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 are all interdependent and affected by the individual patient’s airway physiology. In each of these modes, different combinations of these variables are controlled by the machine, and the patient’s respiratory function determines the uncontrolled variables.
In AC mode, the ventilator delivers a set tidal volume with each breath. A default respiratory rate is set, but the patient may trigger breaths above that default rate. In AC mode, the machine will deliver the full set tidal volume on either a patient‐ or machine‐triggered breath. SIMV is very similar to AC, and, in fact, in patients without spontaneous respiratory effort the two are effectively identical. The major difference is that in SIMV the machine does not deliver the full set tidal volume in response to a patient‐triggered breath, but rather allows the patient’s effort to determine the volume of the breath. In SIMV mode, the ventilator will synchronize ventilator‐triggered breaths with patient‐triggered breaths, assuring that the set rate is met or exceeded. In both AC and SIMV modes, care must be taken to monitor the airway pressures developed during the respiratory cycle. In contrast, PS mode delivers a set inspiratory pressure above a baseline PEEP with each patient‐triggered breath. The patient’s respiratory drive determines the rate and the patient’s lung compliance and airway resistance determine the tidal volume developed. ASV combines several modes of ventilation in an adaptive manner dynamically to adjust levels and modes of support to the patient’s requirements.
Ventilator Settings and Troubleshooting
Once the mode of ventilation is selected, EMS clinicians will need to set several variables. In AC and SIMV modes, tidal volume, respiratory rate, and PEEP are all determined by the clinician. Tidal volumes are normally chosen to be 6–10 mL/kg of ideal body weight based on patient height. Tidal volumes of 6–8 mL/kg are preferred for patients with acute respiratory distress syndrome, whereas 8–10 ml/kg may be better for other conditions such as trauma and COPD [11]. Respiratory rate should be adjusted based upon the patient’s clinical situation, with low tidal volume strategies usually associated with higher rates. Higher than standard minute ventilation should be assured in patients who are dependent upon respiratory compensation of a metabolic acidosis.
PEEP may also be applied to improve oxygenation via mechanisms similar to NIPPV, discussed above, and is often initially set at 5–10 cm H2O. For patients with obstructive physiology (e.g., asthma and COPD), care should be taken to maximize expiratory time to avoid incomplete expiration and breath stacking, which can lead to increased airway pressures. If air trapping is suspected, excess pressure can be alleviated by disconnecting the endotracheal tube from the ventilator for a few seconds and compressing the patient’s chest.
Peak inspiratory pressure (PIP) represents the maximum pressure developed during the inspiratory phase. Changes in PIP are a common source of ventilator alarms. Low PIP usually indicates a leak in the ventilator circuit. High PIP may represent either an increase in airway resistance (e.g., blocked tube, bronchospasm, secretions) or a decrease in lung compliance (e.g., pulmonary edema, atelectasis, pneumothorax, pleural effusion, hyperinflation). These two states can be distinguished by performing an inspiratory hold test to measure a plateau pressure. This test is performed by pressing the hold button on the ventilator for approximately 5 seconds during inspiration without allowing the patient to exhale. This effectively eliminates the airway resistance from the measured pressure and allows independent assessment of pressure being developed in the lungs with a given tidal volume. This is equivalent to a measurement of lung compliance. If the plateau pressure rises along with PEEP, clinicians should look for correctable causes of decreased lung compliance.
