Critical care - Mechanical ventilation: Nursing

Mechanical ventilation is a process that partially or fully assumes breathing for patients who can’t independently maintain effective gas exchange. It can be used in the short-term, such as during surgery when a patient is sedated, or in the long-term, like in cases of respiratory failure, airway obstruction, or when a patient is comatose.

Mechanical ventilation requires the placement of an artificial airway, such as an endotracheal tube, or ETT, which is a tube that’s inserted through the mouth and into the trachea through a procedure called intubation. Another artificial airway is a tracheostomy tube, which is inserted directly through a surgical opening made in the skin of the neck, that creates an opening into the trachea. Once inserted, the tube is connected to a ventilator, which is a mechanical device that pushes oxygen-rich air into the patient’s lungs.

Now, mechanical ventilators have modifiable settings to meet the needs of each patient based on their health status, comorbidities, and goals of care. First, the respiratory rate or frequency determines the number of breaths the ventilator supplies and is typically set between 6 to 20 breaths per minute to mimic normal respiration. Next, tidal volume, or VT, is the volume of air delivered during each breath, which for most patients is between 4 to 8 mL/kg of body weight. So, let’s say your patient weighs 60 kilograms and is started on a VT of 6ml/kg. The ventilator will deliver a VT of 360 milliliters of air with each breath.

Then there’s oxygen concentration, or the fraction of inspired oxygen, called FiO2 for short, which refers to the concentration of oxygen being delivered. FiO2 can be set anywhere from 21 to 100 percent to maintain a partial pressure of arterial oxygen, or PaO2, between 60 and 100 mmHg or oxygen saturation at 90 percent or more.

Next, the inspiratory to expiratory ratio, or I:E ratio, is the length of inspiration compared to the length of expiration with a typical I:E ratio of 1:2 to mimic normal spontaneous ventilation, since inspiration is shorter than expiration.

Next is positive-end expiratory pressure, called PEEP, which is the application of positive pressure at the end of exhalation to prevent the collapse of the small airways and alveoli. PEEP tends to be set between 3 to 5 cm H2O.

Lastly, sensitivity determines how much effort a patient needs to make when attempting a breath for the ventilator to be triggered, which will then assist the patient with the breath. Ideally, the setting will enable detection of a weak effort while avoiding auto-cycling where the ventilator delivers breaths without a patient effort, which can lead to patient-ventilator asynchrony.