While hypoxemia in the setting of adequate ventilation can be treated with supplemental oxygen and augmentation of ventilatory function, inadequate ventilation requires immediate intervention. The EMS clinician should rapidly determine the likely cause (Box 6.1) of the patient’s ventilatory insufficiency and determine if it can be quickly corrected. Examples of this are removal of upper airway obstruction, administration of bronchodilators for bronchospasm, sealing of sucking chest wounds, administration of naloxone for opioid overdose, and needle decompression of tension pneumothorax. Some conditions cannot be immediately alleviated, particularly in the prehospital setting, such as muscle weakness from Guillain‐Barré syndrome or severe physical fatigue, vital capacity reduction from a large pleural effusion, and non‐reversible drug toxicity. In other cases, medical interventions may not be sufficiently effective immediately, such as for acute pulmonary edema or severe asthma. Whenever ventilation is compromised and cannot be immediately alleviated, mechanical ventilatory support must be provided.
Noninvasive Positive‐Pressure Support
Patients who are awake, protecting their airways, have respiratory drive, and can cooperate may be given ventilatory support with noninvasive modalities that provide positive airway pressure. These include continuous positive airway pressure (CPAP), which delivers constant pressure above that of the atmosphere throughout the ventilation cycle, and bi‐level positive airway pressure (BiPAP), which delivers different pressures during the inspiratory and expiratory phases. Portable devices for the delivery of CPAP in the prehospital setting are generally of three types. Two of these require a high‐pressure (50 psi) oxygen source. One continuously delivers oxygen under pressure to a mask with a pop‐off valve that opens when the desired pressure is reached. The other uses a controller that essentially acts as a demand valve, adjusting flow to maintain the desired pressure. The third type of device uses oxygen flow from a standard regulator and a Venturi valve to create a virtual valve resulting in elevated pressure. Usually, treatment is started at 5–10 cm H2O and increased as needed to a maximum of 20 cm H2O. Prehospital devices generally deliver near 100% FiO2, while more advanced devices allow the FiO2 to be titrated. Until recently, BiPAP has been only available to EMS personnel who carry full‐function mechanical ventilators, but newer devices can now provide this modality for the field.
NIPPV has been shown to improve oxygen delivery. This is thought to be the result of the hydrostatic pressure effects of increasing the gas diffusion gradient, promoting displacement of fluid in the alveoli back into the capillary bed, and stenting open small bronchioles (which do not have cartilaginous walls), thereby increasing both the volume of air exchanged and the number of alveoli ventilated. NIPPV also decreases the work of breathing. The ultimate clinical effects are that patients will often have improved oxygenation, improved ventilation, marked improvement in respiratory distress, and significantly lower likelihood of needing intubation and mechanical ventilation [9].
Bag‐Valve‐Mask Ventilation
Patients with marked respiratory failure may need more intensive ventilatory support than NIPPV. This is true for patients with inadequate ventilatory effort and those with depressed mental status who cannot protect their airways. Immediate assistance should be provided for these patients using a bag‐valve‐mask device to either assist spontaneous ventilations or provide full mechanical ventilation. Proper positioning (head and neck tilt, sniffing position), mechanical airway opening (jaw thrust or modified jaw thrust), and placement of a nasal or oral airway can markedly improve airflow. High‐flow oxygen should fill the bag device, preferably with a reservoir bag. Using this device can be difficult for a single clinician, using one hand to seal the mask and the other to squeeze the bag. Whenever possible, a two‐person technique should be used, with one person using both hands and a jaw thrust maneuver to make a firm seal around the mask and open the airway, while the other person squeezes the bag.
EMS clinicians must be cognizant of volume and rate when assisting ventilation. Patients who are severely hypoxic or hypercarbic may initially require hyperventilation, as do those with severe metabolic acidosis, such as from diabetic ketoacidosis or sepsis. However, absent such conditions, unnecessary hyperventilation will have detrimental effects, including decreased cerebral perfusion, venous return, and cardiac output, and metabolic impairment from respiratory alkalosis. Standard adult bags are typically 1,500–1,600 ml, so a full squeeze will provide excessive tidal volume and likely high peak airway pressure, and facilitate inadvertent hyperventilation. One study found that most adults could be ventilated with a pediatric bag, but a small adult size with 1,000 ml is available [10]. Some devices can be equipped with high inspiratory pressure pop‐off valves and positive end‐expiratory pressure (PEEP) valves (Figure 6.3). These adjuncts improve proper ventilation and oxygenation.
Mechanical Ventilation
While bag‐valve‐mask ventilation can be an effective and life‐saving initial measure, it is difficult to maintain effectiveness in the longer term, especially in a moving vehicle. Additionally, it provides no protection from aspiration of stomach contents, blood, or other secretions. When adequately trained personnel are available, a more definitive airway should be sought in patients who have marked depression of consciousness, inability to protect their airway, or who require full mechanical ventilatory support to maintain oxygenation and ventilation. Usually, this entails placement of an endotracheal tube or a supraglottic advanced airway device (see Chapters 2 and 3). Patients can then be ventilated either manually (i.e., with a bag device) or with a portable mechanical ventilator.
Figure 6.3 Bag‐valve mask device
Management of mechanical ventilators is a complex topic and a comprehensive tutorial is beyond the scope of this chapter. However, a basic understanding of the modes, settings, and troubleshooting of mechanical ventilators is important. Mechanical ventilators are typically used in the prehospital setting by air medical services and by ground critical care teams during interfacility transports. Greater portability is facilitating their deployment within EMS systems for use during longer transports. EMS clinicians may also encounter patients who are chronically on ventilators in residential or long‐term care settings.
Modes of Ventilation
There are two basic modes of mechanical ventilation, volume control and pressure control, with multiple variations and combinations of these. Common modalities include assist control (AC), synchronized intermittent mandatory ventilation (SIMV), adaptive support ventilation (ASV), and pressure support (PS). The key to understanding these modes is recognizing that the time of the respiratory cycle (i.e., respiratory rate), tidal volume, flow rate, and pressure developed in the airways