Definition of assisted ventilation
Assisted ventilation is where the practitioner offers ventilatory support to a patient whose alveolar ventilation is inadequate to maintain normal partial pressures of oxygen and carbon dioxide. This can be achieved by using either mechanical or manually generated positive pressure and may be a lifesaving treatment. In prehospital care the pressure may be generated using expired air ventilation (mouth-to-mouth/nose), a mechanical ventilator such as the Pneupac® paraPAC, or a bag-valve-mask (BVM) (Figures 2.1 and 2.2). Either may be used with a mask or endotracheal tube/laryngeal mask airway.
Normal inspiration occurs when the diaphragm (primary muscle of inspiration) contracts, causing an increase in the size of the thoracic cavity. As a result, intrathoracic pressure falls to below that of atmospheric pressure and air is drawn into the lungs. Assisted ventilation creates a positive pressure that pushes air into the lungs. Failure to provide adequate ventilation for an indicated patient will lead to hypoxia, retention of CO2, development of acidosis and cardio-respiratory arrest.
Figure 2.1 Pneupac® paraPAC. Reproduced with permission of Smiths Medical International.
Figure 2.2 Silicone bags
- Make a list of the problems you may encounter when attempting to support the ventilation of this patient and identify some techniques you may be able to use to overcome them.
- Would you use a bag-valve-ventilator or mechanical ventilator? Provide a rationale for your answer.
Indications for assisted ventilation
Inadequate ventilation is the overriding indication for assisted ventilation. In cases of apnoea, the indication is unequivocal but, for patients with depressed ventilation, the requirement may be less obvious. Artificial ventilation should be provided as soon as possible in any patient in whom spontaneous ventilation is inadequate or absent.1
Patients with a life-threatening respiratory emergency will present in either respiratory failure or respiratory distress.2 Those with respiratory distress are still able to compensate for the effects of their illness, and urgent treatment may prevent their further deterioration. These patients present with signs and symptoms indicative of increased work of breathing but may show few signs of the systemic effects of hypoxia or hypercapnia. Patients with respiratory failure tend not to show evidence of increased work of breathing as exhaustion overrides their ability to compensate. These patients will normally exhibit signs of the systemic effects of hypoxia and hypercapnia, and immediate treatment will be required to prevent cardiac arrest. Box 2.12 shows the key findings associated with increased work of breathing and weak respiratory effort; these are indicative of a patient with a life-threatening respiratory condition. For these patients, it is suggested that assisted ventilation should be undertaken if the respiratory rate is <10 or >29/min (adult), titrated to SpO2.2 However, some patients presenting with respiratory rates between 10 and 29/min will still benefit from assisted ventilation; it is a clinical judgement for the paramedic to make based upon ALL physical observations not just the respiratory rate.
- Stridor associated with other key findings
- Use of accessory muscles
- Adopting orthopnic position (sat upright)
- Tracheal tug
- Intercostal recession
- Expiratory wheeze associated with other key findings
- Cessation of expiratory wheeze without improvement in condition
- Inability to speak in whole sentences
- Respiratory rate <10 or >29
- Decreased, asymmetrical, or absent breath sounds
- Oxygen saturation <92% on air or <95% on high concentration oxygen
- PEFR <33% of normal
- Hypercapnia (measured with end tidal CO2 monitor where available)
- Tachycardia (≥120) or bradycardia (late and ominous finding)
- Arrhythmias
- Pallor and/or cyanosis (particularly central cyanosis)
- Cool clammy skin
- Falling blood pressure (late and ominous finding)
- Changed mental status – confusion, feeling of impending doom, combativeness
- Falling level of consciousness
- Exhaustion (+/−muscular chest pain)
Fatigue induced by a prolonged, severe asthma attack, head injury induced hypoventilation, and drug induced respiratory depression (e.g. opiates) are typical causes of inadequate ventilation requiring assisted ventilation. Apnoea is an absolute indication for assisted ventilation irrespective of the cause.
The literature and complications associated with assisted ventilation
The most common initial form of ventilatory assistance in the emergency clinical setting is usually accomplished using bag-valve-ventilation (BVV) techniques. The BVV mask was developed in 1955 by Henning Ruben in Denmark and has been the most common method of ventilating a patient in respiratory or cardiac arrest since that time.3 Performing ventilation with a bag-valve-mask device is regarded as a relatively simple task that all healthcare personnel should be able to perform with little training. Ventilation utilising this technique is accepted worldwide and considered to be the standard of care in a variety of clinical settings.4,5
Even though the technique is considered to be safe and effective and has been used in the emergency setting for many years, it has some potentially fatal complications. Among them are decreased oxygenation, lung aspiration due to gastric dilatation, or even gastric rupture.6 The main complications should be recognised early but it is questionable as to whether problems are always rectified. Gastric over-distention, aspiration of gastric contents, and barotrauma can lead to the premature death of a patient.7 By utilising too much pressure whilst ventilating, the rescuer may overcome the lower oesophageal sphincter pressure and produce aspiration.8
In a patient with an unprotected airway, the distribution of inspiratory gas volume between lungs and stomach during bag-valve-mask ventilation depends on several variables. These variables include: upper airway pressure, inspiratory flow rate, airway resistance and compliance, and lower oesophageal sphincter pressure.9 The lower oesophageal sphincter pressure is normally 20 to 25 cm H2O in a healthy adult, but is significantly reduced in patients with cardiac arrest.10 Bag-valve-mask (BVM) ventilation is often applied with a high flow rate over a short inflation time, which inevitably produces a high peak airway pressure. If the peak airway pressure exceeds the lower oesophageal sphincter pressure during ventilation, the stomach is inflated. Thus, it is essential to keep the peak airway pressure to a minimum during ventilation of a non-intubated patient.
Several strategies are available to reduce peak inspiratory flow rates, and therefore, peak airway pressure. These include the use of a paediatric bag-valve-mask instead of an adult one, use of cricoid pressure, or a mechanical ventilator.11,12 Studies looking at the efficacy and safety of mechanical ventilators suggest that pulmonary barotrauma may result from excessive peak inspiratory flow rates, so a recommendation has been made that lower peak inspiratory flow rates should be used.13 There have been few studies investigating the effectiveness of mechanical ventilators although one study does suggest significant benefits of a mechanical ventilator over BVV. The study found that when compared with the resuscitation ventilator, the bag-valve-mask resulted in significantly higher peak airway pressure and significantly lower oxygen saturation.14 This study suggests that the mechanical ventilator may be a suitable alternative to BVM even in the non-intubated patient.
A further option is offered by the SMART BAG®, which has a pressure-responsive flow-limiting valve. If properly squeezed, there are no differences in performance between this valve and a standard valve. The piston provides both a tactile and visual feedback to the provider when excessive pressure is applied and prevents excessively high peak airway pressure. In simulated scenarios, this bag provided ventilation performance that was more consistent with current guidelines and delivered similar tidal volumes when compared with ventilation with a traditional bag-valve-mask resuscitator.15 However, in the patient with low compliant lungs it is possible that this bag will not allow for adequate ventilation.
The ventilation rate is also very important as hyperventilation of a patient during cardiac arrest is linked with adverse haemodynamic effects and decreased cerebral perfusion, which translate into increased mortality.16 Several studies have documented high respiratory rates during pre-hospital resuscitation,17,18 despite the recommended rate of 10 breaths per minute.1
There are many potential reasons for the apparent failings in assisted ventilation, including stress, fatigue, the inability to convert training into real world situations, failings in training manikins, and an erroneous belief that in cardiac arrest the patient can never receive too much oxygen. The use of a correctly set-up mechanical ventilator may overcome many of these issues although the question of bag or machine remains unanswered.
When using a mask rather than a tube, the seal between face and mask is the key component to minimising the risk of leakage and hypoventilation (Figure 2.3). Traditionally, ambulance staff have been expected to perform this as a single-handed operator in the rear of a moving vehicle, but it is unlikely that this is an effective procedure due to movement, stress and the need to undertake other tasks. This is particularly true in the case of obese patients or those without teeth or with abundant facial hair. In addition, practitioners with smaller hands may not be able to squeeze the bag sufficiently to ventilate the patient.19 When using a ventilator with a mask it is best practice for one person to hold the mask in place whilst maintaining the patient’s airway and the second person squeezes the bag, paying particular attention not to overinflate20 (Figure 2.4). This would require a second person to squeeze a BVV device (not always available), so it may be necessary to connect the mask to a mechanical ventilator in order to achieve this.
Figure 2.3 Making a seal (single-operator).
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