Continuing Education Activity

Patients with respiratory failure or requiring airway protection may need artificial ventilation. A bag-valve-mask device is unreliable and utilizes clinician resources. Although mechanical ventilation is less commonly used in the prehospital setting, it offers superior ventilatory control, especially during prolonged transports. Recognition of basic respiratory physiology and ventilator settings is essential to initiate mechanical ventilation safely and correct the underlying respiratory derangements. Improper ventilator management can worsen the acute disease process and create an inflammatory cascade, thereby worsening lung injury. 

Participating clinicians within emergency medicine, critical care, and pulmonology benefit from interprofessional collaboration in cases of complex airway management. This activity reviews the physiology of positive pressure ventilation, various ventilator modes, and troubleshooting techniques, providing healthcare professionals with the knowledge and skills to manage patients requiring mechanical ventilation in the prehospital setting.

Objectives:

• Identify the appropriate indications for portable ventilators in emergency medical services based on patient assessment and clinical presentation.

• Screen patients effectively to determine the need for portable ventilation, considering factors such as respiratory rate, oxygen saturation, and clinical signs of respiratory distress.

• Apply evidence-based protocols and guidelines to select and use specific portable ventilation modalities for diverse patient populations.

• Implement effective strategies to improve care coordination among interprofessional team members to facilitate safe mechanical ventilation and handoffs.

Introduction

Although emergency medical services are known to adopt many innovative procedures and treatments, manual ventilation with a bag-valve-mask device remains the standard of care throughout the US.[1] This method requires the dedication of a team member and provides inconsistent respiratory rates and tidal volumes.[2] The benefits of mechanical ventilation are well-known in the hospital setting, and portable devices are available at a relatively low cost. A 2022 National Association of Emergency Medical Services Physicians (NAEMSP) position statement urges greater prehospital mechanical ventilation adoption.[3] While many reasons may prevent ventilator adoption, unfamiliarity with respiratory physiology likely prevents many clinicians from advocating for ventilator use. According to the US National Emergency Medical Services Information System (NEMSIS) public research dataset, the use of ventilators by emergency medical services during intubations is only 1%.[4] 

Anatomy and Physiology

The respiratory system consists of several organs responsible for gas exchange. The upper airway includes the nasal passages, mouth, pharynx, and larynx. The primary function of the upper airway is to warm, humidify, and filter incoming air. The lower airway comprises the trachea, bronchi, and bronchioles. These structures conduct air to and from the alveoli, where gas exchange occurs. Alveoli are tiny, air-filled sacs within the lungs where oxygen and carbon dioxide are exchanged with the bloodstream.Respiration involves ventilation (V) and perfusion (Q). Ventilation refers to the movement of air between the environment, through the upper and lower airways, and the alveoli. Perfusion is the movement of blood through the alveolar capillaries. The ventilation-perfusion (V/Q) ratio represents the balance between air ventilation and lung blood perfusion.[5] Changes in the V/Q ratio affect the exchange of oxygen and carbon dioxide. While removing carbon dioxide requires diffusion and ventilation, it primarily relies on ventilation. Ventilation involves the management of the patient's respiratory rate and volume. Oxygenation depends on the partial pressure of oxygen within the alveoli, where it rapidly diffuses into the capillaries.[6] Oxygenation can be modified during mechanical ventilation by adjusting the fraction of inspired oxygen (FiO₂) and positive end-expiratory pressure (PEEP). Pulse oximetry is a dynamic measurement of oxygenation, and end-tidal capnography is an active measurement of ventilation.

Using a ventilator allows the modification of a greater number of parameters compared to a bag-valve-mask device. Slight changes can have significant effects on patient physiology. PEEP has a notable impact on cardiac preload. When PEEP is applied, it increases the pressure in the thoracic cavity. This increased pressure within the thoracic cavity decreases the pressure gradient between the thorax and the vena cava, resulting in a decrease in cardiac preload. However, the effects of PEEP on preload can be complex and vary depending on factors such as the patient's intravascular volume status, compliance of the heart, and the severity of lung disease. The titration of FiO₂ is crucial, as the effects of hypoxia and hyperoxia are evident.[7] Rate control and tidal volumes can be titrated to maintain EtCO₂ levels between 35 and 45 mmHg.

Mechanical ventilation is an essential tool for prehospital clinicians and paramedics in managing critically ill patients with respiratory distress or failure. Understanding the anatomy and physiology of the respiratory system and mechanical ventilation principles is essential. Prehospital clinicians can significantly improve patient outcomes and contribute to their overall well-being by selecting appropriate ventilation parameters and closely monitoring patients.

Indications

Mechanical ventilation should be considered whenever a patient requires positive pressure ventilation, such as during orotracheal intubation or laryngeal mask airway ventilation. Moreover, using an appropriate ventilator for noninvasive positive pressure ventilation can improve patient compliance and comfort. NAEMSP recommends using mechanical ventilation in the prehospital with any patient experiencing respiratory failure or requiring airway protection.[3] If a patient exhibits ventilation failure (hypercapnia) or oxygenation failure (hypoxia), mechanical ventilation allows the clinician to correct these derangements by reducing the work of breathing, controlling minute ventilation, increasing alveolar recruitment to improve gas exchange, and mitigating the V/Q mismatch.[8][9]

Contraindications

Using mechanical ventilation requires a trained and credentialed clinician to manage the ventilator parameters safely. Although the benefits of using a ventilator may not be pronounced during shorter transport times, it is not strictly contraindicated.

Preparation

General Guidelines for Ventilator Settings

Patients must receive appropriate sedation and analgesia before initiating mechanical ventilation in prehospital settings.[3] The underlying physiology and the indication for mechanical ventilation are essential for establishing proper ventilator settings, and the initial setup should target that derangement. Maintaining normal oxygenation and appropriate carbon dioxide levels in specific settings is crucial for future neurologic function and patient outcomes. Mechanical ventilation facilitates better control of external parameters, maintaining appropriate levels of oxygen and carbon dioxide. Hypoxia in the prehospital settings is known to be associated with poor outcomes in trauma patients.[10] More liberal use of oxygen, and resulting hyperoxia, is also known to be associated with increased mortality.[11] Therefore, FiO₂ should be titrated with a goal SpO₂ greater than 95%.[11]

When a patient is intubated for airway protection, ventilator settings should closely mimic normal physiology. Management of hypoxic respiratory failure should increase FiO₂ and PEEP to improve oxygenation. If the patient is hypercapnic but requires minimal oxygen supplementation, the strategy is to augment ventilation to return CO₂ to normal and keep excess oxygenation to a minimum. The clinician must generally match the minute ventilation the patient demands before intubation. This is especially crucial for a patient with severe metabolic acidosis, necessitating markedly high minute ventilation to maintain appropriate respiratory compensation. An example of this is a patient in diabetic ketoacidosis with tachypneic, hyperpneic respirations requiring both high tidal volume and a high rate to meet the minute ventilation demand.

Initial setup: Most parameters recommended for prehospital care by emergency medical services are extrapolated from hospital protocols or hospital-based literature and should be initiated as soon as feasible. While a short transport may be a reasonable indication for a bag-valve-mask device use, lung injury from using large tidal volumes can develop within 20 minutes and is associated with increased mortality.[12]

The hospital-based literature clearly states that mechanical ventilation is better than BVM and should be initiated as soon as feasible. Choosing a ventilation mode is somewhat arbitrary and will depend on the clinician's experience, comfort level with various modes, and local protocols. This review strictly discusses volume assist or control, as transport ventilators vary widely in their settings, but assist or control is standard across all.[13]

• Determine tidal volume. The initial parameters include tidal volumes of 4-6 mL/kg of ideal body weight (IBW), a plateau pressure (Pplat) <30 cm H₂O, and a driving pressure (Pplat - PEEP) <15 cm H₂O. IBW is a predicted weight based on the patient's sex and height. A common IBW calculation is below, although there are several.[14] IBW should be used instead of actual weight because lung volume does not change with body mass. Lung volume is based on the thoracic cavity size, independent of a patient's weight. In other words, a 65" tall male weighing 200 kg has a similar-sized thoracic cavity as another male of the same height weighing 70 kg and will have a similar tidal volume as well. Therefore, obtaining the patient's height or asking the family for accurate height is crucial. In many resuscitation bays, tape measures are available to ensure a correct IBW is obtained for appropriate tidal volumes.[15]

IBW (Men) = 50 kg + (2.3 kg × (height [in] − 60))

IBW (Women) = 45.5 kg + (2.3 kg × (height [in] − 60))

• Set respiratory rate. An average starting point for patients with respiratory failure is 14 to 16 bpm. However, it is recommended to consider higher rates for patients with metabolic acidosis, as discussed above, who will require much higher minute ventilation. Initial EtCO₂ after intubation should be noted, and this dynamic measure should be used as a guide to monitor the adequacy of ventilation.

• Start with PEEP of 5 cm H₂O. This is typically sufficient to overcome the intrinsic resistance of the ventilator circuit and maintain a physiological amount of PEEP.

• Titrate FiO₂. Most will default to 100% FiO₂ initially, but prolonged exposure can lead to hyperoxemia, which may have long-term detrimental effects on the patient. Therefore, it is recommended to quickly titrate FiO₂, aiming to use the minimum amount of oxygen required to maintain SpO₂ between 92% and 98%.[11]

• Ensure adequate sedation. Ventilator synchrony maximizes ventilation effectiveness, increases patient comfort, and prevents downstream lung injury. The choice of sedative will vary depending on the local protocol.

Technique or Treatment

Mechanical ventilation techniques and parameters for prehospital care should be disease-specific and similar to the in-hospital standard of care.[3]

Basic Variables of Mechanical Breath

Before discussing the mechanics of the ventilator, it is crucial to understand the basic variables that determine the generation of a mechanical breath.[16]

• Tidal volume: The air delivered with each breath, measured in milliliters (mL).

• Respiratory rate: The number of breaths delivered per minute.

• Minute ventilation: The volume of air exchanged over 1 minute, calculated by multiplying the tidal volume by the respiratory rate, measured in liters per minute (L/min).

• Peak airway pressure: The maximum pressure exerted on the airways during inspiration, measured in cm H₂O.

• Flow rate: The inspiratory flow rate required to overcome the resistance of the circuit to deliver the inspiratory breath, measured in liters per minute (L/min).

• I:E ratio: The ratio of inspiratory time to expiratory time.

• Fraction of inspired oxygen (FiO₂): The percentage of oxygen in the inspired air.

• Positive end-expiratory pressure (PEEP): The airway pressure applied at the end of exhalation, measured in cm H₂O.

Modes of Ventilation

Mechanical ventilators have several modes of ventilation that deliver breaths based on 3 preset factors: trigger, target, and termination.

• Trigger: How the breath is initiated, either ventilator-initiated based on a specific parameter or patient-initiated based on negative pressure generated by the patient.

• Target: A preset inspiratory flow rate or pressure limit that the ventilator targets to generate the breath.

• Termination: The endpoint of the inspiratory breath, which could be a preset duration of inspiration, volume target, or inspiratory flow rate.

Types of Breaths

Three different types of breaths delivered during mechanical ventilation vary based on the trigger of the breath and how the work is performed.

• Mandatory: These breaths are triggered by the ventilator, which performs the work of inspiration.

• Assisted: The patient triggers assisted breaths, but the ventilator performs the work of inspiration.

• Spontaneous: The patient triggers spontaneous breaths and performs the work of inspiration.

Categories of Mechanical Ventilation

The breath strategy or the target and termination of the inspiration determine 2 broad categories of mechanical ventilation.

• Volume control (volume limited): Volume-limited breaths target a preset flow rate, and inspiration is terminated when that volume is achieved. The circuit's intrinsic resistance and airway compliance determine airway pressures.

• Pressure control (pressure limited): Pressure-limited breaths target a preset inspiratory pressure, and inspiration is terminated when a set inspiratory time is achieved. Tidal volume and minute ventilation depend on lung compliance and airway resistance.

Ventilator Mode

Each ventilator mode varies based on the trigger, the breath strategy, and the types of breaths delivered.[17]

• Controlled mechanical ventilation (CMV): All breaths are mandatory and triggered by the ventilator based on the preset respiratory rate, with the target and termination depending on volume or pressure-limited strategy. No assisted or spontaneous breaths are required.

• Assist or control ventilation (AC): The ventilator triggers mandatory breaths at a preset minimum respiratory rate; however, this mode also allows for patient-triggered assisted breaths. The target and termination are determined by volume or pressure-limited breath strategy. Spontaneous breathing is not enabled. 

• Pressure-regulated volume control (PRVC): This mode is similar to assist or control; however, the ventilator adjusts the inspiratory flow rate to regulate the pressure delivered to the airways.

• Intermittent mechanical ventilation (IMV): The ventilator triggers mandatory breaths at a preset rate. However, this mode allows for patient-triggered breaths, which can be spontaneous or assisted, depending on the settings. The ventilator can provide pressure support to spontaneous breaths to reduce the work of breathing, or breaths can be entirely spontaneous. The target and termination of mandatory breaths vary depending on the breath strategy.

• Pressure support (PS): Breaths are fully patient-triggered, and the ventilator delivers a set driving pressure with each breath. Inspiration is determined by the cessation of inspiratory force generated by the patient. Tidal volume varies depending on compliance and resistance.

• Continuous Positive Airway Pressure (CPAP): No ventilator cycling is needed. The ventilator provides a fixed amount of airway pressure, and breaths are spontaneous.

Monitoring

After initiating mechanical ventilation, close and continuous monitoring is required for the patient, and settings are adjusted accordingly. In the absence of the capability to check blood gases in the field, clinicians rely on pulse oximetry and end-tidal capnometry to make necessary adjustments in patient management. 

End-tidal capnometry (EtCO₂): This provides an approximate measure of the PaCO₂ or the pressure of carbon dioxide in the blood as it passes through the alveoli. The target EtCO₂ for most patients ranges from 40 to 45 mm Hg. Actual PaCO₂ may be higher than EtCO₂, but trending is as important as the absolute number. For example, if the patient was intubated for respiratory failure secondary to chronic obstructive pulmonary disease exacerbation and the initial EtCO₂ was 80 mm Hg after mechanical ventilation, this value should trend downward toward normal. If EtCO₂ trends upward, this is a sign of inadequate ventilation, and the minute ventilation must be increased by increasing the respiratory rate and then the tidal volume. Tidal volumes exceeding 8 mL/kg should be avoided to prevent lung injury.

Pulse oximetry (SpO₂): This serves as a dynamic monitor of oxygenation. The ideal range for SpO₂ should be 92% to 98%, using the minimum amount of oxygen. Adding PEEP is another method of increasing oxygenation, as this helps prevent the collapse of the alveoli and increases the diffusion gradient for oxygen. 

Complications

Hemodynamic Considerations

Under normal conditions, the chest cavity is under negative pressure. The negative inspiratory pressure generated by the expansion of the chest cavity not only pulls air into the lungs but also augments venous return to the heart. When the patient transitions to positive pressure ventilation, venous return to the heart (and thus preload) is reduced, which may lower blood pressure. This is often associated with the concurrent drop in blood pressure caused by many sedative agents. The clinician's response to this drop in blood pressure ultimately depends on the patient and is beyond the scope of the chapter; however, a common error is setting the tidal volume too high, and reducing the tidal volume may lead to lower pressures and help mitigate some of this effect.

Diagnostics

Whenever a ventilator requires troubleshooting or there is a concern for oxygen desaturation or instability, clinicians should consider removing the patient from the ventilator. This action removes a highly complex device and can help determine the source of potential issues. During this time, the patient should be carefully ventilated with a bag-valve-mask device. A loss of consistent EtCO₂ waveform can be due to obstruction of the device, tube dislodgement, poor ventilation, or device failure. Among these possibilities, device failure is improbable. Clinicians should assess the tube placement and ensure adequate chest rise and fall. The replacement of the airway device should be considered, and attention should be given to ensure nothing is blocking the EtCO₂ tubing if the waveform still does not correct. The sensor should be checked, as secretions may impair its function. Finally, if EtCO₂ cannot be corrected, the EtCO₂ monitor should be replaced, followed by an alternative monitor.

An increase in minute ventilation or tidal volume may decrease CO₂ levels, while decreased or shallow breathing may elevate CO₂ levels. However, ventilated patients rely on changes to ventilator settings to manage their systemic pH balance. Patients with increasing CO₂ levels should first be removed from the ventilator and assessed for tube displacement, pulmonary edema, and pneumothorax. The patients should receive bag-valve-mask respirations and ensure CO₂ improvement with increased respiratory rate. This should be addressed if an underlying issue, such as septic shock, may cause the patient's hypercapnia. When interrogating the ventilator settings, elevated CO₂ levels will likely be improved by increased respiratory rate or tidal volume.

In cases of sudden worsening of hypoxia, it is essential to confirm tube placement, assess breath sounds, and check for any signs of tracheal deviation or subcutaneous emphysema suggestive of pneumothorax development. If hypoxia is confirmed, increasing PEEP and FiO₂ is prudent. In addition, it is crucial to disconnect the patient from the ventilator and manually ventilate them with a bag-valve-mask device and 100% oxygen.

When an alarm for high inspiratory pressures is detected, it is advisable to check the circuit for any obstructions and ensure adequate sedation and ventilator synchrony. Moreover, if the high peak pressure alarms are triggered with normal plateau pressure, underlying obstructive ventilatory defects should be suspected and treated using bronchodilators. 

Special Considerations

Breath stacking occurs when the patient does not fully exhale. As a result, with each successive breath, the volume of air in the lungs, and consequently airway pressure, increases.[18] This can be dangerous and put the patient at risk of barotrauma. The stacking can occur when patients have severe obstructive disease, particularly asthma. Some ventilators display the volume waveform or the inspired and exhaled volumes. The volume of exhaled air should match the inspired volume, or the waveform should return to zero. If breath stacking is detected, the ventilator circuit should be briefly disconnected, the patient's chest should be gently pushed to exhale all the excess volume, and the ventilator should be reconnected. The respiratory rate should be reduced to allow more time between breaths for exhalation, and if the machine has the capability, the inspiratory time of the breath should be decreased. Permissive hypercapnia is often required to obtain appropriate oxygenation and prevent breath stacking.[19]

Patients with severe metabolic acidosis, such as diabetic ketoacidosis, rely on respiratory compensation to mitigate the acidemia and thus require significant minute ventilation above normal levels. Intubation should be avoided in these patients whenever possible, and if it is unavoidable, the respiratory rate should be set to match the patient's breathing rate before intubation. The initial end-tidal CO₂ levels should be noted immediately after intubation, and efforts should be made to maintain or lower that number. Failure to do so results in worsening acidemia and further deterioration of the patient's condition.

Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome is a complicated condition characterized by severe lung injury and inflammation. Ventilation strategies for these patients should be aimed at minimizing lung injury. Tidal volumes in these patients should be reduced to 6 mL/kg of IBW or lower with higher PEEP and FiO₂. Typically, PEEP should be increased to higher than 10 cm H₂O. PEEP should be titrated and monitored to ensure patient compliance and improvement. In these patients, lower SpO₂ values of 88% to 90% can be tolerated with permissive hypercapnia due to low tidal volumes.[20] Acute respiratory distress syndrome is not readily identifiable in prehospital settings due to specific diagnostic criteria; however, it can be suspected when patients require markedly high levels of oxygen.[21]

Clinical Significance

Patients intubated in the field require some form of artificial ventilation. Bag-valve-mask ventilation is unreliable and inconsistent and requires a dedicated clinician to continue ventilation. Using mechanical ventilators in the prehospital environment facilitates precise ventilation control, particularly in areas with longer transport times. Understanding basic respiratory physiology and ventilator settings is crucial for safely initiating mechanical ventilation and addressing the underlying respiratory derangements. Improper ventilator management can worsen the acute disease process and trigger an inflammatory cascade, thereby worsening lung injury.[22]

Enhancing Healthcare Team Outcomes

Before initiating intubation and mechanical ventilation, the caregivers should try to obtain the patient's preferences. Medical Orders for Life-Sustaining Treatment (MOLST) and do-not-resuscitate (DNR) orders should be considered, and efforts should be made to engage with major healthcare stakeholders in the community to ensure appropriate communication of patient preferences.

Mechanical ventilation is a complex process that requires teamwork and interprofessional coordination. Effective communication among team members is crucial. When possible, it is essential to inform the receiving facility in advance to ensure a smooth transition of care and continued ventilation. Prehospital clinicians should coordinate their activities with the rest of the interprofessional healthcare team and seek consultations with clinicians if necessary. Interprofessional care coordination and open communication are essential for achieving the best possible outcomes for ventilated patients. Finally, patients undergoing mechanical ventilation should be reviewed patient-centered to identify potential barriers to care and ensure the delivery of high-quality care.