Ventilators

There are two general kinds of ventilators: negative pressure and positive pressure.

Negative Pressure

The original ventilators used negative pressure to remove and replace gas from the ventilator chamber. Examples of these include the iron lung, the Drinker respirator, and the chest shell. Rather than connecting to an artificial airway, these ventilators enclosed the body from the outside. As gas was pulled out of the ventilator chamber, the resulting negative pressure caused the chest wall to expand, which pulled gas into the lungs. The cessation of the negative pressure caused the chest wall to fall and exhalation to occur. While it’s an advantage that these ventilators didn’t require insertion of an artificial airway, they were noisy and made nursing care difficult. These ventilators are no longer commonly used in the critical care environment.

Positive Pressure

Positive pressure ventilation was a result of the polio epidemic in 1955, when the demand for mechanical ventilation exceeded the available number of negative pressure ventilators. There was such a shortage in Sweden that medical students worked in 8-hour shifts, manually ventilating polio patients. The Emerson Company in Boston developed the positive pressure ventilator, which was first used at Massachusetts General Hospital. It was an immediate success, and launched a new era of intensive care medicine.

Positive pressure ventilators require an artificial airway (endotracheal or tracheostomy tube), and use positive pressure to force oxygen into a patient’s lungs. Inspiration can be triggered either by the patient or the machine. There are four types of positive pressure ventilators: volume cycled, pressure cycled, flow cycled, and time cycled.

Volume-cycled ventilators are designed to deliver a preset tidal volume, then, allow passive expiration. This is ideal for patients with bronchospasm since the same tidal volume is delivered regardless of the amount of airway resistance. This type of ventilator is the most commonly used in critical care environments.

Pressure-cycled ventilators deliver gases at preset pressure, then, allow passive expiration. The benefit of this is a decreased risk of lung damage from high inspiratory pressures. The disadvantage of these ventilators is that the patient may not receive the complete tidal volume if he or she has poor lung compliance and increased airway resistance. This type of ventilation is usually used for short-term therapy (less than 24 hours). Some ventilators have the capability to provide both volume-cycled and pressure-cycled ventilation. These combination ventilators are also commonly used in critical care environments.

Flow-cycled ventilators deliver a breath until a preset flow rate is achieved during inspiration. Time-cycled ventilators deliver a breath over a preset time period.

Operation and Maintenance

Many ventilators are now computerized and have a user-friendly control panel. To activate various modes, settings, and alarms, the appropriate key need only be pressed. There are windows on the face panel that show the current settings and the alarm parameters. Some ventilators have dials instead of computerized keys; for example, the smaller, portable ventilators used for transporting patients.

Ventilators are electrical equipment and must be plugged in. They do have battery back-up, but this isn’t designed for long term use. It should be ensured they are plugged into an outlet that will receive generator power if there is an electrical outage. Ventilators are a method of life-support; if the ventilator stops working, the patient’s life may be in jeopardy. There should be a manual resuscitation bag at the bedside of every patient receiving mechanical ventilation, so they can be manually ventilated, if needed. When mechanical ventilation is initiated, the ventilator goes through a self-test to ensure that it’s working properly.


Ventilator Settings

Ventilator settings are ordered by the physician and are individualized for each patient. Ventilators are designed to monitor many components of the patient’s respiratory status. Various alarms and parameters can be set to warn healthcare providers that the patient is having difficulty with the settings.

Respiratory Rate (RR)

The respiratory rate is the number of breaths the ventilator delivers to the patient each minute. The rate chosen depends on the tidal volume, the type of pulmonary pathology, and the patient’s target PaCO2. The respiratory rate parameters are set above and below this number and the alarm will then sound if the patient’s actual rate is outside of the desired range.

(The following are guidelines.) For patients with obstructive lung disease, the rate should be set at 6-8 breaths/minute to avoid the development of auto-PEEP and hyperventilation, or “blowing off CO2”. Patients with obstructive lung disease often adapt to a higher PaCO2, so lowering it back to the “normal” range of 35-45 mm Hg may not be beneficial. Patients with restrictive lung disease usually tolerate a range of 12-20 breaths/minute. Patients with normal pulmonary mechanics can tolerate a rate of 8-12 breaths/minute. The patient should be monitored on the initial rate setting and adjustments made as necessary.

Tidal Volume (VT)

The tidal volume is the volume of gas the ventilator delivers to the patient with each breath. The usual setting is 5-15 cc/kg, based on compliance, resistance, and type of pathology. Patients with normal lungs can tolerate a tidal volume of 12-15 cc/kg, whereas patients with restrictive lung disease may need a tidal volume of 5-8 cc/kg. The tidal volume parameters are set above and below the desired number, and the alarm will sound if the patient’s actual tidal volume is outside of the desired range. This is especially helpful if the patient is breathing spontaneously between ventilator-delivered breaths, since the patient’s own tidal volume can be compared with the tidal volume delivered by the ventilator.

Fractional Inspired Oxygen (FIO2)

The fractional inspired oxygen is the amount of oxygen delivered to the patient. It can range from 21% (room air) to 100%. It’s recommended that the FIO2 be set at 1.0 (100%) upon the initiation of mechanical ventilation to allow the patient to get used to the ventilator without experiencing hypoxia. However, 100% oxygen should not be used continuously for long periods of time because of the risk of oxygen toxicity. Oxygen toxicity causes structural changes at the alveolar-capillary membrane, pulmonary edema, atelectasis, and decreased PaO2. Once the patient is stabilized, the FIO2 can be weaned down based on pulse oximetry and arterial blood gas values. The FIO2 should only be as high as is necessary to keep the PaO2 in the desired range.

Most ventilators have a temporary 100% oxygen setting that delivers 100% oxygen for only a few breaths. This should always be used prior to and after suctioning; during bronchoscopy, chest physiotherapy, or other stressful procedures; and during patient transport.

Inspiratory:Expiratory (I:E) Ratio

The I:E ratio is usually set at 1:2 or 1:1.5 to approximate the normal physiology of inspiration and expiration. Occasionally, a longer inspiratory than expiratory time is desired to allow more time to oxygenate the patient’s lungs. This is called inverse ratio ventilation, and will be discussed later.

Pressure Limit

The pressure limit regulates the amount of pressure the volume-cycled ventilator can generate to deliver the preset tidal volume. High pressures can cause lung injury. High pressure is usually resolved with suctioning. It can also be caused by the patient coughing, biting on the ETT, breathing against the ventilator, or by a kink in the ventilator tubing.

Flow rate

The flow rate is the speed with which the tidal volume is delivered. The usual setting is 40-100 liters per minute.

Sensitivity/Trigger

The sensitivity determines the amount of effort required by the patient to initiate inspiration. It can be set to be triggered by pressure or flow. Flow triggering is a better setting for patients who can breathe spontaneously because it reduces the work of breathing.

The following is a summary of the settings that nurses deal with the most.

Setting Function Usual Parameters

Respiratory Rate (RR)

Number of breaths delivered by the ventilator per minute

Usually 4-20 breaths per minute

Tidal Volume (VT)

Volume of gas delivered during each ventilator breath

Usually 5-15 cc/kg

Fractional Inspired Oxygen (FIO2)

Amount of oxygen delivered by ventilator to patient

21% to 100%; usually set to keep PaO2 > 60 mmHg or SaO2 > 90%

Inspiratory:Expiratory (I:E) Ratio

Length of inspiration compared to length of expiration

Usually 1:2 or 1:1.5 unless inverse ratio ventilation is required

Pressure Limit

Maximum amount of pressure the ventilator can use to deliver breath

10-20 cm H2O above peak inspiratory pressure