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.
Pneumothorax
Pneumothorax is air in the otherwise “virtual space” between the parietal and visceral pleurae. The volume and pressure of the air in this space determine the clinical effect, which can range from asymptomatic to life‐threatening. Early signs and symptoms may be subtle and the condition is often not expected. It is therefore important for EMS clinicians to maintain a high index of suspicion for pneumothorax in a variety of presenting complaints and to be aware of potential predisposing or associated conditions (Box 6.3).
Patients may present with pleuritic pain, sudden onset of a sharp pain, minimal to severe shortness of breath, and hypoxemia. Physical exam findings that should prompt consideration of pneumothorax include decreased or absent unilateral breath sounds, subcutaneous emphysema, or evidence of thoracic trauma. Pulse oximetry may or may not decrease depending on the size of the pneumothorax and the underlying pulmonary function and comorbidities of the individual patient. Similarly, EtCO2 may or may not appreciably change and its interpretation may be further complicated by compensatory hyperventilation or other comorbid conditions. A potentially more sensitive indicator in patients already receiving mechanical ventilation may be decreases in tidal volumes and increases in peak pressures. Ultrasound, if available, can also be used to identify pneumothorax.
The one case that must be recognized clinically is a tension pneumothorax. This occurs when the intrathoracic pressure is so great that ventilation and venous return to the heart are obstructed, leading to respiratory compromise and shock. Besides unilateral decreased or absent breath sounds and subcutaneous emphysema, tracheal deviation and jugular venous distension may be present, but these should not be relied upon. Tension physiology must be recognized and treated immediately.
Tension pneumothorax must be treated immediately with needle thoracostomy (needle decompression). Following skin cleansing, a large‐bore intravenous catheter (14 gauge or larger) should be inserted through the chest wall. When the needle enters the pleural space, a rush of air is often heard. The needle is then removed, leaving the catheter in place. Patients may require decompression with several needle thoracostomies in the prehospital environment as air reaccumulates in the pleural space. Needle decompression traditionally was performed in the second intercostal space at the mid‐clavicular line. Increasing evidence has shown that use of this site is prone to great vessel injury and failure to reach the pleural space [12–14]. Therefore, the Committee on Trauma of the American College of Surgeons now recommends using the fourth or fifth intercostal space between the anterior and mid‐axillary lines [15]. The chest wall should never be penetrated inferior to the nipple line due to risk of splenic or hepatic puncture. Treatment failure is typically due to using too short a needle or the catheter becoming occluded, which requires placement of additional needle(s). The hub of the catheter should either be left open or attached to a Heimlich (one‐way) valve.
Box 6.3 Conditions associated with pneumothorax
Trauma:
Blunt
Penetrating
Medical:
Acute asthma, especially if cardiac arrest
Chronic obstructive pulmonary disease or other underlying lung disease
Decompression‐associated barotrauma
Marfan syndrome (or marfanoid habitus)
Thoracic endometriosis (catamenial)
Finger thoracostomy is an additional technique for emergent chest decompression [16, 17]. Some EMS physicians consider this more reliable than needle decompression and less likely to cause lung injury. This technique should be performed only by experienced EMS clinicians with specific training and credentialing. The procedure includes antisepsis of skin, identification of mid‐axillary line just above the nipple line, making a 3–4 cm skin incision with a scalpel, spreading subcutaneous and intercostal tissue with hemostats, and puncture of parietal pleura with finger. The site should then be covered as for a sucking chest wound. Vigilance for reaccumulation of the pneumothorax is essential just as after needle decompression.
Subsequently, the patient should receive a formal thoracostomy tube placed on suction with water seal. This is typically deferred until arrival in the emergency department but may be considered