2: Critical Care

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Section 2 Critical Care

Edited by Anthony F.T. Brown

2.1 Airway and ventilation management

Introduction

Assessment and management of the airway is the first step in the resuscitation of a critically ill patient in the emergency department (ED). Evaluation of the airway commences with a ‘look, listen, feel’ approach to detect partial or complete airway obstruction. If airway compromise is suspected, initial basic airway manoeuvres include the jaw thrust, chin lift and head tilt (providing there is no suspicion of cervical spine injury), and placement of an oropharyngeal airway such as the Guedel (see Chapter 1.1 on Basic Life Support).

Gentle direct inspection of the upper airway using a laryngoscope may be necessary to detect a foreign body, which may be removed using a suction catheter and/or Magill’s forceps for solid material. Once the airway is cleared, supplemental oxygen by face mask is commenced while consideration is given to the breathing status.

Non-invasive ventilation

Many patients in respiratory failure with hypoxaemia and/or hypercapnia may benefit from a trial of non-invasive ventilation.1 The use of NIV involves administration of a controlled mixture of oxygen and air delivered at a set positive pressure via a tightly sealed face mask. The pressure is maintained between 5 and 10 cmH2O during both inspiration and expiration. This continuous positive airways pressure (CPAP) recruits lung alveoli that were previously closed, improving the ventilation/perfusion ratio and helping to correct hypoxaemia. There is also a reduction in the work of breathing as a result of an increase in pulmonary compliance. More recently, NIV machines have become available that administer positive pressure (i.e. 5–20 cmH2O) above the elevated baseline pressure during inspiration, known as bilevel NIV. This additional inspiratory support is thought to further reduce the work of breathing.

Clinical indications for non-invasive ventilation in the ED

Patients who present with severe acute pulmonary oedema (APO) should receive CPAP to improve cardiac and pulmonary function while medical therapy with nitrates and diuretics is initiated.2 However, the use of bilevel NIV in patients with APO gives no additional benefit and may increase the rate of myocardial infarction. On the other hand, patients who present with an exacerbation of chronic obstructive pulmonary disease (COPD) do benefit from bilevel NIV rather than CPAP alone.3

There is also some evidence to support the use of NIV in patients with respiratory failure due to other common ED conditions, such as community-acquired pneumonia4 or asthma.5 Thus it is common ED practice to now administer a trial of NIV in many patients with respiratory failure, prior to instituting ETI and mechanical ventilation. Contraindications to NIV include comatose or combative patients, poor tolerance of a tight-fitting face mask, and the lack of familiarity or lack of trained medical staff to institute and monitor the NIV.

Endotracheal intubation

Endotracheal intubation provides secure, definitive airway management and allows assisted mechanical ventilation. Patients with respiratory failure who are either ineligible for NIV or fail a trial of NIV should receive ETI and mechanical ventilation.

There are additional challenges to emergency endotracheal intubation in the ED compared to elective ETI in the operating theatre. There is often inadequate time for a complete clinical assessment of the upper airway or thorough consultation with the patient and/or family, and details of current medications, previous anaesthetics and allergies may not be available. Also, the status of the cervical spine in patients with an altered conscious state following trauma is unknown, even if initial plain imaging and even CT scanning appear normal.

There are a number of possible techniques for ETI, which are reviewed below. The selection of the appropriate technique depends on physician preference, experience and the clinical setting.

Intubation process

The conscious patient should receive explanation and reassurance. Pre-oxygenate with 100% oxygen to prevent oxygen desaturation during the procedure. Ideally, administer NIV with 100% oxygen for a 3-minute period.6 If this is not possible, then breathing through a tight-fitting oxygen mask circuit using 15 L/min oxygen flow is an alternative way to pre-oxygenate the patient. Position the patient in the ‘sniffing the morning air’ position with the neck flexed and the head extended, using a pillow under the head. If the patient has suspected spinal column injury, immobilize the neck in the anatomically neutral position. Ensure there is reliable intravenous access, as well as equipment for suctioning the airway and a tipping trolley.

Drugs used in RSI

The drugs required will depend on physician preference and the clinical situation. Common choices for induction include propofol at 1–2 mg/kg,7 a narcotic such as morphine 0.15 mg/kg with a benzodiazepine such as midazolam 0.05–0.1 mg/kg, followed by a rapid-onset depolarizing neuromuscular blocking drug such as suxamethonium 1.5 mg/kg (Table 2.1.1). An alternative when suxamethonium is contraindicated is the rapid acting non-depolarizing drug rocuronium 1 mg/kg. Contraindications to suxamethonium include known allergy, hyperkalaemia or risk of from burns, spinal cord injury or crush injury (not in the acute setting), and a history of malignant hyperthermia (rare).8 Details of the indications, dosages and side effects of all the commonly used drugs for RSI intubation are shown in Table 2.1.1.

Complications of RSI intubation

Hypotension following endotracheal intubation is common and must be addressed promptly. The causes include the vasodilator and/or negative inotropic effects of the sedative drug(s) given, and/or the reduction in preload from positive-pressure ventilation decreasing venous return and cardiac output. Treatment consists of administration of a fluid bolus of 10–20 mL/kg and/or inotrope, depending on the clinical setting. Alternatively, in the setting of bronchospasm hypotension may be due to gas trapping, with dynamic hyperinflation from excessive ventilation and the development of auto-PEEP (positive end-expiratory pressure), or even to a tension pneumothorax occurring after the commencement of positive-pressure ventilation. Hypertension usually indicates inadequate sedation and should be treated with supplemental sedation.

The following additional measures need to be considered during intubation in patients with severe head injury. An assistant must hold the head in the neutral position as there is the possibility of cervical spine instability, which increases the difficulty of visualizing the larynx. Also, laryngoscopy may raise intracranial pressure, although the benefit of pretreatment with lignocaine (lidocaine) 1.5 mg/kg is uncertain.12 Thiopentone or propofol must be used cautiously in patients with shock, or with severe head injury and possible hypovolaemia, as precipitate and prolonged hypotension may occur. Doses as small as one-tenth of normal may be necessary, e.g thiopentone 0.5 mg/kg or propofol 0.2 mg/kg.

The technique of RSI is not recommended for patients with a grossly abnormal upper airway, and/or impending upper airway obstruction. In this setting, the larynx may not be visualized and ventilation of the apnoeic patient may become impossible, leading to the extreme emergency of the ‘can’t intubate, can’t ventilate’ situation. An initial awake technique, such as using local anaesthesia, or a fibreoptic assisted intubation, should be performed in these patients. Alternatively, an inhalational anaesthetic agent or a short-acting intravenous agent such as propofol is used, as the sedative effects will rapidly reverse and spontaneous respirations resume if intubation and ventilation prove impossible.

The difficult intubation

Endotracheal intubation under direct vision may be easy or difficult, depending on the view of the larynx during laryngoscopy. This view has been classified by Cormack and Lehane13 into grades 1–4.

Failed intubation drill

Attempts at blind placement of the ETT down the trachea when the larynx is not visualized are unlikely to be successful, and repeated attempts may result in direct pharyngeal or laryngeal trauma (making the situation even more difficult) and hypoxaemia. In this situation a failed intubation drill must be initiated.15 A failed intubation algorithm suitable for use in the ED is shown in Figure 2.1.1. Depending on local hospital staffing and resources, an urgent call for assistance from another physician with additional experience should also be made.

Simple initial manoeuvres to improve visualization of the larynx include adding a second pillow to further flex the neck (unless cervical spine injury is suspected), the use of a straight Mackintosh laryngoscope blade, and ‘backward/upward/rightward external pressure’ (BURP) on the thyroid cartilage. A new approach to laryngoscopy using the GlideScope Video Laryngoscope (GVL) (Verathon, Bothell, WA, USA) provides a real-time view of the larynx on a colour monitor, which has been shown in a large case series to convert a Cormack and Lehane grade 3–4 view to a grade 1–2 view 77% of the time.16 In the absence of a GlideScope, and if the larynx still cannot be visualized, blind placement of a gum-elastic bougie and subsequent insertion of the ETT by railroading it over the bougie should be attempted as the preferred next manoeuvre.17 Rotating the ETT through 90° in an anticlockwise direction may be helpful if resistance to its passage occurs at the larynx.

If these initial steps are unsuccessful, adequate oxygenation must be maintained using a bag/mask with an oral airway at all times. Alternative equipment suitable for use in the ED should be prepared.18 A summary of these devices for a failed intubation drill is given below (see Fig. 2.1.1). However, if oxygenation can not be maintained during the attempted use of these devices, immediate cricothyroidotomy is indicated. Make sure additional help has also been summoned.

Laryngeal mask airway

The laryngeal mask airway (LMA) is now used routinely for airway management during elective general anaesthesia. During a failed intubation drill, the LMA may be superior to a bag/mask and oral airway for oxygenation and ventilation.15,17 However, the LMA has had a limited role in the ED, for two reasons. First, if pulmonary compliance is low or airway resistance is high, there will be a leak around the cuff of the LMA when peak inspiratory airway pressures exceed 20–30 mmHg. Second, there is the potential risk of aspiration pneumonitis as the airway remains unprotected. The LMA ProSeal (Vitaid Ltd, Toronto, Ontario, Canada) modification of the standard LMA minimizes this risk, and includes a double cuff to improve the seal and a distal drainage tube to provide access for suctioning the upper oesophagus. The LMA may also be used to assist in orotracheal intubation, using either a 6 mm ETT passed blindly through the LMA, or an ETT placed over a fibreoptic bronchoscope which is then passed through the LMA into the trachea.

Fibreoptic bronchoscope-assisted intubation

A fibreoptic bronchoscope assists in the intubation of the patient when RSI fails or is contraindicated. In particular, fibreoptic bronchoscope-assisted intubation (FBI) is the technique of choice in suspected traumatic injury to the larynx, and in the obstructed airway, particularly with distorted anatomy such as with an upper airway burn or tumour. The FBI may diagnose the severity of the laryngeal injury or pathology and the possible requirement for surgery. However, it requires considerable training and should only be performed by an experienced operator. Equipment sterilization, maintenance and checking procedures must also be in place (see later).

Retrograde intubation

When other techniques fail the technique of retrograde intubation may occasionally be used in the ED if time permits.21 The cricothyroid membrane is punctured by a needle/cannula and a guide-wire is passed through the cannula, directed cephalad. The wire is then brought out through the mouth using Magill’s forceps. There are a number of techniques used to then guide the ETT over the wire and back into the larynx, such as a proprietary device (Cook, Cook Medical Inc, Bloomington, IN, USA), or the introducer of a Minitrach II kit (Portex Ltd, Hythe, Kent, UK).22 Alternatively, the wire may be passed inside the end of the ETT and then out through the ‘Murphy eye’. Resistance may be felt when the ETT reaches the larynx, and some anticlockwise rotation may be required to facilitate passage into the larynx. When the level of the cricothyroid is reached, the guide-wire is removed and the ETT passed further down the trachea. The technique of retrograde intubation takes time and experience to perform and is usually unsuitable in a critical airway emergency.

Blind nasotracheal intubation

Blind nasotracheal intubation (BNTI) is a technique that is now rarely used in the operating theatre, but may occasionally be useful in the ED, either as the initial technique of choice or as part of a failed intubation drill once spontaneous respirations have resumed. Contraindications include a fractured base of skull or maxillary fracture, a suspected laryngeal injury, coagulopathy or upper airway obstruction.

High-flow oxygen is administered by mask and the nasal passages are inspected to assess patency. The larger nasal passage is prepared with a pledget soaked in local anaesthetic and vasoconstrictor, such as 5 mL lignocaine (lidocaine) 2% with epinephrine 1:100 000. After several minutes the pledget is removed and sterile lubricant applied. Local anaesthetic may also be sprayed into the upper airway, and/or intravenous sedation may be administered if required and clinically appropriate. An ETT one size smaller than the predicted oral size is passed via the nose to the pharynx and advanced slowly towards the larynx, with the operator listening for breath sounds.

The head may need to be flexed, extended or rotated to facilitate entry into the larynx, the ETT rotated clockwise through 90°, and/or a suction catheter used to guide the ETT. When the tube passes into the trachea, louder spontaneous respirations heard from the ETT, or the onset of coughing down the tube, confirm successful placement. However, there are significant complications with BNTI, including epistaxis,23 injuries to the turbinates, perforation of the posterior pharynx, laryngospasm and injury to the larynx. In addition, an already jeopardized airway may be made worse, leaving the situation impossible to then control.

Cricothyroidotomy

Cricothyroidotomy is an essential skill for all emergency physicians and must be considered immediately in the situation of ‘can’t intubate, can’t ventilate’. There are several possible techniques for emergency cricothyroidotomy.

Techniques for emergency cricothyroidotomy

First, there are proprietary kits that allow a cricothyroidotomy tube to be placed using the Seldinger technique. In this approach, the cricothyroid membrane is punctured with a needle mounted on a syringe; free aspiration of air confirms placement in the airway. A guide-wire is passed through the needle down the trachea. The needle is then removed and a dilator passed along the wire, then a 4.5–6 mm cricothyroidotomy tube is mounted on a guide and passed along the wire and into the trachea. The position of the cricothyroidotomy tube must be carefully checked, as it is easy to misplace it anterior to the trachea. However, if the cricothyroidotomy tube is uncuffed, interpretation of a capnograph waveform can be difficult as much of the exhaled gas may pass into the upper airway, and not through the cricothyroidotomy tube during exhalation, resulting in a false-negative end-tidal CO2 trace.

Mechanical ventilation

Once intubation has been achieved, the patient is connected to a mechanical ventilator to provide continued ventilatory support. Because ventilated patients may initially be managed for some time in the ED, it is important that recommendations for optimal mechanical ventilation are implemented in the ED.

Recommendations for optimal mechanical ventilation

A tidal volume of 10 mL/kg and a respiratory rate of 10–14 breaths per minute are considered safe for most patients. However, patients with acute lung injury may have reduced pulmonary compliance and hence elevated peak inspiratory pressures. These patients should receive a ‘protective lung strategy’.25 This involves limiting the tidal volume to 6 mL/kg, with the respiratory rate setting increased to 16–20 breaths per minute to prevent excessive hypercapnia. Deliberate hyperventilation using a respiratory rate of 16–20 breaths per minute may also be indicated to provide hypocapnia in other situations, such as in patients with severe metabolic acidosis, and in patients with raised intracranial pressure, in whom transient hypocapnia of 30–35 mmHg (4.0–4.7 kPa) may temporarily reduce intracranial pressure while other treatments for intracranial hypertension are being implemented.

Conversely, patients with severe airways obstruction such as asthma or COPD should receive a standard tidal volume of 10 mL/kg, but a decreased respiratory rate from 4 to 8 breaths per minute to allow sufficient time for adequate passive exhalation.26 This reduces the risk of pulmonary hyperinflation, with the development of auto-PEEP leading to hypotension, even electromechanical dissociation. Thus when ventilating a critical asthmatic the PaCO2 level will rise (known as ‘permissive hypercapnia’), with the aim being to initially concentrate only on oxygenation.

References

1 Hill NS, Brennan J, Garpestad E, Nava S. Noninvasive ventilation in acute respiratory failure. Critical Care Medicine. 2007;35:2402-2407.

2 Peter JV, Moran JL, Phillips-Hughes J, et al. Effect of non-invasive positive pressure ventilation (NIPPV) on mortality in patients with acute cardiogenic pulmonary oedema: a meta-analysis. Lancet. 2006;367:1155-1163.

3 Ram FS, Lightowler JV, Wedzicha JA. Non-invasive positive pressure ventilation for treatment of respiratory failure due to exacerbations of chronic obstructive pulmonary disease. Cochrane Database Systematic Review. 1, 2004. CD004104

4 Keenan SP, Sinuff T, Cook DJ, et al. Does non-invasive positive pressure ventilation improve outcome in acute hypoxemic respiratory failure? A systematic review. Critical Care Medicine. 2004;32:2516-2523.

5 Ram FS, Wellington S, Rowe B, et al. Non-invasive positive pressure ventilation for treatment of respiratory failure due to severe acute exacerbations of asthma. Cochrane Database Systematic Review. 3, 2005. CD004360

6 Baillard C, Fosse JP, Sebbane M, et al. Noninvasive ventilation improves preoxygenation before intubation of hypoxic patients. American Journal of Respiratory and Critical Care Medicine. 2006;174:171-177.

7 Wilbur K, Zed PJ. Is propofol an optimal agent for procedural sedation and rapid sequence intubation in the emergency department? Canadian Journal of Emergency Medicine. 2001;3:302-310.

8 Sluga M, Ummenhofer W, Studer W, et al. Rocuronium versus succinylcholine for rapid sequence induction of anesthesia and endotracheal intubation: a prospective, randomized trial in emergent cases. Anesthesia and Analgesia. 2005;101:1356-1361.

9 Ellis DY, Harris T, Zideman D. Cricoid. Pressure in emergency department rapid sequence tracheal intubations: a risk-benefit analysis. Annals of Emergency Medicine. 2007;50:653-665.

10 Deiorio NM. Continuous end-tidal carbon dioxide monitoring for confirmation of endotracheal tube placement is neither widely available nor consistently applied by emergency physicians. Emergency Medicine Journal. 2005;22:490-493.

11 Schaller RJ, Huff JS, Zahn A. Comparison of a colorimetric end-tidal CO2 detector and an esophageal aspiration device for verifying endotracheal tube placement in the prehospital setting: a six-month experience. Prehospital and Disaster Medicine. 1997;12:57-63.

12 Robinson N, Clancy M. In patients with head injury undergoing rapid sequence intubation, does pretreatment with intravenous lignocaine/lidocaine lead to an improved neurological outcome? A review of the literature. Emergency Medicine Journal. 2001;18:453-457.

13 Cormack RS, Lehane J. Difficult intubation in obstetrics. Anaesthesia. 1984;39:1105-1111.

14 Shiga T, Wajima Z, Inoue T, Sakamoto A. Predicting difficult intubation in apparently normal patients: a meta-analysis of bedside screening test performance. Anesthesiology. 2005;103:429-437.

15 Henderson JJ, Popat MT, Latto IP, et al. Difficult Airway Society. Difficult Airway Society guidelines for management of the unanticipated difficult intubation. Anaesthesia. 2004;59:675-694.

16 Cooper RM, Pacey JA, Bishop MJ, McCluskey SA. Early clinical experience with a new videolaryngoscope (GlideScope) in 728 patients. Canadian Journal of Anaesthesia. 2005;52:191-198.

17 Jabre P, Combes X, Leroux B, et al. Use of gum elastic bougie for prehospital difficult intubation. American Journal of Emergency Medicine. 2005;23:552-555.

18 Bair AE, Filbin MR, Kulkarni RG, et al. The failed intubation attempt in the emergency department: analysis of prevalence, rescue techniques, and personnel. Journal of Emergency Medicine. 2002;23:131-140.

19 Ferson DZ, Rosenblatt WH, Johansen MJ, et al. Use of the intubating LMA-Fastrach in 254 patients with difficult-to-manage airways. Anesthesiology. 2001;95:1175-1181.

20 Timmermann A, Russo SG, Rosenblatt WH, et al. Intubating laryngeal mask airway for difficult out-of-hospital airway management: a prospective evaluation. British Journal of Anaesthesia. 2007;99:286-291.

21 Weksler N, Klein M, Weksler D, et al. Retrograde tracheal intubation: beyond fibreoptic endotracheal intubation. Acta Anaesthesiologica Scandinavica. 2004;48:412-416.

22 Slots P, Vegger PB, Bettger H, et al. Retrograde intubation with a Mini-Trach II kit. Acta Anaesthesiologica Scandinavica. 2003;47:274-277.

23 Piepho T, Thierbach A, Werner C. Nasotracheal intubation: look before you leap. British Journal of Anaesthesia. 2005;94:859-860.

24 Sulaiman L, Tighe SQ, Nelson RA. Surgical vs wire-guided cricothyroidotomy: a randomised crossover study of cuffed and uncuffed tracheal tube insertion. Anaesthesia. 2006;61:565-570.

25 Girard TD, Bernard GR. Mechanical ventilation in ARDS: a state-of-the-art review. Chest. 2007;131:921-929.

26 Shapiro JM. Management of respiratory failure in status asthmaticus. American Journal of Respiratory and Critical Care Medicine. 2002;1:409-416.

27 Meade M, Guyatt G, Cook D, et al. Predicting success in weaning from mechanical ventilation. Chest. 2001;120:400S-4424S.

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2.2 Oxygen therapy

Physiology of oxygen

Oxygen transport chain

Oxygen proceeds from inspired air to the mitochondria via a number of steps known as the oxygen transport chain. These steps include:

Pulmonary gas exchange

Oxygen diffuses across the alveoli and into pulmonary capillaries, and carbon dioxide diffuses in the opposite direction. The process is passive, occurring down concentration gradients. Fick’s law summarizes the process of diffusion of gases through tissues:

image

where image = rate of gas (oxygen) transfer, ∝ = proportional to, A = area of tissue, T = tissue thickness, Sol = solubility of the gas, MW = molecular weight, PA = alveolar partial pressure, and Ppa = pulmonary artery partial pressure.

In healthy patients oxygen passes rapidly from the alveoli to the blood, and after 0.25 seconds pulmonary capillary blood is almost fully saturated with oxygen, resulting in a systemic arterial oxygen partial pressure (PAO2) of approximately 13.3 kPa (100 mmHg). The difference between the PAO2 and the PaO2 is known as the alveolar to arterial oxygen gradient (A–a gradient). It is usually small and increases with age.

Oxygen carriage in the blood

Three steps are required to deliver oxygen to the periphery:

The haemoglobin–oxygen (Hb–O2) dissociation curve

The haemoglobin-oxygen (Hb–O2) dissociation curve is depicted in Figure 2.2.1, which also summarizes the factors that influence the position of the curve. If the curve is shifted to the left, this favours the affinity of haemoglobin for oxygen. These conditions are encountered when deoxygenated blood returns to the lung. A shift of the curve to the right favours unloading of oxygen and subsequent delivery to the tissues.

A number of advantages are conferred by the shape of the Hb–O2 dissociation curve that favour uptake of oxygen in the lung and delivery to the tissues:1

Oxygen is carried in the blood as dissolved gas and in combination with haemoglobin. At sea level (101.3 kPa), breathing air (FIO2 = 0.21), the amount of oxygen dissolved in plasma is very small (0.03 mL oxygen per litre of blood for each 1 mmHg PaO2). This dissolved component assumes greater significance in a hyperbaric situation, where at 284 kPa and FIO2 = 1.0 up to 60 mL oxygen can be carried in the dissolved form per litre of blood.

Haemoglobin carries 1.34–1.39 mL oxygen per gram when fully saturated. Blood with a haemoglobin concentration of 15 g/L carries approximately 200 mL oxygen per litre.