Mechanical ventilation has a long and storied history, but until recently the process required little from the emergency physician. In the modern emergency department, critically ill patients spend a longer period under the care of the emergency physician, requiring a greater understanding of ventilator management. This article serves as an introduction to mechanical ventilation and a user-friendly bedside guide.
Key points
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Mechanical ventilation is a commonly used but sometimes poorly understood modality in the emergency department.
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In the modern emergency department, where patients remain under the care of the emergency physician for a longer duration, physicians must become comfortable with treating ventilated patients for extended periods.
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Emergency physicians need to understand how to initiate, titrate, and manage mechanical ventilation.
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Emergency physicians must have the ability to adapt ventilatory strategies for specific patients, understand the potential harms of mechanical ventilation, and take action to reduce their incidence.
History of mechanical ventilation
Mechanical ventilation has a long, storied history. Descriptions of positive-pressure ventilation can be found in the Old Testament, in writings dating from 800 bc . A passage from Kings 4:34 to 35 describes the Prophet Elisha performing mouth-to-mouth ventilation on a dying child :
And he went up, and lay upon the child, and put his mouth upon his mouth, and his eyes upon his eyes, and his hands upon his hands: and stretched himself upon the child; and the flesh of the child waxed warm.
Hippocrates described the process of endotracheal intubation in his book Treatise on Air , published in 460 bc : “One should introduce a cannula into the trachea along the jaw bone so that air can be drawn into the lungs.”
Despite these early forays into positive-pressure ventilation, our application of mechanical ventilation took a dark turn in England in the 18th century. Doctors used bellows and a long tube inserted in the rectum to blow smoke into a drowned patient’s gastrointestinal tract. Their intention was to stimulate the failing myocardium and dry the recently submerged body from the inside. As medical knowledge advanced, this practice lost favor because of its obvious absurdity. Although some would say our understanding of mechanical ventilation has grown immensely since that era, others would argue that we have merely learned the appropriate orifice through which to ventilate!
This article reviews the common modes of mechanical ventilation that emergency physicians are likely to experience in their practice, discusses the strengths and weaknesses of the various approaches, and proposes a strategy of how best to initiate and maintain mechanical ventilation in the wide range of patients who are intubated in the emergency department.
History of mechanical ventilation
Mechanical ventilation has a long, storied history. Descriptions of positive-pressure ventilation can be found in the Old Testament, in writings dating from 800 bc . A passage from Kings 4:34 to 35 describes the Prophet Elisha performing mouth-to-mouth ventilation on a dying child :
And he went up, and lay upon the child, and put his mouth upon his mouth, and his eyes upon his eyes, and his hands upon his hands: and stretched himself upon the child; and the flesh of the child waxed warm.
Hippocrates described the process of endotracheal intubation in his book Treatise on Air , published in 460 bc : “One should introduce a cannula into the trachea along the jaw bone so that air can be drawn into the lungs.”
Despite these early forays into positive-pressure ventilation, our application of mechanical ventilation took a dark turn in England in the 18th century. Doctors used bellows and a long tube inserted in the rectum to blow smoke into a drowned patient’s gastrointestinal tract. Their intention was to stimulate the failing myocardium and dry the recently submerged body from the inside. As medical knowledge advanced, this practice lost favor because of its obvious absurdity. Although some would say our understanding of mechanical ventilation has grown immensely since that era, others would argue that we have merely learned the appropriate orifice through which to ventilate!
This article reviews the common modes of mechanical ventilation that emergency physicians are likely to experience in their practice, discusses the strengths and weaknesses of the various approaches, and proposes a strategy of how best to initiate and maintain mechanical ventilation in the wide range of patients who are intubated in the emergency department.
Introduction to variables
In 1493, Paracelsus, a Swiss German renaissance physician, inserted a tube connected to fire bellows into a patient’s mouth to assist with ventilation. A person pumped the bellows, delivering breaths to the patient. The force, rate, and timing of each breath were left to the prerogative of whoever was squeezing the bellows. In modern ventilators, we have outsourced these tasks to a mechanical circuit with adjustable settings.
Imagine that you have just intubated a patient and now must provide adequate ventilator support. Instead of a modern ventilator, you have a turn-of-the-century bellows device. You also have an assistant, who will pump the device in your absence. How would you instruct him to ventilate your patient?
Management of Ventilation
Trigger
When should your eager assistant pump the bellows? After all, you cannot stand next to the bedside, commanding him to pump the bellows each time you want to give your patient some air. If you instructed him to pump the bellows once every 6 seconds, then your patient would receive a fixed respiratory rate independent of his or her own respiratory efforts. On the other hand, if you instructed your assistant to pump the bellows only when the patient initiated a spontaneous breath, you would simply be augmenting your patient’s own respiratory rate. The trigger is simply the stimulus that notifies your assistant or the modern-day ventilator when to deliver a breath.
Limit
Now that you have instructed your assistant when to pump the bellows, how much air should he deliver? Should he deliver the maximum volume the bellows contains, or would it be prudent to tailor the amount of gas delivered to the size and requirements of the patient? Traditionally, the quantity of breath delivered is controlled in 2 ways: volume controlled and pressure controlled. Volume-controlled ventilation delivers a fixed volume of gas with each breath. But, in some cases, control over the volume of breath delivered is not ideal (discussed in more detail later). In such scenarios, the assistant should be instructed to pump the bellows until a specific pressure is reached on the manometer attached to the bellows device. Once this threshold is reached, he should halt his delivery, independent of the volume of gas delivered. This is pressure-controlled ventilation.
Cycle
Finally, how fast do you instruct your assistant to deliver each breath? For example, if you have instructed him to deliver 10 breaths each minute, then he will pump the bellows once every 6 seconds. How long should it take for him to deliver the breath? Remember that your assistant must allow time for the patient to passively exhale. In our natural spontaneous breathing pattern, we spend much more time in exhalation than in inhalation. Therefore, we commonly ask our assistants and ventilators to deliver a breath quickly, allowing adequate time for exhalation. The relationship between these intervals is the inhalation-to-exhalation ratio (I/E). The I/E is controlled by 2 factors: first, the amount of time spent in each breath cycle (number of breaths taken per minute) and, second, the speed at which the breath is delivered. If we instruct our assistant to deliver 10 breaths per minute, then he has 6 seconds to complete every breath cycle. If he delivers each breath over a 2-second period, he has 4 seconds to allow for passive expiration. This yields an I/E ratio of 2/4 or ½. If I asked him to deliver 20 breaths per minute, then each breath cycle would only be 3 seconds long. If he continued to deliver each breath over a 2-second interval, he would only have 1 second for passive exhalation before he had to give the next breath. This would create an I/E of 2/1. If you instructed him to deliver his breaths twice as fast, he would have 2 seconds for passive exhalation, changing the I/E ratio to 1/2. Thus, changing the speed of inhalation directly affects the I/E ratio.
A reduction in expiratory time is normally of little consequence, but it becomes a problem for patients with asthma or chronic obstructive pulmonary disease (COPD), in whom bronchoconstriction prolongs the expiratory phase. Too short of an expiratory time in these patients results in retention of tidal volumes within the lung from a previous breath. The retention of volume within the chest (also known as intrinsic or auto positive end-expiratory pressure [PEEP]) can lead to hemodynamic problems (increased intrathoracic pressure and compression of the great vessels) and pulmonary complications such as barotrauma. Please note that PEEP is defined as positive pressure that exists within the tracheobronchial tree and the lungs at the end of expiration; in this example it can lead to a pathologic condition; however, as will be discussed, PEEP can be used beneficially when ventilating patients.
Management of Oxygenation
Fraction of inspired oxygen
Imagine that your assistant can fill his bellows from 2 sources. One source is 100% oxygen, the other is room air (21% oxygen). Your assistant is able to draw from both sources, giving him whatever concentration of oxygen you request. This concentration is expressed as a ratio, fraction of inspired oxygen (F io 2 ).
Positive end-expiratory pressure
The PEEP is the measured pressure at the end of expiration. In our example, it is measured on the bellows manometer as the patient exhales. PEEP has 2 components: intrinsic PEEP, generated by the patient’s own physiology and extrinsic PEEP, which is added to the system for therapeutic purposes. These 2 pressures are additive. In our case, we can increase the extrinsic PEEP of our system by asking our assistant to tighten a release valve that controls the rate at which gas is released from our system. Likewise, modern ventilators allow you to set the amount of extrinsic PEEP that will be present throughout the respiratory cycle.
Modes of ventilation
Now that we have established the variables that constitute a ventilatory strategy, we turn to the modes of ventilation commonly used in emergency departments and intensive care units.
Volume-Controlled Ventilation
Imagine that you have placed an opaque screen between the patient and your assistant and asked him to deliver a selected number of breaths per minute, at a given volume, with a set inspiratory flow rate. In this mode of ventilation, your assistant is detached from the patient’s intrinsic respiratory efforts and will uniformly deliver the requested number of breaths at the desired volume. If the patient attempts to take a spontaneous breath, it will go unnoticed by the blinded assistant. Likewise, if the patient has yet to finish the previous breath’s exhalation, your assistant will automatically deliver the subsequent breath despite this incomplete expiratory effort. This lack of synchrony can cause significant distress to the patient and, in some cases, serious lung injury. For the aforementioned reasons, it is not a recommended mode of ventilation in the emergency department, unless the patient is fully anesthetized and paralyzed.
Volume Assist–Control Ventilation
Now imagine you have removed the screen separating your assistant and the patient but again ask your assistant to deliver a set volume of gas from the bellows. This time, you ask him to pump the bellows in synchrony with the patient’s intrinsic breaths. Every time the patient begins to inspire, your assistant delivers a predetermined volume of air. As a safety mechanism, because you cannot stand at the bedside watching, you set a baseline respiratory rate, below which your assistant will pump the bellows independent of the patient’s intrinsic efforts.
This mode of ventilation creates a more natural ventilatory cycle for the patient, allowing him to control when a breath is initiated and forcing in breaths only when the patient’s intrinsic rate of breathing is insufficient for ventilation. Despite allowing for the patient’s own initiation of each breath, the process of mechanical ventilation can be a distressing experience, as the rest of the components of the ventilatory cycle are out of the patient’s control. For example, if the patient wishes to take larger volumes of air than you have instructed your assistant to allow, generating enough inspiratory force to inhale the remainder can prove difficult because the patient is working against the resistance of the entire circuit. Likewise, if the rate at which you have instructed your assistant to deliver each breath is slower than the patient’s natural inspiratory inclination, then the work of breathing naturally increases as the patient inhales against resistance.
Pressure Control Ventilation
Similar to volume control ventilation, pressure control places a screen between your assistant and the patient. Again, you instruct your assistant to deliver a selected number of breaths per minute. But instead of instructing him to give a set volume of gas, you now instruct him to pump the bellows until the monometer reaches a specific pressure threshold. Your assistant will continue to deliver this breath at this pressure for 1 second (cycle time). At this point, he will cease breath delivery independent of the volume of gas delivered. You are able to adjust the volume of gas delivered by adjusting the pressure and cycle time. If you instruct your assistant to pump his bellows at a higher continual pressure then the volume of gas delivered will be higher. Concordantly, if you instruct him to provide the same constant pressure for a longer duration (cycle time), this too will increase the volume of gas delivered.
Pressure Assist–Control Ventilation
Once again, you have removed the screen separating your assistant from the patient, instructing him to pump the bellows until the monometer reaches a given pressure value. You ask him to pump the bellows in synchrony with the patient’s intrinsic breaths. Every time the patient begins to inspire, your assistant will pump the bellows until the predetermined pressure level is reached. As a safety mechanism, because you cannot stand at the bedside watching, you set a baseline respiratory rate, below which your assistant will pump the bellows independent of the patient’s intrinsic efforts.
Volume and pressure both have advantages and disadvantages in the determination of the quantity of gas delivered. A volume-controlled mode of ventilation ensures that your patient receives an exact amount of air with each breath. This approach can work against you, as the predetermined volume delivered does not take into account the clinical scenario. For example, say you have instructed your assistant to deliver 400 mL of volume with each breath to an asthmatic who has been intubated for respiratory failure. The patient is still paralyzed from the long-acting paralytic you gave during the intubation process, so you tell your assistant to pump the bellows once every 5 seconds (ie, a respiratory rate of 12). While you are away seeing other patients, your assistant does exactly as instructed. What your assistant does not notice is that, because of the patient’s obstructive physiology, he is exhaling only 300 mL of gas after each breath. As your assistant indiscriminately pumps 400 mL, the patient is trapping 100 mL of air with each breath. The manometer on the bellows is rising with each pump, and your assistant notices that it is becoming increasingly hard to deliver the allotted volume of air. But he is fastidious in his duties and continues to deliver 400 mL of air. This is the phenomenon of breath stacking, which is just as likely to occur with a volume control mode on a modern ventilator. This example highlights the major shortcoming of using volume goals exclusively to ventilate patients. In certain pathologic circumstances, a volume-guided approach to ventilation can be deleterious for the patient, as it does not account for physiologic states that cause high pressures during positive-pressure ventilation. In cases of obstructive physiology, continual delivery of a set volume of air without taking note of the expiratory volume can lead to breath stacking, pneumothorax, and even death. Additionally, in patients with acute lung injury, the intrinsic elasticity of the lung can be damaged, causing “stiff lung” and leading to acute respiratory distress syndrome (ARDS). Ventilating patients at normal lung volumes can further damage their already-injured lung parenchyma. If you continue to ventilate these patients without accounting for the high alveolar pressures created by their pathology, you risk contributing to their disease iatrogenically.
Now you are savvy to the harms of an unchecked volume-controlled method of ventilation in a patient with asthma. You instruct your assistant to pump the bellows until the monometer reaches 40 mm Hg and to continue delivering this pressured breath for 1 second, at which point he will stop. You walk away, satisfied that you have ensured the safety of your patient’s lungs. But when you return a short time later, you find your patient is hypoxic and cyanotic despite your assistant doing just as you instructed. What happened this time? Our assistant had dutifully followed your instruction to the letter. He initiated a breath once every 5 seconds and delivered only enough air to reach a certain pressure limit on the manometer. Because your patient is showing significant obstructive pathology, the peak ventilator pressures will be elevated. Therefore, each time your assistant initiated a breath, the manometer quickly reached the predefined pressure limit, but little air was delivered to the patient. Not surprisingly, the patient’s carbon dioxide (CO 2 ) level began to increase quickly, because no ventilation was being delivered. Eventually, even the patient’s partial pressure of oxygen, arterial (Pa o 2 ) level began to decrease. This example shows that a purely pressure-guided method of mechanical ventilation can be equally problematic.
Pressure-Regulated Volume Control
Now that we have effectively mismanaged our patient in several ways, we must find a way to ensure that he receives adequate volumes of gas while minimizing harm to his lung parenchyma caused by increased alveolar pressures. You instruct your assistant to pump the bellows once every 5 seconds and deliver a breath using the manometer (pressure controlled). Additionally, you inform him of the desired volume you wish the patient to receive. To accomplish this, your assistant will deliver a test breath using a specific pressure and observe the volume of gas it delivers. If this volume is more than or less than the specified volume, your assistant will adjust the pressure used for the subsequent breath. He will continue to adjust his pressures to deliver the desired volume of gas.
This example describes the pressure-regulated volume control mode of ventilation, a pressure-controlled approach in which you assign the volume of gas delivered, the trigger to initiate ventilation, the PEEP, F io 2 levels, and a peak pressure limit. A more appropriate name for this mode of ventilation would be pressure control, volume guaranteed ventilation. This method of ventilation allows you to control both the pressure and volume of gas during ventilation.
Synchronized Intermittent Mandatory Ventilation
Some clinicians have suggested that even pressure-regulated volume control modes of ventilation create a ventilatory environment that is unnatural to the intubated patient. Although the patient guides the initiation of each breath, the inspiratory rate and volume are determined by the ventilator. If either of them is not congruent with the patient’s needs, ventilator dysynchrony is produced. Synchronized intermittent mandatory ventilation can ameliorate these problems. Similar to the previously described modes, you assign a specific tidal volume, respiratory rate, inspiratory flow rate, F io 2 , and PEEP. The assistant is instructed to pump the bellows a given number of times per minute, delivering the amount of gas at the determined flow rate with each breath. You then set the respiratory rate at a frequency below which the patient would typically breathe. You again place the screen between your assistant and the patient, this time providing an escape valve in the circuit that allows the patient to take independent breaths not determined by the bellows. Now the patient will receive 2 forms of ventilation: the positive pressure delivered by your assistant’s bellows and the patient’s own intrinsic breathing. The work associated with these patient-originated breaths is far more taxing, as the patient has to generate enough inspiratory force to overcome the resistance of the ventilator tubing and the endotracheal tube. This increased work can be alleviated by the addition of pressure support during the spontaneous inspiratory efforts; this mode is called synchronized intermittent mandatory ventilation with pressure support ventilation .
Two other forms of ventilation are more esoteric and unlikely to be initiated in the emergency department, but it would behoove the savvy emergency physician to have a certain degree of understanding of them. These methods are inverse ratio ventilation (also called airway pressure release ventilation [APRV]) and high-frequency oscillatory ventilation (HFOV).
Airway Pressure Release Ventilation
APRV uses prolonged periods of continuous positive airway pressure interspersed with brief periods of lower pressure. Proponents of APRV state that the prolonged periods of continuous positive airway pressure cause alveolar recruitment and thereby improve oxygenation and lung volumes without the typical alveolar stress caused by more traditional forms of mechanical ventilation. Most ventilation and CO 2 exchange occurs during the brief periods of low pressure (ie, the release phase). This strategy can be used in both spontaneously breathing and passive patients. This strategy is used most commonly for the management of ARDS and acute lung injury in the hopes of maintaining adequate oxygenation without causing iatrogenic damage to the already injured lung.
High-Frequency Oscillatory Ventilation
HFOV provides an extremely high frequency of small tidal volume breaths around a high level of constant airway pressure. It is akin to instructing your assistant to attach a small engine to the bellows, which creates a propeller-driven system that delivers 30 mm Hg of continuous airway pressure. Above this pressure, the assistant is instructed to pump the bellows rapidly to deliver minuscule tidal volumes at a fast rate (3–15 Hz). Similar to APRV, this method of ventilation hopes to minimize the damage induced by mechanical ventilation by limiting the degree of alveolar distention using small tidal volumes. Gas exchange is achieved by the mechanisms listed in Table 1 .
Related posts:
Early Treatment of Severe Acute Respiratory Distress Syndrome
Management of Acute Exacerbation of Asthma and Chronic Obstructive Pulmonary Disease in the Emergency Department
Respiratory Emergencies in Geriatric Patients
Approach to Adult Patients with Acute Dyspnea
Airway Management of Respiratory Failure
Advances in Point-of-Care Thoracic Ultrasound
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