Mechanical ventilation is required when there is a respiratory failure. There are two situations that lead to respiratory failure:
Ventilatory (Hypercapnia respiratory failure)
It can be caused caused by slowed respiratory function, chest wall abnormalities and respiratory muscle fatigue.
Inefficient Gas Exchange (Hypoxic respiratory failure)
It can be caused by an intrapulmonary shunt, ventilation-perfusion mismatch, decreased Functional Residual Capacity (FRC).
The main objectives of mechanical ventilation are maintaining adequate gas exchange, resolving respiratory distress, reducing breathing effort for patients and allowing lung healing.
Different modes of mechanical ventilation
There are three modes in mechanical ventilation.
Pressure Controlled Ventilation
Volume Controlled Ventilation
Dual Control Modes
Pressure Controlled Modes of Ventilation
In this mode, the ventilator delivers a preset pressure. So, the tidal volume of breath varies according to the resistance and elasticity of the patients respiratory system. The advantage of pressure controlled mode is that the gas is distributed equally throughout the lungs and a major disadvantage in this mode is that tidal volumes may vary because of changes in pulmonary mechanics. This mode needs close monitoring, which limits its usage in many patients in emergency departments.
Pressure-Controlled Ventilation (PCV)
During pressure-controlled ventilation, two pressure levels are kept constant:
The lower pressure level
The upper pressure level
It does not allow the patient to trigger breaths. It delivers breaths at the preset pressure. The volume and the decelerating flow will vary according to the changes in the lung mechanics.
The inspiratory flow pattern decreases exponentially and results in reduction of peak pressures and improvement of gas exchange.
The major disadvantage is that there is no guarantee for specific volume especially when lung mechanics are changing.
Pressure Support Ventilation (PSV)
It is used to enhance spontaneous breathing. This is flow-cycled and the duration of inspiration depends on the lung mechanics of the patient.
The patient can determine the inflation volume and frequency of respiratory, but not pressure.
This mode ensures guaranteed minimum frequency (backup frequency). If the breathing frequency of the patient is lower than the backup frequency or there is no spontaneous breathing, then the machine-triggered flow-cycled mandatory breaths are applied with the set upper pressure.
Pressure Controlled Inverse Ratio Ventilation (PCIRV)
In this mode, pressure is continuously adjusted downward and the time spent by the lungs on inhalation is greater than exhalation for proper gas exchange.
The reduced expiratory time leads to risks of auto lower pressure level and hemodynamic deterioration.
Airway Pressure Release Ventilation (APRV)
Airway pressure release ventilation mode is the time cycled pressure control mode.
PRV releases pressure temporarily on exhalation. This mode results in higher average airway pressures.
This mode enables patients to spontaneously ventilate at both low and high pressures unlike typical ventilation occurring at the high pressure.
Volume controlled modes of ventilation
In volume-controlled ventilation modes, the set tidal volume is supplied by the ventilator at a constant flow. The inspiratory pressure is variable and changes according to lung mechanics of the patient. This mode results in increased patient comfort and AutoFlow can be enabled. A major disadvantage of this method is that the high airway pressures may be generated resulting in barotrauma. Nevertheless, close monitoring and use of pressure limits are helpful in avoiding this problem. Volume-controlled ventilation modes are not available for the neonatal patient category.
Assist-Control Ventilation (ACV)
It is also known as Continuous Mandatory Ventilation (CMV). In this mode, the patient can trigger the ventilator to get breath.
If the patient fails to trigger the device within the preset time, the machine will automatically give breath.
The device delivers the present volume of breath when triggered by the patient.
Synchronized Intermittent-Mandatory Ventilation (SIMV)
In this mode, the machine delivers a preset volume of breathes at a preset rate and guarantees a certain number of breaths.
In addition, the patient is able to breathe spontaneously between machine breaths. Because the mandatory breaths are synchronized to coincide with spontaneous respiration.
Disadvantages of SIMV are increased effort of breathing and a tendency to reduce cardiac output, which may extend ventilator dependency. The addition of pressure support on top of spontaneous breaths can reduce the workload of breathing.
Dual control modes
Pressure Regulated Volume Control (PRVC)
In this mode, the machine delivers a set of tidal volumes at the minimum pressure level needed.
The ventilator automatically adjusts the inspiratory pressure control level according to the changes in the mechanical properties of the lungs.
The limitation of this mode is that the decrease in pressure support levels can result in hypoxemia.