Mechanical ventilation

In architecture and climate control, mechanical or forced ventilation is the use of powered equipment, e.g. fans and blowers, to move air - see ventilation (architecture).

Bag valve mask

In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing when patients cannot do so on their own, and must be done so after invasive intubation with an endotracheal or tracheostomy tube through which air is directly delivered (in contrast to noninvasive ventilation). In many cases, mechanical ventilation is used in acute settings such as in the ICU for a short period of time during a serious illness. For some patients who have certain chronic illnesses that require long-term ventilation assistance, they are also able to do so at home or other nursing/rehabilitation institution with the help of respiratory therapists and physicians. The main form of mechanical ventilation currently is the positive pressure ventilators, which serves to apply pressure to the patient's airway and thus force a specified amount of air into their lungs. This is in contrast to the previously used negative pressure ventilators that serve to creating a negative pressure environment around the patient's chest and thus sucking air into the lungs. However, mechanical ventilation should be weaned off as soon as possible, since it is very possible to develop complications such as volutrauma, barotrauma, ventilator-associated pneumonia, among others.

History

Vesalius was the first person to describe mechanical ventilation by inserting a reed or cane into the trachea of animals and then blowing into this tube.

Negative pressure machines

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 1950s. 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 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' (by Westminster 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. It has mostly been effective with children and is still in use in domiciliary ventilation in West England and Wales.

Positive pressure machines

The design of the modern positive-pressure ventilators were mainly based on the military practice 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. 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 intervention, 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 pushes the tidal volume -- the breath -- out through passive exhalation.

Types of Ventilators

For hand assisted ventilation, see

Main article: bag valve mask

Modern mechanical ventilators are positive pressure ventilators, and are classified by their method of cycling between inspiration and expiration. Every time the machine delivers a breath, it must know when to stop delivering that breath. There are several ways of terminating the breath and the ventilators are named after the parameter that signals the termination of that positive-pressure inspiration cycle.

• In a volume-cycled ventilator -- the most common type used -- the ventilator has a preset amount of volume to be delivered each breath. Once the specified volume of breath is delivered, the positive pressure is terminated.
• In a pressure-cycled ventilator, once a preset pressure is reached within the ventilator, the breath is terminated.
• In a time-cycled ventilator, the termination of the breath occurs after a certain specified time period.

Indications for use

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 resolved 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:

• acute lung injury (including ARDS, trauma)
• apnea with respiratory arrest, including cases from intoxication
• chronic obstructive pulmonary disease (COPD)
• acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) > 50 mmHg and pH < 7.25, which may be due to paralysis of the diaphragm due to Guillain-Barré syndrome, Myasthenia Gravis, spinal cord injury, or the effect of anaesthetic and muscle relaxant drugs
• tachypnea with respiratory rate > 30
• hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FIO2) < 55 mm Hg
• hypotension including sepsis, shock, congestive heart failure

Initial Ventilator Settings

Nasotracheal intubation

Modes of ventilation

Assist-control mode should be the initial mode used in most circumstances, where it uses a preset tidal volume and rate. (Tidal volume is the amount of air a person breathes in/out normally, as opposed to a full breath). This tidal volume is the preset volume used in the volume-cycled ventilator. Patients on assist-control mode can either initiate inspiration or let the ventilator dictate when to administer the preset volume. If the patient attempts to initiate inspiration, the ventilator will sense a decrease in the circuit pressure and administer the preset volume. Thus, the patient only needs to exert minimal effort in his respiratory muscles in order to deliver a breath. If the patient does not make any inspiratory effort at all, the ventilator itself will administer the preset volume at the preset rate to preserve minimum ventilation. Because the rate set in assist-control mode may not be the optimal rate the patient would like to breath, there is still work required for breathing. The amount of work required is inversely proportional to the sensitivity of the pressure circuit that detects the patient's inspiratory effort, which is modifiable. Therefore, this system is very advantageous because it allows the patients to dictate their rate of breathing if they so require it, but it also allows the ventilator to dictate their breathing if the patient cannot do so on his own.

Assist-control mode should not be used in those patients with a potential for respiratory alkalosis, whereby 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.

Tidal Volume, Rate, and Pressures

In the assist-control mode, the preset tidal volume and rate are different for different patient status:

• For patients without existing lung disease -- a tidal volume of 12 mL per kg body weight is set to be delivered at a rate of 12 times a minute (12-12 rule).
• For patients with COPD -- a reduced tidal volume of 10 mL/kg is to be delivered 10 times a minute to prevent overinflation and hyperventilation (10-10 rule).
• For patients with acute respiratory distress syndrome (ARDS) -- an even more reduced tidal volume of 6-8 mL/kg is used with a rate of 10-12/minute. This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) but this elevation does not need to be corrected (termed permissive hypercapnia)

As the amount of tidal volume increases, the pressure required to administer that volume is increased. This pressure is known as the peak airway pressure. If the peak airway pressure is persistently above 45 cmH₂O, the risk of barotrauma is increased (see below). Therefore, the tidal volume should be modified in order to keep the peak airway pressure below 45 cmH₂O.

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. The ventilator is programmed so that after delivery of the tidal volume (inspiration), the patient is not allowed to exhale for a half a second. Therefore, pressure must be maintained in order to prevent exhalation, and this pressure is the plateau pressure. Barotrauma is minimized when the plateau pressure is maintained < 30-35 cmH₂O.

Sighs

A patient breathing spontaneously will usually sigh about 6-8 times/hr to prevent microatelectasis, and this has led some to propose that ventilators should deliver 1.5-2 times the amount of the preset tidal volume 6-8 times/hr 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.

Initial FIO₂

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. For patient safety, the initial FIO₂ should always be 100% until arterial blood gases can document adequate oxygenation. (Note that normal atmosphere has only ~21% oxygen content). An FIO₂ of 100% for an extended period of time can be dangerous, but it offers a couple of advantages if used for only a short period. First, it can protect against hypoxemia from unexpected intubation problems. Second, it is easy to calculate the next FIO₂ to be used and easy to estimate the shunt fraction. The shunt estimated 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 unoxygenated 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:

• Alveolar collapse from major atelectasis
• Alveolar collection of material other than gas, such as pus from pneumonia, water and protein from acute respiratory distress syndrome, water from congestive heart failure, or blood from hemorrhage

Alternative Modes of Ventilation

• Intermittently (IPPV) to simulate the normal tidal breathing pattern.
  • Greatest pressure is provided during the inspiratory phase of ventilation, determined by the Peak Inspiratory Pressure (PIP).
  • Usually a smaller positive pressure is deliverd during the expiration phase (Positive End Expiratory Pressure or PEEP) to prevent atelectasis.

• Continuously, so that inspiratory and expiratory phases of respiration are met with the same pressure resistance - see CPAP.
• High frequency oscillation. Used exclusively in neonates. This specialised form of ventilation does not require specific inspiratory or expiratory phases to ventilate the alveoli.

Within the subset of IPPV ventilation, parameters can be set by varying any 2 of time, pressure and volume. Thus there exists:

• Pressure Controlled Ventilation (PCV, or pressure ventilation). A constant inspiratory pressure is delivered for a set time.
• Volume Controlled Ventilation (VCV, or volume preset ventilation). A set tidal volume is given with each breath.

Note that any IPPV that provides PEEP will provide positive pressure throughout the respiratory cycle. Because of this PEEP, all patient initiated breaths will receive some pressure support. However, in the descriptions that follow, a supported breath refers to an inspiration supported by the higher PIP.

Patient breathing effort can either be supported (by initiating an inspiratory cycle), or ignored. Thus ventilation is further divided into:

• Controlled Mechanical Ventilation (CMV).
  • Inspiratory and expiratory cycles are determined on a preset cycling pattern. Patient respiratory efforts are ignored.
• Pressure Support Ventilation (PSV);
  • All breaths are spontaneous breath types, meaning that the ventilator delivers breaths only in response to patient effort. Thus appropriate apnea alarms must be set on the ventilator.

By utilising the better features of the above 2 ventilation modes, some ventilators can deliver:

• Synchronized Intermittent Mandatory Ventilation (SIMV);
  • As with PSV, only the set number of breaths per minute are supported. Unlike PSV, the exact time between ventilation cycles is able to vary (within limits), in an attempt to co-ordinate with patient inspiratory effort.

Non-invasive methods of ventilation are discussed below.

Types of Ventilator

Intermittent Positive pressure ventilation can be delivered via:

• Hand controlled ventilation. This is always a short term measure. It can be delivered via the following devices.
  • Bag valve mask
  • Anaesthetic (or T-piece) bag
  • Neopuff in neonates
• A mechanical ventilator. Mechanical ventilators can be divided in to:
  • Transport ventilators. These ventilators are smaller, more rugged, and can be powered via:
    • The energy contained within the pressurised gas (oxygen and/or air) being delivered to the patient.
    • a portable battery.
  • ICU ventilators. These ventilators are larger and usually require mains power. This style of ventilator provides much more control of ventilation parameters. Often these can easily switch between different modes of ventilation. Usually these ventilators give more accurate feedback on various patient ventilation parameters.
    • NICU ventilators. designed with the preterm neonate in mind, these are a specialised subset of ICU ventilators which are even more accurate at delivering the smaller volumes, pressures and oxygen concentrations required to ventilate this patient subset.
  • PAP ventilators. these ventilators are specifically designed for non-invasive ventilation. this includes ventilators for use at home, in order to treat sleep apnoea.

Non-invasive ventilation

This refers to all modalities that assist ventilation without the use of an endotracheal tube. Non-invasive ventilation is aimed at minimizing patient discomfort and ventilation-related disease. It is often used in cardiac or pulmonary disesase, sleep apnea, and neuromuscular diseases.

Forms of non-invasive ventialtion include:

• Continuous positive airway pressure (CPAP).
• Bi-level Positive Airway Pressure (BIPAP). Pressures alternate between Inspiratory Positive Airway Pressure (IPAP) and a lower Expiratory Positive Airway Pressure (EPAP), triggered by patient effort. On many such devices, backup rates may be set, which deliver IPAP pressures even if patients fail to initiate a breath.
• Intermittent positive pressure ventilation (IPPV) via mouthpiece or mask

Indications

Initially this technique was used for patients with COPD to avoid intubation, but recent studies have suggested its merit in facilitating weaning, early extubating and cardiogenic pulmonary edema, and in some cases as an alternative to invasive ventilation.

Generally speaking patients eligible for this treatment should be:

• dyspneic due to hypoxic, hypercapnic or mixed respiratory failure
• dysplaying physical signs of respiratory muscle weakness (exhaustion)
• tachypneic, respiratory rate greater than 25
• hemodynamically stable
• able to tolerate or submit to the particular mode of ventilation

Modalities

Several types of interfaces are possible. A nasal, oronasal (i.e. venturi mask), or full-face mask.

Connection to ventilators

There are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration:

• Face mask - In resuscitation and for minor procedures under anesthesia, a face mask is often sufficient to achieve a seal against air leakage. Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway. These are designed to provide a passage of air to the pharynx through the nose or mouth, respectively. Poorly fitted masks often cause nasal bridge ulcers which is a problem for some patients. Face masks are also used for non-invasive ventilation in conscious patients. A face mask does not, however, provide protection against aspiration.
• Laryngeal mask airway - The laryngeal mask airway (LMA), causes less pain and coughing than a tracheal tube. However, unlike tracheal tubes it does not seal against aspiration, making careful individualised evaluation and patient selection mandatory.
• Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration. A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea. In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration. Intubation with a cuffed tube is thought to provide the best protection against aspiration. Tracheal tubes inevitably cause pain and coughing. Therefore, unless a patient is unconscious or anesthetized for other reasons, sedative drugs are usually given to provide tolerance of the tube. Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis.
• Cricothyrotomy - Patients who require emergency airway management, in whom tracheal intubation has been unsuccessful, may require an airway inserted through a surgical opening in the cricothyroid membrane. This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access. [1]
• Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the patient's trachea. A tracheostomy is a surgically created passage into the trachea. Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs. Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease, or in any patient who are expected to be difficult to wean from mechanical ventilation, i.e., patients who have little muscular reserve.

Sources and External Links

• Irwin R, Rippe J, "Intensive care medicine", 5th Edition, 2003 Lippincott Williams & Wilkins
• Marino P, "The ICU Book", 3rd Edition, 2006 Lippincott Williams & Wilkins
• Irwin R, Rippe J, "Procedures and Techniques in Intensive care medicine", 3rd Edition, 2003 Lippincott Williams & Wilkins
• International Ventilator Users Network (IVUN), Resource of information for users of home mechanical ventilation
• NIV Users Group, Group for users of noninvasive ventilation and interested parties (i.e. respiratory therapists, doctors, parents)
• Dr. Bach, a doctor experienced in use of noninvasive ventilation for patients with neuromuscular diseases (note: site is written by a third-party)
• e-Medicine, article on mechanical ventilation along with technical information