science saving life intensivecare foundation. Handbook of Mechanical. Ventilation. A User's Guide. The Intensive Care Foundation. Mechanical ventilation is the most used short-term life support technique worldwide the future of mechanical ventilation, addressing avenues for improvement. How does a ventilator work? The ventilator is connected to the person through a tube. (endotracheal or ET tube) that is placed into the mouth or nose and down.
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2/ Initiating (and de-escalating) mechanical ventilation. text (pdf). THINK: Conventionally, we distinguish between lung damage due to high. An important concept to remember: normal breathing is a negative-pressure phenomenon; mechanical ventilation is. POSITIVE pressure ventilation. The end . Mechanical Ventilation. CMV modes include or Assist Control (AC) includes Pressure Controlled (PC), or. Volume Controlled (VC) ventilation. Pressure.
Assist Control Ventilation AC Assist control is basically when the ventilator will assist the patient by delivering support for every breath the patient takes that is the assist part , and the ventilator will have control over the respiratory rate if it goes below the set rate control part.
In assist control, if the rate is set at 12 and the patient breathes at 18, the ventilator will assist with the 18 breaths, but if the rate drops to 8, the ventilator will take over control of the respiratory rate and deliver 12 breaths in a minute.
Pressure modes of mechanical ventilation: the good, the bad, and the ugly.
In assist control ventilation, the breath can be delivered by either giving volume or giving pressure. This is termed volume-assist control or pressure-assist control ventilation. In order to maintain simplicity, and understanding that given that ventilation is commonly a major problem than pressure and that volume control is used overwhelmingly more commonly than pressure control, the focus for the remainder of this review will use the term "volume control" interchangeably when discussing assist control.
Assist control volume control is the mode of choice used in the majority of intensive care units through the United States because it is easy to use. The volume delivered by the ventilator in each breath in assist control will always be the same, regardless of the breath being initiated by the patient or the ventilator, and regardless of compliance, peak, or plateau pressures in the lungs.
Each breath can be time-triggered if the patient's respiratory rate is below the set ventilator rate, the machine will deliver breaths at a set interval of time or patient triggered if the patient initiates a breath on its own. This makes assist control a very comfortable mode for the patient as each of his or her efforts will be supplemented by the ventilator. After making changes on the vent or after starting a patient on mechanical ventilation, careful consideration on checking arterial blood gases should be made and the oxygen saturation on the monitor should be followed to determine if further changes should be made to the ventilator.
This means that for each breath the patient takes above the set RR, the tidal volume pulled by the patient will depend solely on lung compliance and patient effort. Nonetheless, multiple studies have failed to show any advantages to SIMV.
Furthermore, SIMV generates higher work of breathing than AC, which negatively impacts outcomes as well as generates respiratory fatigue.
A general rule to go by is that the patient will be liberated from the ventilator when he or she is ready, and no specific mode of ventilation will make this faster.
In the meantime, it is better to keep the patient as comfortable as possible and SIMV may not be the best mode to achieve this. As the name implies it is a pressure-driven mode of ventilation.
In this setting all breaths are patient-triggered as the ventilator has no backup rate, so each breath has to be started by the patient. In this mode, the ventilator will cycle between two different pressures PEEP and pressure support. PEEP will be the remaining pressure at the end of exhalation, and pressure support is the pressure above the PEEP that the ventilator will administer during each breath for support of ventilation.
The biggest drawback of PSV is its unreliable tidal volumes that may generate CO2 retention and acidosis as well as the higher work of breathing which can lead to respiratory fatigue. Airway pressure release ventilation APRV As the name suggests, in APRV mode the ventilator will deliver a constant high airway pressure that will deliver oxygenation, and ventilation will be served by releasing that pressure.
This mode has recently gained popularity as an alternative for difficult-to-oxygenate patients with ARDS in whom other modes of ventilation fail to reach the set targets. This reduces the repetitive inflation and deflation of the lungs that happens with other ventilator modes, preventing ventilator-induced lung injury.
During this time T high the patient is free to breathe spontaneously which makes it comfortable but he will be pulling low tidal volumes as exhaling against such pressure is harder.
Then, when T high is reached, the pressure in the ventilator will go down to P low usually zero. This allows for air to be rushed out of the airways allowing for passive exhalation until T low is reached and the vent delivers another breath.
To prevent airway collapse during this time the T low is set short, usually around 0. What happens here is that when the ventilator pressure goes to zero, the elastic recoil of the lungs pushes air out, but the time is not enough for all the air to leave the lungs, so the alveolar and airway pressure does not reach zero and there is no airway collapse.
Disadvantages and contraindications: Given that spontaneous breathing is an important aspect of APRV, it is not ideal for heavily sedated patients No data for APRV use in neuromuscular disorders or obstructive lung disease and its use should be avoided in this patient populations Theoretically, constant high intrathoracic pressure could generate high pulmonary artery pressure and worsen intracardiac shunts in patients with Eisenmenger physiology Strong clinical reasoning should be employed when selecting APRV as the mode of ventilation over more conventional modes such as AC.
Further information on the details of the different modes of ventilation and their set up can be found in the articles related to each specific mode of ventilation. Nonetheless, there are some basic settings for the majority of cases. The most common ventilator mode to use in a newly intubated patient is AC. This mode provides good comfort and easy control of some of the most important physiologic parameters.
Low tidal volume ventilation has been shown to be lung protective not only in ARDS but in other types of diseases. Always use a lung protective strategy as there are not many advantages for higher tidal volumes and they will increase shear stress in the alveoli and may induce lung injury.
Initial RR should be comfortable for the patient bpm should suffice. A very important caveat on this is for patients with severe metabolic acidosis. For these patients, the minute ventilation should at least be matched to their pre-intubation ventilation as failure to do so will worsen acidosis and can precipitate complications such as cardiac arrest.
Pay close attention to blood pressure and patient comfort while doing this. An ABG should be obtained 30 minutes after intubation and changes to the ventilator settings should be made in accordance with ABG findings. Attention should be given to the volume curves in the ventilator display as a reading showing that the curve is not coming back to zero at the time of exhalation is indicative of incomplete exhalation and development of auto-PEEP and corrections to the vent should be made immediately.
To correct for hypoxia increasing any of these parameters should raise the oxygenation. Special attention should be paid to the possible adverse effects of raising PEEP which can cause barotrauma and hypotension. Raising FiO2 does not come without its concerns as high FiO2 can cause oxidative damage in the alveoli. Another important aspect of managing oxygen content is to define a goal for oxygenation.
A sudden drop in oxygen saturation should raise suspicion for tube misplacement, pulmonary embolism, pneumothorax, pulmonary edema, atelectasis, or development of mucus plugs. Hypercapnia: To modify CO2 content in blood one needs to modify alveolar ventilation. Raising the rate or the tidal volume, as well as increasing T low, will increase ventilation and decrease CO2. Consideration has to be made while increasing the rate, as this will also increase the amount of dead space and might not be as effective as tidal volume.
Other important circumstances are those of elevated pressures. As discussed, two pressures are important in the system: peak and plateau. The peak pressure is a measure of airway resistance as well as compliance and includes the tubing and bronchial tree.
Plateau pressures are a reflection of alveolar pressure and thus of lung compliance. If there is an increase in peak pressure, the first step to take is to do an inspiratory hold and check the plateau.
Elevated peak pressure and normal plateau pressure: high airway resistance and normal compliance. PSV, by contrast, has a flow cycle. Note also that the lines between pressure and volume controlled methods are being continually blurred by increasingly complex modes.
Each breath is either an assist or control breath, but they are all of the same volume. The larger the volume, the more expiratory time required.
If the I:E ratio is less than , progressive hyperinflation may result. ACV is particularly undesirable for patients who breathe rapidly — they may induce both hyperinflation and respiratory alkalosis.
Note that mechanical ventilation does not eliminate the work of breathing, because the diaphragm may still be very active. Mandatory breaths are synchronized to coincide with spontaneous respirations.
Disadvantages of SIMV are increased work of breathing and a tendency to reduce cardiac output, which may prolong ventilator dependency.
The addition of pressure support on top of spontaneous breaths can reduce some of the work of breathing. SIMV Personal preference prevails, except in the following scenarios: 1. The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation.
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. If such complications are not present, other causes must be sought after, and positive end-expiratory pressure 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 haemorrhage[ citation needed ] Weaning from mechanical ventilation[ edit ] Timing of withdrawal from mechanical ventilation—also known as weaning—should be carefully considered.
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 are no specific criteria that generalizes to all patients.
Martin Tobin of Loyola University Medical Center is one of the best studied and most commonly used weaning predictors, with no other predictor having been shown to be superior. 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.
In patients who have diffused loss of aeration PEEP can be used provided it does not cause the plateau pressure to rise above the upper inflection point.
Most modern ventilators have basic monitoring tools. There are also monitors that work independently of the ventilator which allow for measuring patients after the ventilator has been removed, such as a T tube test. Artificial airways as a connection to the ventilator[ edit ] Main article: Artificial airway 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 anaesthesia, 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.This is usually protocolized in all intensive care units ICU and should be performed in every patient who is stable and in whom the indication for mechanical ventilation has resolved.
Supraglottic airways differ primarily from tracheal intubation in that they do not prevent aspiration. Timer ventilator-initiated breaths — occur at the set respiratory rate or frequency f.
To prevent and resolve auto-PEEP, enough time should be given for the air to leave the lungs during exhalation. Provides full support to the preset V t volume-controlled or pressure limit pressure-controlled for each ventilator-generated breath.
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