| Mechanical ventilation | |
|---|---|
| Intervention | |
| ICD-9: | 93.90 96.7 |
| MeSH | D012121 |
| OPS-301 code: | 8-71 |
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by a physician, respiratory therapist or other suitable person compressing a bag or set of bellows. Traditionally divided into negative-pressure ventilation, where air is essentially sucked into the lungs, or positive pressure ventilation, where air (or another gas mix) is pushed into the trachea. There are two main divisions of mechanical ventilation; Invasive ventilation, and non-invasive ventilation.[1] There are two main modes of mechanical ventilation within the two divisions; positive pressure ventilation and negative pressure ventilation.
The Roman physician Galen may have been the first to describe mechanical ventilation: "If you take a dead animal and blow air through its larynx [through a reed], you will fill its bronchi and watch its lungs attain the greatest distention."[2] Vesalius too describes ventilation by inserting a reed or cane into the trachea of animals.[3] In 1908 George Poe demonstrated his mechanical respirator by asphyxiating dogs and seemingly bringing them back to life.[4]
Mechanical ventilation is often a life-saving intervention, but carries many potential complications including pneumothorax, airway injury, alveolar damage, and ventilator-associated pneumonia.[5]
In many healthcare systems prolonged ventilation as part of intensive care is a limited resource (in that there are only so many patients that can receive care at any given moment). It is used to support a single failing organ system (the lungs) and cannot reverse any underlying disease process (such as terminal cancer). For this reason there can be (occasionally difficult) decisions to be made about whether it is suitable to commence someone on mechanical ventilation. Equally many ethical issues surround the decision to discontinue mechanical ventilation.[6]
It can be used as a short term measure, for example during an operation or critical illness (often in the setting of an intensive care unit). It may be used at home or in a nursing or rehabilitation institution if patients have chronic illnesses that require long-term ventilatory assistance. Owing to the anatomy of the human pharynx, larynx, and esophagus and the circumstances for which ventilation is required then additional measures are often required to secure the airway during positive pressure ventilation to allow unimpeded passage of air into the trachea and avoid air passing into the esophagus and stomach. Commonly this is by insertion of a tube into the trachea which provides a clear route for the air. This can be either an endotracheal tube, inserted through the natural openings of mouth or nose or a tracheostomy inserted through an artificial opening in the neck. In other circumstances simple airway maneuvres, an oropharyngeal airway or laryngeal mask airway may be employed. If the patient is able to protect their own airway and non-invasive ventilation or negative-pressure ventilation is used then a airway adjunct may not be needed.
The iron lung, also known as the Drinker and Shaw tank, was developed in 1929 and was one of the first negative-pressure machines used for long-term ventilation. It was refined and used in the 20th century largely as a result of the polio epidemic that struck the world in the 1940s. The machine is effectively a large elongated tank, which encases the patient up to the neck. The neck is sealed with a rubber gasket so that the patient's face (and airway) are exposed to the room air.
While the exchange of oxygen and carbon dioxide between the bloodstream and the pulmonary airspace works by diffusion and requires no external work, air must be moved into and out of the lungs to make it available to the gas exchange process. In spontaneous breathing, a negative pressure is created in the pleural cavity by the muscles of respiration, and the resulting gradient between the atmospheric pressure and the pressure inside the thorax generates a flow of air.
In the iron lung by means of a pump, the air is withdrawn mechanically to produce a vacuum inside the tank, thus creating negative pressure. This negative pressure leads to expansion of the chest, which causes a decrease in intrapulmonary pressure, and increases flow of ambient air into the lungs. As the vacuum is released, the pressure inside the tank equalizes to that of the ambient pressure, and the elastic coil of the chest and lungs leads to passive exhalation. However, when the vacuum is created, the abdomen also expands along with the lung, cutting off venous flow back to the heart, leading to pooling of venous blood in the lower extremities. There are large portholes for nurse or home assistant access. The patients can talk and eat normally, and can see the world through a well-placed series of mirrors. Some could remain in these iron lungs for years at a time quite successfully.
Today, negative pressure mechanical ventilators are still in use, notably with the polio wing hospitals in England such as St Thomas' Hospital in London and the John Radcliffe in Oxford. The prominent device used is a smaller device known as the cuirass. The cuirass is a shell-like unit, creating negative pressure only to the chest using a combination of a fitting shell and a soft bladder. Its main use is in patients with neuromuscular disorders who have some residual muscular function. However, it was prone to falling off and caused severe chafing and skin damage and was not used as a long term device. In recent years this device has re-surfaced as a modern polycarbonate shell with multiple seals and a high pressure oscillation pump in order to carry out biphasic cuirass ventilation.
The design of the modern positive-pressure ventilators were mainly based on technical developments by the military during World War II to supply oxygen to fighter pilots in high altitude. Such ventilators replaced the iron lungs as safe endotracheal tubes with high volume/low pressure cuffs were developed. The popularity of positive-pressure ventilators rose during the polio epidemic in the 1950s in Scandinavia and the United States and was the beginning of modern ventilation therapy. Positive pressure through manual supply of 50% oxygen through a tracheostomy tube led to a reduced mortality rate among patients with polio and respiratory paralysis. However, because of the sheer amount of man-power required for such manual intervention, mechanical positive-pressure ventilators became increasingly popular.
Positive-pressure ventilators work by increasing the patient's airway pressure through an endotracheal or tracheostomy tube. The positive pressure allows air to flow into the airway until the ventilator breath is terminated. Subsequently, the airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume — the breath—out through passive exhalation.
Mechanical ventilation is indicated when the patient's spontaneous ventilation is inadequate to maintain life. It is also indicated as prophylaxis for imminent collapse of other physiologic functions, or ineffective gas exchange in the lungs. Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease, the patient's underlying condition should be correctable and should resolve over time. In addition, other factors must be taken into consideration because mechanical ventilation is not without its complications (see below)
Common medical indications for use include:
Barotrauma — Pulmonary barotrauma is a well-known complication of positive pressure mechanical ventilation.[7] This includes pneumothorax, subcutaneous emphysema, pneumomediastinum, and pneumoperitoneum.[7]
Ventilator-associated lung injury — Ventilator-associated lung injury (VALI) refers to acute lung injury that occurs during mechanical ventilation. It is clinically indistinguishable from acute lung injury or acute respiratory distress syndrome (ALI/ARDS).[8]
Diaphragm — Controlled mechanical ventilation may lead to a rapid type of disuse atrophy involving the diaphragmatic muscle fibers, which can develop within the first day of mechanical ventilation.[9] This cause of atrophy in the diaphragm is also a cause of atrophy in all respiratory related muscles during controlled mechanical ventilation.[10]
Motility of mucocilia in the airways — Positive pressure ventilation appears to impair mucociliary motility in the airways. Bronchial mucus transport was frequently impaired and associated with retention of secretions and pneumonia.[11]
Ventilators come in many different styles and method of giving a breath to sustain life. Hand controlled — Manual ventilators such as Bag valve masks and anesthesia bags require the user to hold the ventilator to the face or to an artificial airway and maintain breaths with their hands.
Mechanical ventilators typically require power by a battery or a wall outlet (DC or AC) though some ventilators work on a pneumatic system not requiring power.
Transport ventilators — These ventilators are small, more rugged, and can be powered pneumatically or via AC or DC power sources.
Intensive-care ventilators — These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure). This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time). Many ICU ventilators also incorporate graphics to provide visual feedback of each breath.
Neonatal ventilators — Designed with the preterm neonate in mind, these are a specialized subset of ICU ventilators which are designed to deliver the smaller, more precise volumes and pressures required to ventilate these patients.
Positive airway pressure ventilators (PAP) — These ventilators are specifically designed for non-invasive ventilation. this includes ventilators for use at home for treatment of chronic conditions such as sleep apnea or COPD.
Initiation of a mechanical ventilation breath can be triggered by number of variables including flow, volume, pressure and time.
The process of gas delivery to the patient depends on the mode of ventilation (see below). Typically a breath can be classified as either volume controlled or pressure controlled depending on which variable the ventilator actively controls during ventilation.
Termination of a mechanical ventilation breath can occur either at the completion of the inspiratory phase (see above), or as an alarm condition. Alarm conditions typically cause the ventilator to immediately cycle from inspiration to expiration. An example of this is when the measured pressure exceeds the high pressure alarm value.
Exhalation in mechanical ventilation is almost always completely passive. The ventilator's expiratory valve is opened, and expiratory flow is allowed until the baseline pressure (PEEP) is reached. Expiratory flow is determined by patient factors such as compliance and resistance.
Mechanical ventilation utilizes several separate systems for ventilation referred to as the "mode". Modes come in many different delivery concepts but all modes generally fall into one of the few main flagship categories such as volume controlled continuous mandatory ventilation, volume controlled intermittent mandatory ventilation, pressure controlled continuous mandatory ventilation, pressure controlled intermittent mandatory ventilation, continuous spontaneous ventilation and the high frequency ventilation systems.
Controlled mechanical ventilation (CMV) — In this mode the ventilator provides a mechanical breath on a preset timing. Patient respiratory efforts are ignored. This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient. It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing. Since CMV is no longer contained in its original form the term volume controlled continuous mandatory ventilation has consumed it into its definition and overall has combined any CMV mode for mechanical ventilation into the more accepted term in nomenclature for mechanical ventilation.
Volume controlled continuous mandatory ventilation — In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath. Traditional assist-control used only a pre-set tidal volume—when a preset peak pressure is used this is also sometimes termed intermittent positive pressure ventilation (IPPV). However, the initiation timing is the same—both provide a ventilator breath with every patient effort. In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic. Although a maximum rate is not usually set, an alarm can be set if the ventilator cycles too frequently. This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt).
Volume controlled intermittent mandatory ventilation (VC-IMV). Formerly known as synchronized intermittent mandatory ventilation (SIMV). In this mode the ventilator provides a pre-set mechanical breath (volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 seconds — thus a respiratory rate of 12 results in a 5 second cycle time). Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor. When the ventilator senses the first patient breathing attempt within the cycle, it delivers the preset ventilator breath. If the patient fails to initiate a breath, the ventilator delivers a mechanical breath at the end of the breath cycle. Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath. However, SIMV may be combined with pressure support (see below). SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate, which requires the patient to take additional breaths beyond the SIMV triggered breath.
Pressure controlled continuous mandatory ventilation (PC-CMV) — mechanical ventilation with preset inspiratory pressure (PIP) and inspiratory time (Ti). Every breath is machine initiated and mandatory.
Pressure controlled intermittent mandatory ventilation (formerly known as SIMV) — In this mode the ventilator provides a pre-set pressure limited mechanical breath every specified number of seconds SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate, which requires the patient to take additional breaths beyond the SIMV triggered breath. PC-IMV is fundamentally the same as VC-IMV with an emphasis on pressure support and control instead of volume. An example of PC-IMV is in the mode pressure regulated volume control.
High frequency ventilation refers to ventilation that occurs at rates significantly above that found in natural breathing. High frequency ventilation is further defined as any ventilation with a respiratory rate (Vf) greater than 150 respirations per minute. Within the category of high-frequency ventilation, the two principal types are high-frequency ventilation (passive) (i.e. high-frequency jet ventilation) and high-frequency ventilation (active) (i.e. high-frequency oscillatory ventilation).
Pressure support ventilation (PSV). When a patient attempts to breathe spontaneously through an endotracheal tube, the narrowed diameter of the airway results in higher resistance to airflow, and thus a higher work of breathing. PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that "supports" ventilation during inspiration. Thus, for example, SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported. However, while the SIMV mandated breaths have a preset volume or peak pressure, the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (e.g. 10–25%). New generation of ventilators provides user-adjustable inspiration cycling off threshold, and some even are equipped with automatic inspiration cycling off threshold function. This helps the patient ventilator synchrony.[12] The peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath. PSV can be also be used as an independent mode.
Continuous positive airway pressure (CPAP). A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation, decrease the work of breathing, and decrease the work of the heart (such as in left-sided heart failure CHF). Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths. In addition, no additional pressure above the CPAP pressure is provided during those breaths. CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs. Non-invasive ventilation has become more common for treatment of acute respiratory failure.[1]
Assist-control mode minimizes patient effort by providing full mechanical support with every breath. This is often the initial mode chosen for adults because it provides the greatest degree of support. In patients with less severe respiratory failure, other modes such as SIMV may be appropriate. Assist-control mode should not be used in those patients with a potential for respiratory alkalosis, in which the patient has an increased respiratory drive. Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease, hyperventilatory sepsis, and head trauma. Respiratory alkalosis will be evident from the initial arterial blood gas obtained, and the mode of ventilation can then be changed if so desired.
Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients. It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis.
High frequency oscillation is used most frequently in neonates, but is also used as an always alternative mode in adults with severe ARDS.
Pressure regulated volume control is another option.
The following are general guidelines that may need to be modified for the individual patient.
As a general rule, whenever possible, spontaneous breathing must be maintained or supported, to avoid muscular atrophy of the diaphragm (Ventilator Induced Dysfunction of Diaphragm, VIDD) . To limit VALI and VILI, protective ventilation pattern should be applied to the patient. If this results in severe hypercapnia, exceeding accepted levels for permissive hypercapnia (pH below 7.2), measures for extracorporeal CO2 removal (iLA Membranventilator, Novalung) should be installed at an early stage of mechanical ventilation, to terminate cascades of inflammatory response from the lung tissue, resulting in multiorgan failure respective.
As the tidal volume increases, the pressure required to administer that volume also increases. This pressure is known as the peak airway pressure. If the peak airway pressure is persistently above 45 cmH2O (4.4 kPa) for adults, the risk of barotrauma is increased (see below), and efforts should be made to try to reduce the peak airway pressure (such as acceptable alarm limits). In infants and children it is unclear what level of peak pressure may cause damage. In general, keeping peak pressures below 30 cmH2O (2.9 kPa) is desirable.
Monitoring for barotrauma can also involve measuring the plateau pressure, which is the pressure after the delivery of the tidal volume but before the patient is allowed to exhale. Normal breathing pattern involves inspiration, then expiration. To determine the plateau pressure, the ventilator can be programmed to perform a breath hold maneuver. In this maneuver, the patient is not allowed to exhale for a set period after inspiration occurs. The static lung pressure measured during the set breath hold period is the plateau pressure. Barotrauma is minimized when the plateau pressure is maintained < 30–35 cmH2O.
An adult patient breathing spontaneously will usually sigh about 6–8 times per hour to prevent microatelectasis, and this has led some to propose that ventilators should deliver 1½–2 times the amount of the preset tidal volume 6–8 times per hour to account for the sighs. However, such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma. Currently, accounting for sighs is not recommended if the patient is receiving 10–12 mL/kg or is on PEEP. If the tidal volume used is lower, the sigh adjustment can be used, as long as the peak and plateau pressures are acceptable.
Sighs are not generally used with ventilation of infants and young children.
Because the mechanical ventilator is responsible for assisting in a patient's breathing, it must then also be able to deliver an adequate amount of oxygen in each breath. The FiO2 stands for fraction of inspired oxygen, which means the percent of oxygen in each breath that is inspired. (Note that normal room air has ~21% oxygen content or an FiO2 of 0.21). In adult patients who can tolerate higher levels of oxygen for a period of time, the initial FiO2 may be set at 100% until arterial blood gases can document adequate oxygenation. An FiO2 of 100% for an extended period of time can cause atelectasis, but it can protect against hypoxemia from unexpected intubation problems. For infants, and especially in premature infants, avoiding high levels of FiO2 (>60%) is very important to avoid lung injury and apnea of prematurity. Fraction of inspired oxygen should always be titrated to meet the patients needs and to avoid unnecessary lung injury.
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC). At the end of expiration, the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure. The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange, thus reducing the size of the shunt. For example, if a large shunt is found to exist based on the estimation from 100% FiO2 (see above), then PEEP can be considered and the FiO2 can be lowered (< 60%) in order to maintain an adequate PaO2, thus reducing the risk of oxygen toxicity.
In addition to treating a shunt, PEEP may also be useful to decrease the work of breathing. In pulmonary physiology, compliance is a measure of the "stiffness" of the lung and chest wall. The mathematical formula for compliance (C) equals change in volume divided by change in pressure. The higher the compliance, the more easily the lungs will inflate in response to positive pressure. An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC, since the partially inflated lung takes less energy to inflate further. Excessive PEEP can however produce overinflation, which will again decrease compliance. Therefore it is important to maintain an adequate, but not excessive FRC.
Indications. PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function. As such, it should be judiciously used and is indicated for adults in two circumstances.
If used, PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2. As PEEP increases intrathoracic pressure, there can be a resulting decrease in venous return and decrease in cardiac output. A PEEP of less than 10 cmH2O (1 kPa) is usually safe in adults if intravascular volume depletion is absent. Lower levels are used for pediatric patients. Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2O for hemodynamic monitoring. More recent literature has failed to find outcome benefits with routine PA catheterisation when compared to simple central venous pressure monitoring.[13] If cardiac output measurement is required, minimally invasive techniques, such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives.[14][15] When withdrawing, it is decreased through 1–2 cmH2O decrements while monitoring haemoglobin-oxygen saturations. Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation.
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia. It improves FRC, drainage of secretions, and ventilation-perfusion matching (efficiency of gas exchange). It may improve oxygenation in > 50% of patients, but no survival benefit has been documented.
Most intubated patients receive intravenous sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress. Sedation also helps the patient tolerate the constant irritation of the endotracheal tube in their mouth, pharynx and trachea. Without some form of sedation and analgesia, it is common for patients to "fight" the ventilator. This fighting increases work of breathing and may cause further lung injury. Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning. These interruptions are frequently described as "sedation vacations" and have been shown to reduce the time patients stay on mechanical ventilation.[16]
It is not uncommon for patients on a mechanical ventilator to be given a muscle relaxant or paralytic to aid in ventilation. These "neuromuscular blockades" prevent skeletal muscle from contracting and thereby stop all patient movement including respiratory efforts. These types of pharmaceutical agents must always be given in conjunction with sedation as the effects of the paralytics is not only uncomfortable but would cause significant psychological stress and anxiety.
In adults when 100% FiO2 is used initially, it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction. The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation. In normal physiology, gas exchange (oxygen/carbon dioxide) occurs at the level of the alveoli in the lungs. The existence of a shunt refers to any process that hinders this gas exchange, leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood).
When using 100% FiO2, the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg. For each difference of 100 mmHg, the shunt is 5%. A shunt of more than 25% should prompt a search for the cause of this hypoxemia, such as mainstem intubation or pneumothorax, and should be treated accordingly. If such complications are not present, other causes must be sought after, and PEEP should be used to treat this intrapulmonary shunt. Other such causes of a shunt include:
Withdrawal from mechanical ventilation—also known as weaning—should not be delayed unnecessarily, nor should it be done prematurely. Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation, and this should be assessed continuously. There are several objective parameters to look for when considering withdrawal, but there is no specific criteria that generalizes to all patients.
Trials of spontaneous breathing have been shown to accurately predict the success of spontaneous breathing.[17]
One of the main reasons why a patient is admitted to an ICU is for delivery of mechanical ventilation. Monitoring a patient in mechanical ventilation has many clinical applications: Enhance understanding of pathophysiology, aid with diagnosis, guide patient management, avoid complications and assessment of trends.[18]
Most of modern ventilators have basic monitoring tools. There are also monitors that work independently of the ventilator, which allow to measure patients after the ventilator has been removed, such as a T tube test.
There are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration:
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