Depending on the dictionary used, ventilation has numerous definitions, with the second or third definition as "the process of the exchange of air between the lungs and ambient air." This is the definition that we will use in this article. We will also confine this article to the mechanical ventilation of a patient and how it is accomplished. The key is to think simply and not overcomplicate the technology.

The simplest of the mechanical ventilators is the Ambu-bag. This is widely used in hospitals when transporting patients, during codes (CPR), and in operating rooms. Basic operation principles apply: Squeeze the bag, which pushes air through a valve system into a mask placed over the patient's nose and mouth, or into an airway and down to the lungs. The feel of the bag or bag pressure is an indication as to how stiff the lungs are. As the hand pressure is relaxed on the bag it expands, drawing air into it via a one-way valve. The relaxing of the pressure on the bag also allows for the patient to be able to exhale via another valve that vents the patient's exhaled air into the atmosphere. This is the simplest form of mechanical ventilation, and by adding technology to perform the squeezing of the bag the modern ventilator was born.

Design Challenges

Basically, the designers had to consider:

• How much air to move per breath, since a person's size and lung condition are factors to be considered in determining the volume.
• How many times per minute the air should be delivered. On an older adult it can be as few as five or six times per minute, up to close to 200 for a neonate.
• At what pressure the air should be delivered. Again, there is a wide range of pressures, and remember that most ventilators are set up to show CM of H₂O, not psi or mmHg.
• If the machine would control or assist the patient's ventilation. By controlling, we mean—as an example—15 breaths per minute, at 800 cc per breath at a pressure of 50 CMH₂O. In this setting, the patient must be unresponsive and without any inspiratory efforts. By assisting a patient, the ventilator will start flowing to the patient when an inspiratory effort is detected. Generally, the volume delivered is determined by the upper pressure limit set, but occasionally it can be set on the delivered volume. Tidal volume is the volume of air inspired or expired during each normal respiratory cycle.
• If the machine is to assist a patient's breathing. If so, it must detect when the patient makes an inspiratory effort, and the machine must deliver the breath. There are two ways in use to detect inspiratory efforts—withdrawn volume or negative pressure. Both of these are small and may be difficult to measure during testing.
• How to modify gasses. Compressed gases coming from the wall or an onboard compressor are dry and cold, so they must be modified. This is done using a nebulizer that warms the gas and adds humidity (water) to the gases so as not to cause bronchial spasms from the cold, dry gasses entering the lungs.

The first six points were on how/why/when to get gas into the lungs; now we have to look at getting air out of the lungs. Gases in the lungs need to be moved to the atmosphere, and this happens in one of several ways:

• The chest will try to relax, forcing gas from the lungs.
• The inspiratory/expiratory valve at the patient connection allows gas to exit the lungs.
• Some gas will remain in the lungs, called residual volume, to keep the lungs from totally deflating. In some patients, the machine will create a positive pressure called positive end expiratory pressure (PEEP). A lower-technology version of this is called continuous positive airway pressure (CPAP) and is used in the treatment of sleep apnea. There are machines that will assist a patient in exhaling by the use of a venturi effect gas flow past the patient connection, which will draw out more gas from the lungs. This is also referred to as negative end expiratory pressure (NEEP).

A key monitoring tool for patients on ventilators is the pulse oximeter, which provides the therapist or physician with an indication of the oxygen level in the arterial blood. Before pulse oximetry became common, ventilated patients had arterial blood drawn three or more times per day. An arterial "stick" is a painful procedure, so most ventilator patients also had an arterial blood pressure line, enabling the sample to be drawn from a three-way stopcock in the line. The blood sample then had to be quickly sent to the central lab or to a blood gas machine in the ICU. In most cases, the blood gas testing is way down from what it was in the early 1990s, and that is real progress. This has increased patient comfort by eliminating arterial "sticks," and lowered infection rates have been gained through technology. Now, if we could get the staff to wash their hands between patients, the infections rates will be even lower.

Many of you reading this article work on ventilators and received service training from the vendors or another source, but probably the majority of us do not fully understand all the controls and what has to be done with the device by the respiratory people after we fix and calibrate it. Please take some time and talk with them on the whys of what they do. It will surprise you to see how much you know and how much more there is to know,

I will not even try to go over the various ventilators that you may encounter, as I am not current with some of the new machines. Please take the time and talk with the respiratory people, as they have a lot of offer.

David Harrington, PhD, is a health care consultant, Medway, Mass, and is a member of 24x7's editorial advisory board. For more information, contact .