired oxygen (FiO₂) is reduced, although irreversible pulmonary fibrosis may occur (see Box 15.1). The concentration and duration of oxygen exposure that induces oxygen toxicity varies between patients;⁴ the lowest possible FiO₂ should therefore be used to achieve the target PaO₂ or SpO₂.

Oxygen Administration Devices

Initial management of hypoxia in a spontaneously-breathing patient with an intact airway is low-flow oxygen via nasal cannulae (up to 6 L/min) or face mask (up to 15 L/min). Although oxygen devices have traditionally had FiO₂ ascribed to specific flow rates, the FiO₂ delivered to the alveoli is influenced by:

• patient factors: inspiratory flow rate, respiratory rate, tidal volume, respiratory pause

• oxygen device factors: oxygen flow rate, volume of mask/reservoir, air vent size, tightness of fit

Normal inspiratory flow in a healthy adult ranges between 25 and 35 L/min. Patients with respiratory failure tend to increase their flow demand from 50 up to 300 L/min. Patients in respiratory distress are characterised by high respiratory rates and low tidal volumes ^4,⁵ that can significantly decrease the FiO₂ available via an oxygen delivery device, depending on the type in use.

All oxygen delivery devices use some type of reservoir to support oxygen delivery and prevent CO₂ rebreathing. In the case of a face mask, the reservoir is the mask; for nasal cannulae it is the patient’s pharynx. Patients with high inspiratory flow and tidal volume will deplete the reservoir faster than it can be replenished, resulting in air entrainment and dilution of the oxygen concentration.

Variable Flow Devices

A range of low or variable flow oxygen delivery devices are available to meet a patient’s physiological needs. These devices range from nasal cannulae and oxygen masks with different features, through to bag–mask ventilation.

Low-flow Nasal Cannulae

Traditional low-flow nasal cannulae sit at the external nares and deliver 3–4 L/min of oxygen. Higher flows may cause discomfort and damage from the drying effect on respiratory mucosa. They are generally well-tolerated by the patient. Increased flow demand with respiratory distress dilutes the oxygen, reducing the FiO₂ to the alveoli. Mouth-breathing and talking can also render nasal cannulae ineffective.

High-flow Nasal Cannulae

High-flow nasal cannulae (HFNC) have slightly larger prongs that facilitate oxygen flow of up to 60 L/min leading to less air entrainment effect than with other oxygen delivery systems.^5,⁶ HFNC generate low levels of end-expiratory pressure and can therefore reduce tachypnoea and work of breathing.^7,⁸ The high gas flow may flush CO₂ from the anatomical dead space preventing CO₂ rebreathing and thereby decreasing PaCO₂, although this is not well supported by the literature.^9,¹⁰ These systems are also generally well-tolerated by the patient, but must be used with heated humidification to avoid drying the respiratory mucosa.⁸ HFNC are now used more frequently in clinical practice to avoid more invasive therapies but there is limited high-quality evidence on their use in adults.

Practice tip

Patients using any type of nasal cannulae should avoid mouth-breathing and talking to minimise diluting oxygen delivery to the lungs.

Practice tip

Transparent face masks are recommended for bag–mask ventilation as they allow immediate recognition if a patient vomits.

Airway Support

The most common cause of partial airway obstruction in an unconscious patient is loss of oropharyngeal muscle tone, particularly of the tongue. This may be alleviated by tilting their head slightly back and lifting the chin, or thrusting the jaw forward. The head-tilt/chin-lift manoeuvre is not used if cervical spine injury is suspected.¹⁵ The jaw-thrust manoeuvre may require two hands to maintain.¹⁶ If more prolonged support is required, an oro- or nasopharyngeal airway can be used that may also facilitate bag–mask ventilation.

Oro- and Nasopharyngeal Airways

The Guedel oropharyngeal airway is available in various sizes (a medium-sized adult requires a size 4). The airway is inserted into the patient’s mouth past the teeth, with the end facing up into the hard palate, then rotated 180 degrees, taking care to bring the tongue forward and not push it back. Oropharyngeal airways are poorly tolerated in conscious patients and may cause gagging and vomiting.¹⁴

A nasopharyngeal airway (see Figure 15.1) is inserted through the nares into the oropharynx; it can be difficult to insert and require generous lubrication to minimise trauma. This type of airway should not be used for patients with a suspected head injury. As well as opening the airway, suction catheters can be passed to facilitate secretion clearance. Once inserted these airways are better tolerated than an oropharyngeal airway.

FIGURE 15.1 Nasopharyngeal airways.²⁷²

Laryngeal Mask Airway and Its Intubation

The classic laryngeal mask airway (cLMA) (see Figure 15.2) is positioned blindly into the pharynx to form a low-pressure seal against the laryngeal inlet. It is easier and quicker to insert than an endotracheal tube, and is particularly useful for operators with limited airway skills; the cLMA does not carry the same potentially fatal complications such as oesophageal intubation although the risk of aspiration remains.¹⁷

FIGURE 15.2 Laryngeal mask airways.²⁷²

Mechanical ventilation can be delivered with low-airway pressures (less than 20 cmH₂O) via a cLMA. This device is widely used in elective general anaesthesia,¹⁵ and can be used in critical care as an alternative to bag–mask ventilation¹⁷ or endotracheal intubation when initial attempts at intubation have failed.¹⁸ The ‘intubating’ LMA is most commonly used when a difficult intubation is anticipated or encountered. This device has a handle and is more rigid, wider and curved than the cLMA, enabling passage of a purpose-made endotracheal tube.¹⁷

Combitube

The combitube is more widely used in North America for emergency situations than in Australia and the UK.¹⁵ It is a dual-lumen, dual-cuff oesophageal-tracheal airway that enables ventilation if inserted into either the oesophagus or trachea. Inexperienced operators may find a combitube more difficult to insert correctly than a cLMA.¹⁹ Complications may occur in up to 40% of patients and include aspiration pneumonitis, pneumothorax, airway injuries and bleeding, oesophageal laceration and perforation and mediastinitis.²⁰

Intubation

Endotracheal intubation is the ‘gold standard’ for airway support, providing airway protection in the presence of an airway oedema, absent gag, cough or swallow reflex. Intubation facilitates delivery of mechanical ventilation and pulmonary secretion clearance.¹⁶

Endotracheal Tubes

Endotracheal tubes (ETT) are available with internal diameters ranging from 2–10 mm (common adult sizes are 7–9 mm), and are up to 30 cm long. A longitudinal radio-opaque line allows visualisation of tube placement on a chest X-Ray. Markings at 1 cm intervals indicate the length from the distal end. Tubes are available with and without a distal cuff. Adults typically require a cuffed ETT to seal their trachea, facilitating positive pressure ventilation and preventing aspiration of oropharyngeal contents. Cuffs come in a range of profiles and volumes, but are commonly high-volume, low-pressure (see Figure 15.3).

FIGURE 15.3 Endotracheal tube.²⁷²

Endotracheal tubes may be reinforced with a wire coil embedded within the plastic for the entire length of the tube to prevent kinking and occlusion. These tubes are more commonly used in the operating room.²¹ The wire coils can however be irreversibly compressed by a strong bite that occludes the airway. Reinforced tubes also increase the risk of tracheal damage and therefore should be replaced with a standard endotracheal tube on arrival in the ICU. Most endotracheal tubes have a ‘Murphy eye’, an oval-shaped hole in the side of the tube between the cuff and the end of the tube that provides a patent aperture if the distal opening is occluded.²²

Preparation for Intubation

Adequate preparation of the patient, equipment and environment, as well as strong knowledge of emergency procedures is important to ensure safe and efficient intubation. Up to 50% of patients undergoing endotracheal intubation in ICU will experience a complication; 28% will have a serious complication, including hypoxaemia, circulatory collapse, cardiac arrhythmia, cardiac arrest, oesophageal intubation, aspiration and death.²³

Patient Preparation

If appropriate, and time permits, explain the procedure to the patient and family. Prepare the patient with:

• reliable intravenous access established to allow rapid fluid and drug administration

• accurate blood pressure monitoring (preferably intra-arterial)

• continuous oxygen saturation and ECG monitoring

• nasogastric tube (if in situ) should be aspirated and placed on free drainage

• positioned supine in the ‘sniff’ position

Procedure

The patient is preoxygenated to minimise desaturation during apnoea and laryngoscopy, commonly via bag and mask, although other methods such as non-invasive ventilation have been suggested.²⁴ Intubation in ICU is usually performed via laryngoscopy with insertion of an oral ETT. Intubation may be performed using a fibreoptic bronchoscope when difficulty is encountered, or for nasal intubation.

Oral vs Nasal Intubation

Oral intubation is preferred unless there are specific indications for nasal intubation. Oral intubation is easier to perform and allows use of a larger diameter tube. While nasal intubation provides better splinting for the ETT and facilitates oral hygiene, it can damage nasal structures, is contraindicated in skull fractures and increases the risk of maxillary sinusitis and ventilator-associated pneumonia.²⁵

Cricoid Pressure

Cricoid pressure (Sellick manoeuvre) was introduced in the 1960s to prevent aspiration of gastric contents during intubation. The oesophagus lies behind and in line with the trachea. The cricoid cartilage, situated below the thyroid prominence, is a closed tracheal ring which, when compressed, closes the oesophagus while the trachea remains open. Cricoid pressure is performed by placing your thumb on one side of the patient’s trachea, middle finger on the other side and index finger directly on the cricoid.²⁶ Although widely used over the last 50 years, its efficacy is being questioned as technique is frequently poor,²⁷ and there is wide anatomical variation in the exact orientation of the oesophagus in relation to the trachea.²⁸

Backwards, Upwards, Rightward Pressure Manoeuvre

The backwards, upwards, rightward pressure (BURP) manoeuvre on the thyroid cartilage was introduced in the mid-1990s to improve visualisation during difficult laryngoscopy. The patient’s jaw is thrust forward, so their head is in the ‘sniffing’ position. Place your thumb and third finger on either side of the thyroid cartilage and index finger on top. Pressure is applied in the sequence backwards (towards the spine), upwards (towards the head), rightward (towards the patient’s right side). This is easier to perform following administration of muscle relaxants.^29,³⁰

Cuff Management

Endotracheal and tracheostomy tube cuffs prevent airway contamination by pharyngeal secretions and gastric contents and loss of tidal volume during mechanical ventilation. The cuff does not secure the tube in the trachea. Cuff inflation pressures should be maintained at 20–30 cmH₂O.^31,³² Cuff inflation pressures ≤20 cmH₂O (15 mmHg) are associated with an increased risk of aspiration and a 2.5-fold increase in ventilator-associated pneumonia (VAP).³³ Conversely, tracheal wall damage may occur if cuff pressure exceeds the capillary perfusion pressure in the trachea (27–40 cmH₂O/20–30 mmHg).

There are four methods described for assessing cuff inflation:

1. minimal occluding volume (MOV)

2. minimal leak test (MLT)

3. cuff pressure measurement (CPM)

4. palpation.

In Australia and New Zealand, CPM is the most common form of cuff pressure assessment,³⁴ in contrast to the UK³⁵ and North America³⁶ where CPM is used infrequently. Cuff pressure varies with head and body position, tube position and airway pressures.³⁷ The optimum frequency of cuff pressure monitoring is unclear; at a minimum it should be done post-intubation, on arrival in ICU and once per nursing shift. A persistent cuff leak or pressures of ≥30 cmH₂O (22 mmHg) to generate a seal should be reviewed and referred to medical staff.

If performing MOV and MLT, aspiration should be prevented by semi-recumbent positioning, suctioning at the back of the mouth (as far back as tolerated), aspiration of the nasogastric tube and discontinuation of feeds before cuff deflation.

Practice tip

If the pilot tube for the ETT is accidentally cut, cannulate the tubing with a 23- or 24-gauge needle to reinflate the cuff and clamp the tubing. If using a clamp with serrations, place gauze between the tube and the clamps to avoid further damage to the pilot tube.

Endotracheal Tube Fixation

The purpose of ETT fixation is to maintain the tube in the correct position, prevent unintended extubation and facilitate mechanical ventilation while maintaining skin integrity and oral hygiene.³⁸ ETT fixation methods include:

• tying cotton tape around the tube, then around the patient’s neck

• taping the tube to the patient’s face using medical adhesive tape

• commercial tubeholders of varying designs.

There is no evidence supporting a preferred method ³⁹ with each having specific strengths and weaknesses. Two nurses are required to prevent ETT dislodgement during fixation. Although there is also no evidence to recommend a preferred frequency, ETT fixation is generally changed at least daily, to allow assessment of the underlying skin with particular attention to the tops of the ears and corners of the mouth and to facilitate oral hygiene.³⁸ The ETT position in the mouth is alternated at this time.

Practice tip

Adhesive devices may become dislodged as facial hair grows under them.

Confirmation of Tube Position

The correct position of the ETT distal end is 3–5 cm above the carina. A lip level of 20 cm for women and 22 cm for men should prevent endobronchial intubation, with the proximal end fixed at either the centre or the side of the mouth.⁴⁰ Confirmation of the ETT position is required immediately following intubation and at regular intervals thereafter as movement of the tube can occur.

Chest auscultation is the traditional method to confirm ETT position. Observation of chest expansion is, however, unreliable, as the chest may appear to rise with oesophageal intubation. Conversely the chest may not rise with a correctly positioned tube if the patient is obese or has a rigid chest wall. Patients with left main bronchus intubation may exhibit bilateral breath sounds.⁴¹ End-tidal CO₂ monitoring is the ‘gold standard’ method for confirming ETT placement. Disposable devices that change colour in the presence of CO₂ are inexpensive and easy to use, but may be inaccurate during cardiopulmonary resuscitation, or if contaminated. Capnography is the most reliable technique to identify ETT placement in both arrest and non-arrest situations.¹⁸ Continuous end-tidal CO₂ monitoring during intubation is recommended as a minimum standard by the College of Intensive Care Medicine of Australia and New Zealand.⁴²

Practice tip

Always ensure there is someone who is skilled at intubation immediately available when extubating a patient.

Tracheostomy

Tracheostomy may be required for upper airway obstruction, although it is most commonly performed for ICU patients who require prolonged mechanical ventilation. The advantages of tracheostomy over endotracheal intubation include decreased risk of laryngeal damage and subglottic stenosis, reduced airway resistance and deadspace which decreases the work of breathing and therefore supports weaning,⁴³ and improved patient tolerance enabling reduction of sedation. The optimum time to perform tracheostomy remains contentious, and is often influenced by a patient’s diagnosis.⁴⁴

Procedure

Tracheostomy can be performed using a surgical technique (ST) or percutaneous dilatational technique (PDT). PDT is contraindicated in patients with anatomical anomalies of the neck and serious bleeding disorders, and should be undertaken with caution in patients who are obese, have a cervical spine injury, coagulopathy, difficult airway or require high levels of ventilatory support.⁴⁵ PDT is more commonly performed than ST in Australian and New Zealand ICUs.⁴⁵

A variety of tracheostomy tubes are available that facilitate secretion clearance, communication and differing patient anatomy. Inner cannulas (re-usable or disposable) prevent secretion build up on the tracheostomy tube, while fenestrated and talking tracheostomies facilitate communication, as do Passe Muir valves used with the cuff deflated.

Tracheostomy Care

The aim of tracheostomy care is to keep the site free of infection, and prevent tube blockage or dislodgement. The site is cleaned with normal saline and fixation devices changed at least 12-hourly with two nurses to safely perform tape changes.⁴⁶ Velcro tapes are easier to change and more comfortable than cotton tape.⁴⁷ Lint-free or superabsorbent foam dressings may be placed under the flange to absorb secretions. Adequate humidification and suctioning will usually prevent tube obstruction (see later in this chapter). The use of inner cannulae has obviated the need for frequent tracheostomy tube changes. Single lumen (no inner cannula) tracheostomy tubes should be changed every 7–10 days.⁴⁶

Complications of Endotracheal Intubation and Tracheostomy

Tube blockage, tube dislodgement and aspiration are major complications. Partial ETT or tracheostomy tube dislodgement can cause greater harm than complete removal because of delays in diagnosis and resultant aspiration or worsening gas exchange. Tube dislodgement is most likely to occur when turning the patient, if the patient is agitated or when nursing staff are distracted or on breaks.⁴⁸ While physical restraint may be considered to prevent tube dislodgement, multiple studies noted patients were restrained at the time of self-extubation or device removal.⁴⁹^–⁵⁴ Effective levels of analgesia and sedation is therefore most appropriate in minimising the risk of self-extubation.

Complications during and immediately after endotracheal intubation and tracheostomy include cardiovascular compromise, bleeding, injury to the tracheal wall, damage to the vocal cords, pneumothorax, pneumomediastinum and subcutaneous emphysema. Late complications of tracheostomy include tracheal stenosis, tracheomalacia and tracheo-oesophageal fistula and infection. As noted earlier, damage to the trachea is exacerbated by high cuff pressures.⁵⁵ PDT results in fewer wound infections, decreased incidence of bleeding and reduced mortality compared to ST.⁵⁶

Tracheal Suction

Patients with an ETT or tracheostomy tube require tracheal suction to remove pulmonary secretions that can lead to atelectasis or airway obstruction and impair gas exchange.⁵⁷ Suction should be performed as clinically indicated, with assessment of visible or audible secretions, rising inspiratory pressure, decreasing V_T or increased work of breathing.⁵⁸ A sawtooth pattern on the flow-volume waveform may also indicate the need for suction (discussed later in this chapter).⁵⁹

Preoxygenation using a FiO₂ of 1 for 60 seconds prior to performing suction minimises hypoxia and the potential for cardiac arrhythmias. Manual hyperinflation is discouraged due to the risk of barotrauma and lack of benefit. Similarly, installation of saline is not supported due to increased risk of flushing pathogens into distal lung regions.⁶⁰

Methods

The three methods of suctioning are:

• open suction: a suction catheter is passed under aseptic technique directly into the ETT/tracheostomy after disconnection from the ventilator circuit. Disadvantages include loss of PEEP resulting in loss of alveolar recruitment and increased risk of transmission of infective organisms. A surgical mask and protective eyewear should be worn.⁶¹

• semi-closed suction: a suction catheter is passed through a swivel connector with a self-sealing rubber flange.

• closed suction: in-line system is attached between the ETT/tracheostomy tube and the ventilator circuit where the suction catheter is contained in an integrated plastic sleeve. Alveolar derecruitment occurs to a lesser degree than with open suction.

There is no difference between techniques in relation to development of ventilator-associated pneumonia (VAP) and quantity of secretions removed.

The diameter of the suction catheter should not be greater than half the diameter of the airway, using the formula: suction catheter size [Fr] = (ET tube size [mm] − 1) × 2. The suction catheter should be inserted to the carina, then withdrawn 2 cm before suction is applied to prevent damage to the carina. Suction should only last 15 seconds, using continuous, rather than intermittent, suction. Use of ETTs or tracheostomy tubes with integrated subglottic suction ports may assist in preventing VAP, especially when performed with other prevention strategies such as semirecumbant positioning and good cuff seal management.

Adverse Effects

Adverse effects of suction can include hypoxaemia, introduction of infective organisms, tracheal trauma, bradycardia, hypertension and increased intracranial pressure. Tracheal suctioning causes discomfort, and should therefore be performed only when clinically indicated, such as audible presence of secretions and desaturation.⁵⁸

Extubation

Following successful weaning from mechanical ventilation (see later in this chapter), assessment of the patient prior to extubation should include adequate gas exchange, respiratory rate and work of breathing on minimal support for prolonged periods, respiratory muscle strength, the ability to cough and clear secretions spontaneously and a stable haemodynamic status and mental status.⁶² Serious post-extubation complications of laryngospasm and stridor cannot be reliably predicted,⁶³ so the ease/grade of intubation should be considered prior to extubation and provision made for immediate re-intubation.⁶⁴

Mechanical Ventilation

As stated in the introduction, 41% of patients in Australian and New Zealand ICUs received invasive mechanical ventilation and 8% received non-invasive ventilation (NIV) in 2007–08.¹ The median duration of invasive mechanical ventilation for these patients was 2.5 days. In the most recent international study of mechanical-ventilation practices, reporting data from 4968 patients in 349 ICUs and 23 countries found the median duration of ventilation to be 4 days (interquartile range 2–8 days).⁶⁵ In this patient cohort the three most common reasons for mechanical ventilation were postoperative respiratory failure, coma and pneumonia. This international report did not include data from Australia and New Zealand. A study describing ventilation and weaning practices of 55 ICUs in Australia and New Zealand in 2005 reported a similar profile for the most common indications for mechanical ventilation.⁶⁶

Principles of Mechanical Ventilation

Mechanical ventilation describes the application of positive or negative pressure breaths using non-invasive or invasive techniques. Indications for initiation of mechanical ventilation are discussed below. Table 15.1 lists the patient parameters typically observed in acute and chronic respiratory failure that may be influential in the decision to ventilate. During positive pressure ventilation, the type of ventilation used most commonly in critical care, the ventilator delivers a flow of gas into the lungs during inspiration using a pneumatic system. Expiration is passive.

TABLE 15.1 Physiological indications suggesting the need for mechanical ventilation

The Equation of Motion

The equation of motion for the respiratory system is a mathematical model that relates pressure volume and flow during the delivery of a breath, with the pressure required to deliver a volume of gas to the lungs determined by the elastic and resistive properties of the respiratory system ⁶⁷ (see Table 15.2).

TABLE 15.2 Equation of motion

Equation: P_T (P_airway + P_muscle) = V_T/Cr + V_T/T_I × R + PEEP_T
Abbreviations:	P_T = total pressure: the sum of the pressure in the proximal airway and the pressure generated by the respiratory muscles V_T = tidal volume Cr = compliance T_I = inspiratory time R = resistance PEEP_T = total positive end expiratory pressure: alveolar pressure at the end of expiration and is the sum of PEEP applied by the ventilator and any intrinsic (auto) PEEP.

Notes:

V_T/Cr: describes the elastic properties the respiratory system

V_T/T_I: reflects flow in the system

V_T/T_I × R: resistance of the respiratory system.

Compliance and Elastance

Compliance refers to the ease with which lung units distend. Elastance is the tendency of the lung units to return to their original form once stretched. Compliance is defined as the change in volume that occurs due to a change in pressure.

Lung tissue and the surrounding thoracic structures contribute to respiratory compliance. Normal compliance for a mechanically ventilated patient ranges from 35–50 mL/cmH₂O.⁶⁸

Resistance

Resistance refers to the forces that oppose airflow. Resistance in the airways is affected by the diameter and length of the airways, including the artificial airway, the gas flow rate and the density and viscosity of the inspired gas. During mechanical ventilation, bronchospasm, airway oedema, endotracheal tube lumen size, increased secretions, and inappropriate setting of flow rates can influence airway resistance. Normal resistance for intubated patients is 6 cmH₂O/(L/sec).⁶⁸

Ventilator Circuits

Delivery of mechanical ventilation requires a ventilator circuit to transport gas flow to the patient. To prevent condensation from cooling of warm humidified gas, inspired gas is heated via a wire inside the wall of the circuit in either the inspiratory limb alone or both the inspiratory and expiratory limbs.⁶⁹ Historically ventilator circuits were changed frequently (48–72 hours) to decrease the risk of VAP.⁷⁰ Current guidelines for prevention of VAP found evidence that the frequency of ventilator circuit changes had no relationship to the incidence of VAP and therefore recommended routine circuit changes were not necessary and circuits should only be changed when soiled or damaged.⁷¹

Humidification

Humidification techniques warm and moisten gas to facilitate cilia action and mucus removal as well as to prevent drying and irritation of respiratory mucosa and solidification of secretions. During endotracheal intubation and mechanical ventilation, the normal humidification processes of the nasopharynx are bypassed. This, in combination with the use of dry medical gas at high flow rates, means alternative methods of humidification are required. The best conditions for mucosal health and function over prolonged periods are when inspired gas is warmed to core body temperature and is fully saturated with water.⁷²

Absolute and Relative Humidity

Absolute humidity refers to the amount of water vapour in a given volume of gas at a given temperature. Absolute humidity rises with increasing temperature; during mechanical ventilation gas is heated to increase the amount of water vapour it will hold. Relative humidity is expressed as a percentage, and is the actual amount of water vapour in a gas compared to the maximum amount this gas can hold (ratio of absolute to maximal humidity). Ideal humidification is achieved when:

1. the inspired gas delivered into the trachea is at 37°C with a water content of 30–43 g/m³ (relative humidity is 100% at 37°C in the bronchi)

2. the set temperature remains constant without fluctuation

3. humidification and temperature are unaffected by a large or differing types of gas flow

4. the device is simple to use

5. the humidifier can be used with spontaneously breathing and ventilated patients

6. safety alarms prevent overheating, overhydration and electrocution

7. the resistance, compliance and dead space characteristics do not adversely affect spontaneous breathing modes

8. the sterility of the inspired gas is not compromised.⁷³

Humidification is applied using either a heat–moisture exchanger (HME) or a heated water bath reservoir device in combination with a heated ventilator circuit.

Heat–moisture Exchanger

Heat–moisture exchangers conserve heat and moisture during expiration, and enable inspired gas to be heated and humidified. Two types of HMEs exist: hygroscopic and hydrophobic. Hygroscopic HMEs absorb moisture onto a chemically impregnated foam or paper material and have been shown to be more effective than hydrophobic HMEs.⁷⁴ HMEs are placed distally to the circuit Y-piece in line with the endotracheal tube and increase dead space by an amount equal to their internal volume.⁷⁵ HMEs should be changed every 24 hours or when soiled with secretions and are usually reserved for short term humidification.

Heated Humidification

Generally, heated humidification (HH) is used for patients requiring greater than 24 hours of mechanical ventilation. Various models of heater bases and circuits are on the market and we recommend their use in accordance with manufacturer instructions. A recent systematic review and meta-analysis reported no overall effect on artificial airway occlusion, mortality, pneumonia, or respiratory complications when HMEs were compared to HHs, although it noted that PaCO₂ and minute ventilation were increased and body temperature was lower with the use of HMEs.⁷⁶

Non-Invasive Ventilation

Non-invasive ventilation (NIV) is an umbrella term describing the delivery of mechanical ventilation without the use of an invasive airway, via an interface such as an oronasal, nasal, or full face mask or helmet. NIV techniques include both negative and positive pressure ventilation, although in critical care positive pressure ventilation is primarily used.

Terminology

Positive pressure NIV can be further categorised as non-invasive positive pressure ventilation (NIPPV) or continuous positive airway pressure (CPAP). NIPPV is the provision of inspiratory pressure support (also referred to as inspiratory positive airway pressure [IPAP]) usually in combination with positive end expiratory pressure (PEEP) (also referred to as expiratory positive airway pressure [EPAP]). CPAP does not actively assist inspiration but provides a constant positive airway pressure throughout inspiration and expiration.⁷⁷

The terms Biphasic (or bilevel) positive airway pressure (BiPAP®) and non-invasive pressure support ventilation (NIPSV) are also used to refer to NIPPV.⁷⁸ The acronym BiPAP® is registered to Respironics (Murrayville, PA), a company that produces a number of non-invasive ventilators including the BIPAP Vision, a NIV ventilator commonly used in the ICU. The acronym NIPSV is primarily used in European descriptions of NIPPV.

Practice tip

When other members of the ICU team use the term BiPAP/BIPAP, clarify if they are referring to non-invasive or invasive ventilation.

Physiological Benefits

The efficacy of NIV in patients with acute respiratory failure is, at least in part, related to avoidance of inspiratory muscle fatigue through the addition of inspiratory positive pressure thus reducing inspiratory muscle work.⁷⁹ Application of positive pressure during inspiration increases transpulmonary pressure, inflates the lungs, augments alveolar ventilation and unloads the inspiratory muscles.⁸⁰ Augmentation of alveolar ventilation, demonstrated by an increase in tidal volume, increases CO₂ elimination and reverses acidaemia. High levels of inspiratory pressure may also relieve dyspnoea.⁸¹

The main physiological benefit in patients with congestive heart failure (CHF) is attributed to the increase in functional residual capacity associated with the use of PEEP that reopens collapsed alveoli and improves oxygenation.⁸² Increased intrathoracic pressure associated with the application of positive pressure also may improve cardiac performance by reducing myocardial work and oxygen consumption through reductions to ventricular preload and left ventricular afterload.⁸²^–⁸⁴ NIV also preserves the ability to speak, swallow, cough and clear secretions, and decreases risks associated with endotracheal intubation.⁸⁵

Indications for NIV

The success of NIV treatment is dependent on appropriate patient selection.⁸⁶ Table 15.3 outlines indications and contraindications to NIV.

TABLE 15.3 Indications and contraindications for non-invasive ventilation ⁷⁷

Indications
Bedside observations	Increased dyspnoea: moderate to severeTachypnoea: >24 breaths per min [obstructive] >30 breaths per min [restrictive] Signs of increased work of breathing, accessory muscle use and abdominal paradox
Gas exchange	Acute or acute-on-chronic ventilatory failure (best indication), PaCO₂ >45 mm Hg, pH <7.35 Hypoxaemia (use with caution), PaO₂/FIO₂ ratio <200
Contraindications
Absolute	Respiratory arrest Unable to fit mask
Relative	Medically unstable: hypotensive shock, uncontrolled cardiac ischaemia or arrhythmia, uncontrolled upper gastrointestinal bleeding Agitated, uncooperative Unable to protect airway Swallowing impairment Excessive secretions not managed by secretion clearance techniques Multiple (i.e. two or more) organ failure Recent upper airway or upper gastrointestinal surgery

PaCO₂: partial pressure of carbon dioxide in arterial blood; PaO₂: partial pressure of oxygen in arterial blood; PaO₂/FIO₂: ratio of partial pressure of oxygen in arterial blood to fraction of inspired oxygen.

Acute Respiratory Failure

Evidence supporting the role of NIV in patients with hypoxaemic respiratory failure is limited and conflicting.⁸² For patients with community-acquired pneumonia, NIV has been shown to reduce intubation rates, ICU length of stay and 2-month mortality but only in the subgroup of patients with COPD.⁸⁷ Pneumonia also has been identified as a risk factor for NIV failure.⁸⁸

Acute Exacerbation of COPD and CHF

Strong evidence exists to support the use of NIV for patients with acute exacerbation of chronic obstructive pulmonary disease (COPD) and congestive heart failure (CHF). Three meta-analyses have shown a reduction in intubation rates, hospital length of stay and mortality for COPD patients managed with NIPPV compared to standard medical treatment.⁸⁹^–⁹¹ COPD patients most likely to respond favourably to NIPPV include those with an unimpaired level of consciousness, moderate acidaemia, a respiratory rate of <30 breaths/minute and who demonstrate an improvement in respiratory parameters within two hours of commencing NIV.^79,⁹²

Early use of NIV in combination with standard therapy for patients with CHF has also been shown to reduce intubation rates and mortality when compared to standard therapy alone.⁹³^–⁹⁵ A recent meta-analysis found CPAP reduced hospital mortality whereas NIPPV did not have an effect on mortality.⁹⁴ Both NIV modes were shown in this meta-analysis to reduce the need for intubation. An early study comparing NIPPV to CPAP in patients with CHF reported a higher incidence of myocardial infarction.⁹⁶ Based on this finding, practice guidelines from the British Thoracic Society recommend NIPPV should only be used for patients with CHF when CPAP has been unsuccessful.⁹⁷ More recently several studies have found no difference in myocardial infarction rates when comparing the two modes.⁹⁸^–¹⁰¹ A recent large multicentre randomised controlled trial found NIV delivered by either CPAP or NIPPV resulted in symptomatic improvements, but failed to demonstrate a mortality benefit.¹⁰² Practice surveys indicate CPAP may be the preferred method of NIV for patients with CHF in Australia and internationally.^103,¹⁰⁴

NIV in Weaning

NIV may be used as an adjunct to weaning to reduce the duration of invasive ventilation and associated complications.¹⁰⁵ Patients are extubated directly to NIV and then weaned to standard oxygen therapy. This use of NIV differs from its role in preventing reintubation in patients that develop, or who are at high risk of, postextubation respiratory failure.¹⁰⁶ A recent systematic review and meta-analysis of 12 trials of NIV as a weaning adjunct found reductions in mortality, ICU and hospital lengths of stay, duration of ventilation and rates of VAP.¹⁰⁷ Conversely the largest study of NIV use in postextubation respiratory failure reported worsened survival rates hypothesised as a result of delayed reintubation.¹⁰⁸ A subsequent meta-analysis suggested NIV may have a role in preventing the development of respiratory failure postextubation for those at risk, but should be used with caution once respiratory failure has developed and should not delay the decision to reintubate.¹⁰⁶

Other Indications

Other indications for NIV include:

• asthma ¹⁰⁹

• neuromuscular disorders (e.g. muscular dystrophy, amyotrophic lateral sclerosis)

• severe obstructive sleep apnoea

• palliation.

Interfaces and Settings

NIV requires an interface that connects the patient to either a ventilator, portable compressor or flow generator with a CPAP valve. The selection of an appropriate interface can influence NIV success or failure. Oronasal masks cover both the mouth and nose and are the preferred mask type for the management of acute respiratory failure.¹¹⁰ Nasal masks enable speech, eating and drinking, and therefore are used more frequently for long-term NIV use. An oronasal mask enables delivery of higher ventilation pressures with less leak and greater comfort for the patient.¹¹¹ Other interfaces include full-face masks¹¹¹ that seal around the perimeter of the face and cover the eyes as well as the nose and mouth, nasal pillows, mouthpieces that are placed between the patient’s lips, and helmets that cover the whole head and consist of a transparent plastic hood attached to a soft neck collar.^112,¹¹³ These alternative interfaces may increase patient tolerance by reducing pressure ulceration, air leaks and patient discomfort.¹¹⁴

Initiation and Monitoring Priorities

Successful initiation of NIV is dependent on patient acceptance and tolerance. Patient acceptance of NIV may be aided by a brief explanation of the procedure and its benefits. Strategies to enhance patient tolerance include: use of an interface that fits the patient’s facial features, commencing with low pressure levels, holding the mask gently in position prior to securing with the straps/headgear, and ensuring straps prevent major leaks but are not so tight they increase discomfort. Once NIV is commenced, the patient should be monitored for respiratory and haemodynamic stability, response to NIV treatment, ongoing tolerance, and presence of air leaks (Table 15.4). Arterial blood gas analysis should be performed at baseline and within the first one to two hours of commencement.⁹⁷ During the initiation and stabilisation period, patients should be monitored using a nurse-to-patient ratio of 1 : 1 with ongoing coaching to promote NIV tolerance throughout the early stabilisation period.

TABLE 15.4 Monitoring priorities for non-invasive ventilation ¹⁰⁴

Priority	Assessment
Patient comfort	Restlessness Mask tolerance Anxiety level Dyspnoea score Pain score

Conscious level Glasgow coma score Work of breathing

Chest wall motion

Accessory muscle activation

Respiratory rate

Gas exchange parameters

Continuous SpO₂

Arterial blood gas analysis

(Baseline and 1–2 hourly subsequently)

Patient colour

Haemodynamic status

Continuous heart rate

Intermittent blood pressure

Ventilator parameters

Air leak around mask

Adequacy of pressure support (V_T, pH, PaCO₂)

Adequacy peak end expiratory pressure (SpO₂, PaO₂)

SpO₂: saturation of peripheral oxygen; V_T: tidal volume; PaCO₂: partial pressure of carbon dioxide in arterial blood; PaO₂: partial pressure of oxygen in arterial blood.

Practice tip

NIV tolerance may be promoted with a simple explanation of the therapy, reassurance and constant monitoring for your patient. During initiation, allow them to take short breaks from the mask if they are in discomfort or experiencing claustrophobia.

Potential Complications

Masks need to be tight-fitting to reduce air leaks; however, this contributes to pressure ulceration on the bridge of the nose or above the ears (due to mask straps/headgear). Air leaks may cause conjunctival irritation and the high flow of dry medical gas results in nasal congestion, oral or nasal dryness and insufflation of air into the stomach. Claustrophobia associated with the NIV interface may also lead to agitation reducing the efficacy of NIV treatment due to poor coordination of respiratory cycling between the patient and NIV unit.⁷⁹ More serious, yet infrequent, complications include aspiration pneumonia, haemodynamic compromise associated with increased intrathoracic pressures and pneumothorax.⁸⁰

Detecting NIV Failure

Failure to respond to NIV within 1–2 hours of commencement is demonstrated by unchanged or worsening gas exchange, as well as ongoing or new onset of rapid shallow breathing and increased haemodynamic instability.¹¹¹ A decreased level of consciousness may be indicative of imminent respiratory arrest.

Invasive Mechanical Ventilation

Critically ill patients with persistent respiratory insufficiency (hypoxaemia and/or hypercapnia), due to drugs, disease or other conditions, may require intubation and mechanical ventilation to support oxygenation and ventilatory demands.^115,¹¹⁶ Clinical criteria for intubation and ventilation should be based on individual patient assessment and patient response to measures aimed at reversing hypoxaemia.

Mechanical Ventilators

Contemporary ventilators use sophisticated microprocessor controls with sensitive detection, response and control of pressure and gas flow characteristics. These mechanical ventilators are more sensitive to patient ventilatory demands, enabling improved patient–ventilator synchrony during both inspiratory and expiratory breath phases. Parameters commonly manipulated during mechanical ventilation are detailed in Table 15.5. Parameters often observed and documented are discussed below.

TABLE 15.5 Set ventilator parameters

Parameter	Description
Fraction of inspired oxygen (FiO₂)	The fraction of inspired oxygen delivered on inspiration to the patient.
Tidal volume (V_T)	Volume (mL) of each breath.
Set breath rate (f)	The clinician determined set rate of breaths delivered by the ventilator (bpm).
Inspiratory trigger or sensitivity	Mechanism by which the ventilator senses the patient’s inspiratory effort. May be measured in terms of a change in pressure or flow.
Inspiratory pressure (P_insp, P_high)	Clinician determined pressure that is targeted during inspiration.
Inspiratory time (T_insp)	The duration of inspiration (sec).
Inspiratory : expiratory ratio (I : E)	The ratio of the inspiratory time to expiratory time.
Flow (V)	The speed gas travels during inspiration. (L/min).
Pressure support (PS)	The flow of gas that augments a patient’s spontaneously initiated breath to a clinician-determined pressure (cmH₂O).
Positive end-expiratory pressure (PEEP)	Application of airway pressure above atmospheric pressure at the end of expiration (cmH₂O).
Rise time	Time to achieve maximal flow at the onset of inspiration for pressure-targeted breaths.
Expiratory sensitivity	During a spontaneous breath, the ventilator cycles from inspiration to expiration once flow has decelerated to percentage of initial peak flow.
Minute volume (VE)	Generally not set directly but is determined by V_T and f settings. Tidal volume multiplied by the respiratory rate over one minute (L/min).
Airway pressure (P_aw)	The pressure measured in cmH₂O by the ventilator in the proximal airway.
Plateau pressure (P_plat)	The pressure, measured in cmH₂O, applied to the small airways and alveoli. P_plat is not set but can be measured by performing an inspiratory hold manoevre.