Mechanical ventilator (aka assisted ventilation) is the medical terminology for artificial ventilation where mechanical means are used to assist or replace breathing. Mechanical ventilators can be used in invasive mode where an endo-tracheal tube is inserted through the mouth into the patient’s trachea (windpipe). Invasive ventilation could be used during an acute respiratory failure or a surgery where the patient is unable to manage the breathing. Figure 1 shows this mode of mechanical ventilator.
Figure 1: Invasive ventilation through endo-tracheal tube
Non-invasive ventilation uses a mask over a patient’s mouth and nose. In its most simple form, the Ambu bag compression system consists of a flexible bag mask, ventilation tubing, valves, oxygen tube, reservoir bag and a face mask. The bag mask can be compressed by a person or by mechanical means. Figure 2 shows the scenario
Mechanical ventilators are widely used for patients who are suffering from ARDS (Acute Respiratory Distress Syndrome). With the Covid-19 virus creating a pandemic around the world and its flu like symptoms similar to ARDS, the burning question is if there are enough mechanical ventilators to support the patients who experience ARDS symptoms. One study by John Hopkins University (Reference 1) estimates that there are 160,000 mechanical ventilators. 62,000 are full featured and the remaining 98,000 units, although not fully equipped, meet the basic needs of a patient with ARDS. In a pandemic emergency such as the one created by COVID-19, there may be a 25% surge in the need for ventilator (Reference 1). If the pandemic follows the model of the Spanish Flu of 1918, numbers close to 740,000 ventilators will be required (Reference 2).
Figure 3: A more equipped mechanical ventilator
The most recent machines have many features such as LCD or CRT waveform displays, calculated lung mechanics and system diagnostics. There are few parameters of importance for mechanical ventilators that need to be described. They are:
- Tidal Volume: This is defined as the volume of air into and out of the longs during each ventilation cycle and is shown in figure 4 (Reference 6). It is usually in the 400-500 ml range.
Figure 4: Tidal volume is shown on the graph
- I:E ratio: The Inhale to Exhale ratio refers to inspiratory time to expiratory time. In normal spontaneous breathing, this ratio is about 1:2 but in case of patients suffering from ARDS it has to be about 1:2.7 although it also depends on the condition of the lung. The total range is 1:2 to 1:4.
Pressure: It is important to monitor the peak and the plateau pressures in mechanical ventilation. As the tidal volume increases, so does the pressure to force that volume into the lungs. The pressure is measured in units of cm H2O and is normally between 20-30 cm H2O.
Figure 5: Spontaneous breathing
https://www.asensetekca.com/ shop / category / allied - scientific - medical-respiratory - ventilators - non-invasive-and-invasive-16