Transport Ventilators: A Guide For Critical-Care Transportation, Aeromedical & Prehospital Operations

Basic life support is sustained by four very important life functions: ventilation, oxygenation, circulation and perfusion.

Ventilation, the first and most important, is movement of air in and out of the lungs. Ventilation depends on a negative pressure generation within the thoracic cavity, which draws air into the upper airway. This process requires coordinated interaction between the musculoskeletal, respiratory and central nervous systems, and provides fresh air to the lungs with each breath. Mechanical ventilation uses positive pressure to drive air into the airways using the mechanics of a ventilator.

Oxygenation, also known as pulmonary respiration, is the second most important life function. Pulmonary respiration is achieved by oxygen diffusion from the alveoli to the red blood cells through the alveolar pulmonary capillaries. Gas diffusion, which occurs at this point, is defined as the movement of a gas from an area of higher partial pressure concentration to an area of lower partial pressure concentration.

The third and fourth life functions are circulation and perfusion. Circulation is movement of oxygen-rich blood from the lungs throughout the body. Perfusion is the end stage of cellular respiration, which is the exchange of gases at the various tissues. Diffusion occurs again at the cellular level, where oxygen is exchanged for carbon dioxide and waste products. Circulating blood volume returns to the lungs to expel the byproducts of cellular respiration.

Mechanical Ventilation

Some patients may require mechanical ventilation because of a condition where the mechanics or ability to breathe is impaired. This may include drug overdose, brain trauma, neuromuscular disease, musculoskeletal defects or respiratory distress syndrome. A ventilator is defined as a “mechanical device for artificial ventilation of the lungs.” The mechanism may be hand-operated or machine-driven and, in the latter case, may be automatic and very sophisticated with respect to being able to control and monitor airflow to the lungs.

When prehospital providers take definitive control of the airway, they place another type of tube that provides a pathway to replace the upper airway. This may include performing endotracheal intubation, or placing a Combitube or laryngeal mask airway (LMA). Providers can then initiate continuous ventilation, taking into consideration the patient’s immediate needs and overall condition. Mechanical ventilation is not therapeutic. Its function is to maintain a patient’s ventilation and provide time for medical treatments, drugs, surgery or other procedures.

An immediate indication for intubation and continuous ventilation is the apneic patient. In fully monitored patients in the hospital setting, additional indications for intubation may include inspiratory force less than 20 cm H2O, vital capacity less than 10–15 ml/kg, tidal volume less than 5 ml/kg, PaO2 less than 50 torr, PCO2 greater than 55 torr, or a shunt fraction greater than 15%. Providers of critical-care transportation, aeromedical and prehospital care most likely will not have this information, so the decision to intubate is based on the patient’s degree of respiratory distress and the provider’s clinical expertise.

When emergency providers initiate mechanical ventilation, several parameters must be set on the ventilator, depending on the complexity of the ventilator in use. The simplest transport ventilators use the control mode, while more sophisticated units offer a variety of ventilation modes, such as intermittent mandatory ventilation (IMV), assist, control, or a combined assist/control mode. The IMV mode, which is available on some models, is generally used as a weaning tool, allowing the patient to take spontaneous breaths on his own without triggering the ventilator into a positive pressure cycle. A patient requiring transport from one facility to another who is receiving mechanical ventilation and is able to maintain adequate oxygenation with IMV should be managed with this mode.

IMV decreases side effects usually associated with positive pressure ventilations. However, if conditions arise that change oxygenation or ventilation or represent deterioration of the patient, the practitioner may consider a change of modes.

Like IMV, the assist mode may be used when a patient has negative inspiratory forces of his own. Unlike IMV, the assist mode delivers all positive pressure ventilations with each cycle. This mode will deliver a set number of breaths; if the patient triggers the ventilator with additional breaths, it will deliver additional cycles.

The control mode should be used with the patient who is completely apneic, including patients sustained on neuromuscular blocking agents. This mode delivers a set amount of positive pressure ventilations, so patients cannot trigger any additional breaths when this mode is used. Some simpler ventilator units combine the assist and control modes into a single option. Patients who retain some ability to breathe will “buck the vent” and will be very uncomfortable. If this occurs, change the vent settings to the assist/control mode, which is the combined setting that will allow them to trigger the ventilator and receive additional cycled ventilations.

After the ventilation mode has been selected, tidal volume (TV) should be delivered at 10–15 ml/kg of lean body weight. The frequency or breaths per minute (BPM) typically is set at 10–20. Although the range may be set higher, it is generally acceptable to keep the range at this level.

Most transport ventilators have alarms for two pressure limits: low pressure and high pressure. When a low-pressure alarm is activated, the most likely cause is that the patient circuit has been disconnected from either the patient or the ventilator. The alarm also may sound if the circuit has a hole. Standard settings for low-pressure alarms are 20 cm/H2O.

High-pressure limits should be set at 40 cm/H2O, although they may be set as high as 60 cm/H2O. A high-pressure alarm denotes too much pressure in the system, and the patient may develop such complications as pneumothorax or barotrauma. When the high-pressure alarm is activated, some causes include a kink in the ventilatory circuit, or patients “bucking the vent” with coughing, mucus plugs or heavy secretions that are blocking some part of the ventilatory circuit.

Some ventilators have a pop-off valve that vents away some of the tidal volume when a preset level of airway pressure is exceeded. Other units will not deliver additional tidal volume after peak airway pressures reach the same critical level. Using the lowest setting possible yet maintaining adequate volumes can reduce complications of high-pressure ventilation.

Several other controls need to be set before applying a ventilator. The sensitivity control should be set at a level for the patient to pull less than 5 cm/H2O. The standard setting is a negative 2 cm/H2O, which allows the patient to trigger a breath in the assist/control mode.

Normal physiological breathing produces an inspiratory to expiratory ratio of about 1:2. Therefore, in ventilating the patient, the flow control should be set to deliver the patient a 1:2 or 1:3 inspiratory/expiratory ratio. As an example, a 1:2 inspiratory/expiratory ratio means that for every 1 second of inspiratory time, the patient should have an exhalation time of 2 seconds. When the flow is higher, the system pressure will be greater. A sigh can be added and is usually set for multiple breaths several times a minute. A sigh delivers 1.5–2 times the set tidal volume, which will help keep the lower airways open and improve oxygenation. A sigh can also be used after suctioning to open the lower airways, improve oxygenation after suctioning and reduce alveolar collapse associated with suctioning.

The human body naturally has about 5 cm/H2O positive end expiratory pressure (PEEP)—an expiratory setting that remains constant throughout the respiratory cycle and prevents alveolar collapse by maintaining a positive pressure within the lungs. The biggest disadvantage of PEEP therapy is a decrease in venous return caused by increased pressures within the lungs. PEEP therapy greater than 15 cm/H2O can have significant risks when used for patients who already have compromised cardiac output. Standard PEEP therapy is 2–20 cm/H2O.

Generally, practitioners should not use PEEP greater than 10 cm/H2O, although if a patient is being adequately maintained on a pre-established PEEP setting, he should receive the same amount of PEEP en route from one facility to another. Trauma patients, especially those with chest injuries, may have compromised lungs and would best benefit from hyperventilation and no PEEP.

An accurate physical examination with attention to auscultation of lung sounds, evidence of perfusion including skin color, and proper use of noninvasive monitoring will enable the prehospital provider to ensure adequate oxygenation and ventilation during transport. Several noninvasive devices are widely used in emergency care that provide information to the practitioner and help ensure that the patient is well oxygenated.

The ECG rhythm monitor may show a change in ventilation effectiveness. Tachycardia may indicate that a patient is hypoxic, or indicate a decrease in hemoglobin oxygen saturation (PCO2). Tachycardia also may be a result of pain or the discomfort of positive pressure ventilation in both adult and pediatric patients.

Note that if a pediatric patient becomes bradycardic as a result of hypoxia, this is a late and ominous sign of serious trouble.

Pulse oximetry (SpO2), often referred to as the sixth vital sign (after pain), is an easy and reliable tool to measure oxygen saturation in peripheral tissues. With proper probe placement, oximetry readings are typically very accurate, and the incidence of false readings is small. The goal of mechanical ventilation is to maintain SpO2 in the normal range of 95%–100%. Adjustments may have to be made in tidal volume, FiO2 concentration, PEEP and frequency to maintain the SpO2 within normal limits.

Capnography or end-tidal carbon dioxide (ETCO2) detectors, commonly used devices in the prehospital environment, measure the concentration of carbon dioxide in exhaled air. Lack of change in the colorimetric (disposable) ETCO2 device strongly implies that there may be improper placement of the endotracheal tube. In-line electric ETCO2 devices are standard in aeromedical services and are finding their way to ground transport units. Electronic capnography monitors usually register a numerical display with normal limits between 35%–45%. Monitoring end-tidal carbon dioxide is an excellent method of monitoring a patient on a ventilator and defining the elements of normal and abnormal ventilation and perfusion.

With some patients, mechanical ventilation may produce some unwanted side effects, which are not well understood. These side effects may include fluid retention, as a result of increased secretion of antidiuretic hormone, decreased renal perfusion and decreased urinary output; hypotension, hypoventilation or increased intercranial pressure. Lower cardiac output, as a result of decreased venous return and decreased right atrial filling, may also be seen as a result of the increased mean intrathoracic pressure.

Boyle’s law states that, at a constant temperature, the volume of a gas varies inversely with the pressure. Simply put, as the barometric pressure decreases, the volume of a gas increases. Providers who transport patients by air need to remember that when they ascend, the volume inside the air-filled cavities expands. Patients transported in this manner should have increased frequency (breaths per minute) and decreased tidal volume, which will minimize the chances of developing a tension pneumothorax. As the patient is taken to altitude, it is appropriate to decrease the tidal volume from 15 ml/kg to 10 ml/kg.

In summary, providers should become very familiar with the applications and limitations of the transport ventilators at their services. A ventilator should be thought of as a device that puts the good air in and lets the bad air out, and practitioners should not be intimidated by this machine.

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