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Modes of Invasive Mechanical Ventilation and their Indications

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  • Revised on: 2021-05-05

Mechanical ventilation is a method to mechanically assist or replace spontaneous breathing when patients cannot do so on their own. It is implemented with a ventilator machine that can support ventilatory function and improve oxygenation through the application of high-oxygen-content gas and positive pressure.

Currently, a positive pressure ventilation is clinically used. These 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 then airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume then the breath and out through passive exhalation

In this article, we shall cover the indications for mechanical ventilation, methods and modes of mechanical ventilation in detail.

Indications for mechanical ventilation

Mechanical ventilation is primarily indicated for patients who have the following conditions;

Acute lung injury (including ARDS, trauma)

Apnea with respiratory arrest, including cases from intoxication

Chronic obstructive pulmonary disease (COPD)

Respiratory failure, both hypoxemic and hypercarbic respiratory failure.

Hypoxemic, which is present when arterial O2 saturation (Sao2) <90% occurs despite an increased inspired O2 fraction and usually results from ventilation-perfusion mismatch or shunt; Hypercarbic, which is characterized by elevated arterial carbon dioxide partial pressure (PCO2) values (usually >50 mmHg) resulting from conditions that decrease minute ventilation or increase physiologic dead space such that alveolar ventilation is inadequate to meet metabolic demands.

Acute respiratory acidosis which may be due to paralysis of the diaphragm due to Guillain-Barré syndrome, Myasthenia Gravis, spinal cord injury, or the effect of anesthetic and muscle relaxant drugs.

Increased work of breathing

Hypotension including sepsis, shock, congestive heart failure.

Mechanical ventilation is lifesaving when used to support a patient with acute respiratory failure compared when used in chronic respiratory failure

Methods of mechanical ventilation

There are two methods of mechanical ventilation:

  1. Noninvasive ventilation (NIV) and
  2. Invasive (conventional mechanical) ventilation.

Ventilation delivery is by either hand-controlled ventilation using a bag valve mask or continuous-flow or T-piece) bag or by a mechanical ventilators or PAP ventilators.

Modes of mechanical ventilation

When we say a mode of mechanical ventilation we refer to the manner in which ventilator breaths are triggered, cycled, and regulated. For a mechanical ventilator to initiate an assisted breath there has to be a trigger. The trigger can either be an inspiratory effort or a time-based signal.

A mechanical ventilator mode is defined based on three characteristics;

1. Trigger

2. Cycle

3. Limit

A cycle is a factor or a signal that determines the end of inspiratory phase delivered by the vent.

The types of available ventilation cycles are;

  1. Pressure cycling- It works by limiting the pressure provided with each breath. Inspiratory phase ends once the set pressure is achieved.
  2. Volume cycling- works by delivering a pre-set volume of gas with each breath and the inspiratory phase ends once the pre-set volume has been delivered by the ventilator.
  3. Time cycling- It limits the length of the inspiratory cycle (I: E ratio). I-time is set as a percentage of total time (usually 33%)

The limiting factors are operator-specified values, for example, airway pressure. These are monitored by transducers present within the ventilator circuit throughout the respiratory cycle;

If the set values are exceeded, inspiratory flow is terminated, and the ventilator circuit is vented to atmospheric pressure or the specified pressure at the end of expiration PEEP.

In most clinical settings, patients are ventilated with assist-control ventilation (ACMV), intermittent mandatory ventilation (IMV), or pressure support ventilation (PSV).

Classification of modes of ventilation

The modes of ventilation can be classified into three broad categories

  1. Controlled modes
  2. Assisted/support modes
  3. Combined modes
1. Controlled Modes:

These are the commonly used modes in critically ill patients who have a a significantly suppressed or absent respiratory drive.

In these modes, all spontaneous patient breaths sensed by the vent and are assisted with a preset volume or preset pressure that is set by the operator.

These modes are either, patient-/time-triggered or volume-/pressure-cycled and they include;

  • Assist-control ventilation (ACV), aka volume-control ventilation (VCV):
  • Pressure-control (PC) ventilation (PCV)
2. Spontaneous/Supported:

Spontaneous or supported modes are used when there is a significant improvement of in patients who have restored their ability make spontaneous breaths and are considered for weaning.

These modes are patient-triggered and flow-cycled.

These modes include;

  • In Pressure support (PS) ventilation mode, all the spontaneous breaths made by the patient are sensed by the ventilator are supported with a preset pressure that is specified on the vent by the operator.
  • Continuous positive airway pressure (CPAP) with or without pressure support. Here the ventilator provides a continuous circuit pressure.

3. Combined (controlled + spontaneous/supported) modes:

Combined modes are commonly used in patients for maintenance and weaning.

A preset number of patient breaths are assisted by the ventilator (controlled) and the remaining spontaneous patient breaths are supported (spontaneous/supported).

In Pressure support (PS) ventilation mode, all the spontaneous breaths made by the patient are sensed by the ventilator are supported with a preset pressure that is specified on the vent by the operator.

Continuous positive airway pressure (CPAP) with or without pressure support. Here the ventilator provides a continuous circuit pressure.


Combined modes include;

  • Synchronized intermittent mandatory ventilation (SIMV)-VC + PS,
  • Synchronized intermittent mandatory ventilation (SIMV)-PC + PS

Assist-Control Ventilation

The most commonly used mode is Assist-Control Ventilation.

Here, an inspiratory cycle is initiated either by the patient’s inspiratory effort or, if none is detected within a specified time window set on the ventilator.

A specified preset tidal volume or pressure is delivered on every breath delivered. This is independent of whether it is patient triggered or timer-triggered therefore there is always the same setting every time the patient initiates a breath.

Ventilatory rate is determined either by the patient or by the operator-specified backup rate, whichever is of higher frequency.

This mode is commonly used for the initiation of mechanical ventilation due to the fact that it ensures backup minute ventilation in the absence of an intact respiratory drive allowing for synchronization of the ventilator cycle with the patient’s inspiratory effort.

Problems can arise when this mode of mechanical ventilation is used in patients with tachypnea due to nonrespiratory or nonmetabolic factors, such as anxiety, pain, and airway irritation.

Respiratory alkalosis may develop and trigger myoclonus or seizures.

Dynamic hyperinflation leading to increased intrathoracic pressures (auto-PEEP) may occur if the patient’s respiratory mechanics are such that inadequate time is available for complete exhalation between inspiratory cycles.

This hyperinflation can limit venous return, decrease cardiac output, and increase airway pressures, predisposing the patient to barotrauma.

Intermittent Mandatory Ventilation

Here the operator sets the number of mandatory breaths of fixed volume to be delivered by the ventilator to the patient. In between these breaths, the patient can breathe spontaneously.

Synchronized Intermittent Mandatory Ventilation (SIMV)

In the most frequently used synchronized mode (SIMV), mandatory breaths are delivered in synchrony with the patient’s inspiratory efforts at a frequency determined by the operator.

If the patient fails to initiate a breath, the ventilator delivers a preset tidal-volume breath and r