Mechanical Ventilation – Indications, Contraindications

Mechanical ventilation is a lifesaving procedure that is often performed when patients require respiratory support. Assist-Control (AC) mode is one of the most common methods of mechanical ventilation in the intensive care unit[rx].

AC ventilation is a volume-cycled mode of ventilation. It works by setting a fixed tidal Volume (VT) that the ventilator will deliver at set intervals of time or when the patient initiates a breath. The VT delivered by the ventilator in AC always will be the same regardless of compliance, peak, or plateau pressures in the lungs.[rx]

Mechanical ventilation is an established supportive treatment for patients with various forms of respiratory failure. Despite the widespread use and clear benefits of mechanical ventilation, it is not a risk-free intervention. Prolonged mechanical ventilation increases the risk of pneumonia, barotrauma, tracheal injuries and musculoskeletal deconditioning. At the same time, delayed weaning is associated with increased morbidity, mortality, hospital stay and risk of long-term care facility discharge. [rx][rx][rx]

Importance of Mechanical Ventilation

The need for mechanical ventilation is one of the most common causes of admission to the intensive care unit.[rx][rx][rx]

It is first imperative to understand some basic terms to understand mechanical ventilation.

• Ventilation – Exchange of air between the lungs and the air (ambient or delivered by a ventilator), in other words, it is the process of moving air in and out of the lungs. Its most important effect is the removal of carbon dioxide (CO2) from the body, not on increasing blood oxygen content. Ventilation is measured as minute ventilation in the clinical setting, and it is calculated as respiratory rate (RR) times tidal volume (Vt). In a mechanically ventilated patient, the CO2 content of the blood can be modified by changing the tidal volume or the respiratory rate.
• Oxygenation – Interventions that provide greater oxygen supply to the lungs, thus the circulation. In a mechanically ventilated patient, this can be achieved by increasing the fraction of inspired oxygen (FiO 2%) or the positive end-expiratory pressure (PEEP).
• PEEP – The positive pressure that will remain in the airways at the end of the respiratory cycle (end of exhalation) is greater than the atmospheric pressure in mechanically ventilated patients. For a full description of the use of PEEP, please review the article titled “Positive End-Expiratory Pressure (PEEP).”
• Tidal volume -Volume of air moved in and outside the lungs in each respiratory cycle.
• FiO2 – Percentage of oxygen in the air mixture that is delivered to the patient.
• Flow – Speed in liters per minute at which the ventilator delivers breaths.
• Compliance – Change in volume divided by change in pressure. In respiratory physiology, total compliance is a mix of lung and chest wall compliance as these two factors cannot be separated in a live patient.

Since having a patient on mechanical ventilation allows a practitioner to modify the patient’s ventilation and oxygenation, it has an important role in acute hypoxic and hypercapnic respiratory failure as well as in severe metabolic acidosis or alkalosis.[rx][rx]

Physiology of Mechanical Ventilation

Mechanical ventilation has several effects on lung mechanics. Normal respiratory physiology works as a negative pressure system. When the diaphragm pushes down during inspiration, negative pressure in the pleural cavity is generated, this, in turn, creates negative pressure in the airways that suck air into the lungs. This same negative intrathoracic pressure decreases the right atrial (RA) pressure and generates a sucking effect on the inferior vena cava (IVC), increasing venous return. The application of positive pressure ventilation changes this physiology. The positive pressure generated by the ventilator transmits to the upper airways and finally to the alveoli, this, in turn, is transmitted to the alveolar space and thoracic cavity, creating positive pressure (or at least less negative pressure) in the pleural space. Less blood in the right ventricle means less blood reaching the left ventricle and less blood that can be pumped out, decreasing cardiac output. Less preload means that the heart works at a less efficient point in the frank-startling curve, generating less effective work and further decreasing cardiac output, which will result in a drop in mean arterial pressure (MAP) if there is not a compensatory response by increasing systemic vascular resistance (SVR). This is a very important point to have in mind especially in patients who may not be able to increase their SVR, like in patients with distributive shock (septic, neurogenic, or anaphylactic shock).

On the other hand, mechanical ventilation with positive pressure can significantly decrease the work of breathing. This, in turn, decreases blood flow to respiratory muscles and redistributes it to more critical organs. Reducing the work from respiratory muscles also reduces the generation of CO2 and lactate from these muscles, helping improve acidosis.

Proper management of mechanical ventilation also requires an understanding of lung pressures and lung compliance. Normal lung compliance is around 100 ml/cmH20. This means that in a normal lung the administration of 500 ml of air via positive pressure ventilation will increase the alveolar pressure by 5 cm H2O. Conversely, the administration of a positive pressure of 5 cm H2O will generate an increase in lung volume of 500 mL. In real life, we rarely work with normal lungs, and compliance may be much higher or much lower. Any disease that destroys lung parenchyma like emphysema will increase compliance, any disease that generates stiffer lungs (ARDS, pneumonia, pulmonary edema, pulmonary fibrosis) will decrease lung compliance.

There are two important pressures in the system to be aware of when mechanically ventilating a patient:

• Peak pressure is the pressure achieved during inspiration when the air is being pushed into the lungs and is a measure of airway resistance.
• Plateau pressure is the static pressure achieved at the end of a full inspiration. To measure plateau pressure, we need to perform an inspiratory hold on the ventilator to permit for the pressure to equalize through the system. Plateau pressure is a measure of alveolar pressure and lung compliance. Normal plateau pressure is below 30 cm H20, and higher pressure can generate barotrauma.

Indications for Mechanical Ventilation

• The most common indication for intubation and mechanical ventilation is in cases of acute respiratory failure, be it a hypoxic or hypercapnic failure.
• Other important indications include a decreased level of consciousness with an inability to protect the airway, respiratory distress that failed non-invasive positive pressure ventilation, cases of massive hemoptysis, severe angioedema, or any case of airway compromises like airway burns, cardiac arrest, and shock.
• Common elective indications for mechanical ventilation are surgical procedures and neuromuscular disorders.

Contraindications

• There are no direct contraindications for mechanical ventilation as it is a life-saving measure in a critically ill patient, and all patients should be offered the opportunity to benefit from this if needed.
• The only absolute contraindication for mechanical ventilation is if this goes against the patient’s stated wishes for artificial life-sustaining measures.
• The only relative contraindication is if non-invasive ventilation is available and its use is expected to resolve the need for mechanical ventilation. This should be started first as it has fewer complications than mechanical ventilation.

Preparation

In order to initiate mechanical ventilation, certain measures should be taken. The proper placement of the endotracheal tube must be verified. This may be done by end-tidal capnography or a combination of clinical and radiological findings.

Proper cardiovascular support should be ensured with fluids or vasopressors as indicated in a case by case basis.

Ensure that proper sedation and analgesia are available. The plastic tube in the patient’s throat is painful and uncomfortable, and if the patient is restless or fighting the tube or the vent, it will make it much more difficult to control the different ventilation and oxygenation parameters.

Modes of Ventilation

After intubating a patient and connecting to the ventilator, it is time to select the mode of ventilation to be used. Several principles need to be grasped in order to do this consistently for the patient’s benefit.

As mentioned, compliance is the change in volume divided by the change in pressure. When mechanically ventilating a patient, one can select how the ventilator will deliver the breaths. The ventilator can be set up to either deliver a set amount of volume or a set amount of pressure, and it is up to the clinician to decide which would be more beneficial for the patient. When selecting what the ventilator will deliver, you are selecting which will be the dependent and which will be the independent variable in the lung compliance equation.

If we select to start the patient on volume-controlled ventilation, the ventilator will always deliver the same amount of volume (independent variable), and the generated pressure will be dependent on the compliance. If compliance is poor, the pressure will be high, and barotrauma could ensue.

If on the other hand, we decide to start the patient on pressure-controlled ventilation, the ventilator will always deliver the same pressure during the respiratory cycle. However, the tidal volume will depend on lung compliance, and in cases where compliance frequently changes (like in asthma) this will generate unreliable tidal volumes and may cause hypercapnia or hyperventilation.

After selecting how the breath is delivered (by pressure or volume) the clinician has to decide which mode of ventilation to use. This means selecting if the ventilator will assist all the patient’s breaths, some patient’s breaths, or none of them and also selecting if the ventilator will deliver breaths even if the patient is not breathing on its own.

Other parameters that should be borne in mind and that can be adjusted in the ventilator are how fast the breath is delivered (flow), what will be the waveform of that flow (decelerating waveform mimics physiological breaths and is more comfortable for the patient, while square waveforms in which the flow is given at full speed during all inhalation, are more uncomfortable for the patient but deliver quicker inspiratory times), and at what rate will breaths be delivered. All these parameters should be adjusted to achieve patient comfort, desired blood gasses, and prevent air trapping.

There are many different modes of ventilation that vary minimally between each other. In this review, we will focus on the most common modes of ventilation and their clinical use. The mode of ventilation includes assist control (AC), pressure support (PS), synchronized intermittent mandatory ventilation (SIMV) and airway pressure release ventilation (APRV).

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.

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 throughout the United States because it is easy to use. Four settings can be easily adjusted in the ventilator (respiratory rate, tidal volume, FiO2, and PEEP). 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 of 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.

The advantages of AC mode are increased comfort, easy corrections for respiratory acidosis/alkalosis, and low work breathing for the patient. Some disadvantages include that being a volume-cycled mode, pressures cannot be directly controlled which may cause barotrauma, the patient can develop hyperventilation with breath stacking, auto-PEEP, and respiratory alkalosis.

For a full description of Assist Control, please review the article titled “Ventilation, Assist Control.”[rx]

Synchronized intermittent mandatory ventilation (SIMV)

SIMV is another frequently used mode of ventilation, although its use had been falling out of favor given its less reliable tidal volumes and failure to show better outcomes when compared to AC.

“Synchronized” means that the ventilator will adjust the delivery of its breaths with the patient’s efforts. “Intermittent”  means that not all breaths are necessarily supported, and “mandatory ventilation” means that, as with AC, a set rate is selected and the ventilator will deliver these mandatory breaths each minute regardless of the patient’s respiratory efforts. The mandatory breaths can be triggered by the patient or by time if the patient’s RR is slower than the ventilator RR (as with AC). The difference from AC is that in SIMV the ventilator only will deliver the breaths that the rate is set up to deliver, any breath taken by the patient above this rate will not receive a full tidal volume or pressure support. 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. This has been proposed as a method of “training” the diaphragm in order to maintain the muscular tone and wean off patients from the ventilator faster. 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.

Pressure Support Ventilation (PSV)

PSV is a ventilator mode that relies completely on patient-triggered breaths. 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. This means that if a patient is set up in PSV 10/5, the patient will receive 5 cm H2O of PEEP, and during inhalation, he will receive 15 cm H2O of support (10 PS above PEEP).

Because there is no back-up rate, this mode is not for use in patients with decreased consciousness, shock, or cardiac arrest. The tidal volumes will depend solely on the patient’s effort and lung compliance.

PSV often is used for ventilator weaning as it only augments patients breathing efforts but does not deliver a set tidal volume or respiratory rate.

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.

To address this concern, a new algorithm for PSV was created called volume support ventilation (VSV). VSV is a similar mode to PSV, but in this mode, the tidal volume is used as feedback control, as the pressure support given to the patient will be constantly adjusted to the tidal volume. In this setting, if tidal volume is decreasing, the ventilator will increase the pressure support to decrease the tidal volume and if tidal volume increases the pressure support will decrease in order to keep the tidal volume close to the desired minute ventilation. There is some evidence suggesting that the use of VSV may decrease assisted ventilation time, total weaning time and total T-piece time as well as a decreased need for sedation.

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. APRV has been described as a continuous positive airway pressure (CPAP) with an intermittent release phase. What this means is that the ventilator applies a continuous high pressure (P high) for a set amount of time (T high) and then releases that pressure, usually going back to zero (P low) for a much shorter period of time (T low).

The idea behind this is that during T high (which covers 80% to 95% of the cycle), there is constant alveolar recruitment, which improves oxygenation as the time maintained on high pressure is much longer than in other types of ventilation (open lung strategy). 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 0.4-0.8 seconds. 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.  This time is usually set up so that T low ends when the exhalation flow drops to 50% of the initial flow.

Minute ventilation, then, will depend on T low and the patient’s tidal volumes during T high.

 Indications for the use of APRV

• ARDS that is difficult to oxygenate with AC
• Acute lung injury
• Postoperative atelectasis.

 Advantages of APRV

• APRV is a good mode for lung protection ventilation. The ability to set the P high means that the operator has control over the plateau pressure which can significantly lower the incidence of barotrauma
• Because the patient initiates his respiratory efforts, there is better gas distribution secondary to improved V/Q matching.
• Constant high pressure means more recruitment (open lung strategy)
• APRV may improve oxygenation in ARDS patients who are difficult to oxygenate on AC
• APRV may reduce the need for sedation and neuromuscular blocking agents as the patient may be more comfortable than with other modes.

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.

References