Ventilation and Oxygenation Management

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15 Ventilation and Oxygenation Management

Introduction

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.

Complications

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

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.

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.

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.

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

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

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

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.

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:

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.

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:

There is no evidence supporting a preferred method39 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.

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

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

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

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 VT 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:

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.

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.

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

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.

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.

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.8284 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 ventilation77

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.8991 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.9395 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.98101 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

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

Gas exchange parameters Haemodynamic status Ventilator parameters

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

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 (VT) 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 (Pinsp, Phigh) Clinician determined pressure that is targeted during inspiration.
Inspiratory time (Tinsp) 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 VT and f settings. Tidal volume multiplied by the respiratory rate over one minute (L/min).
Airway pressure (Paw) The pressure measured in cmH2O by the ventilator in the proximal airway.
Plateau pressure (Pplat) The pressure, measured in cmH2O, applied to the small airways and alveoli. Pplat is not set but can be measured by performing an inspiratory hold manoevre.

Tidal Volume

Tidal volume (VT) is the volume, measured in mL, of each breath. The VT is calculated using the patient’s ideal body weight using height and gender-specific tables120 to achieve 6–8 mL/kg (see Table 15.6). Strong evidence indicates a mortality benefit for using 6 mL/kg in patients with acute respiratory distress syndrome (ARDS).121 Some evidence also indicates 6 mL/kg as a target for patients without ARDS or acute lung injury (ALI).122,123

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