The majority of patients admitted to critical care will need their breathing supported by a mechanical ventilator at some point in their admission. "To ventilate" means "to breathe". A mechanical ventilator is a machine that breathes for a patient. Although the term artificial "respirator" is sometimes used to describe a mechanical ventilator, this is really an incorrect term. The word respiration means "the exchange of gases".

A mechanical ventilator delivers breaths to patients, usually using positive pressure to assist or push a breath into the lungs.

We can fully ventilate a patient and make sure all of their breaths are delivered by the breathing machine (versus the patient). Assist Control (AC) is the most common method for giving a patient all of their breaths. The number designation after the AC (e.g., AC 14) indicates the minimum number of breaths from the ventilator.

SIMV (Synchronized Intermittent Mandatory Ventilation) is another way to breathe for a patient. The numerical designation identifies the number of mandatory breaths the patient will receive. With SIMV, the patient can breathe over and above the mechanical breaths. Patients can be fully ventilated while on SIMV, or they can have a very low SIMV, requiring the patient to breathe more on their own. SIMV can be used to slowly take the ventilator away while forcing the patient to increase their own breathing.

Pressure Support (or PS) is another type of ventilation. With Pressure Support, the patient must breathe on their own. Pressure Support helps the patient to take larger breaths on his or her own. Pressure Support provides positive pressure at the mouth piece of the breathing tube at the moment the patient begins to take a breath. The amount of Pressure Support (measured in cmH20) ranges between 5 (minimal support) and 30 (total support). The pressure makes it easier to take a breath. Patients who only need 5 - 10 of Pressure Support could likely breathe without the ventilator. Patients on > 15 are receiving full support.

Pressure Control is a way to prevent the patient from receiving too much pressure from the ventilator. Patients are fully ventilated and usually receive an AC type breath. The ventilator pushes the breath in at a fixed pressure. This prevents injury to the lung from high airway pressures.

Oxygen is added to the ventilator and usually given at a level higher than normal. Oxygen is a gas that has a chemical formula of O2. For this reason, oxygen is often called "O-2" as a short form.

The normal concentration of oxygen in the atmosphere is 21%. This is called an FiO2 (Fraction of inspired oxygen) of .21. If a patient is not receiving any additional oxygen, we often say the patient is on "Room Air". Room air is an FiO2 of .21. A patient on >.21 oxygen is receiving supplemental oxygen or Oxygen Therarpy. Most patients will be on an FiO2 of .3-.5. A low level of oxygen in the blood is called hypoxemia.

Oxygen is measured in the blood be a blood test called a "blood gas". The concentration of oxygen molecules in the blood is called the PaO2. We can also measure the amount of oxygen that is bound or attached to hemoglobin (oxygen saturation). Hemoglobin is a protein found inside red blood cells that carries oxygen.

Each hemoglobin molecule is able to carry a maximum of 4 oxygen molecules. Oxygen saturation is a measurement that tells us how much oxygen the hemoglobin molecules are carrying.Oxygen saturation is a percentage; it tells us the percentage of available binding sites on hemoglobin that are "loaded up" with oxygen. Oxygen saturation can never be higher than 100%. An oxygen saturation measurement that is taken from arterial blood is abbreviated the SaO2.

We can measure oxygen saturation directly by doing a blood gas. We can continuously measure oxygen saturation using a clip on the patient's finger called an oxygen saturation probe or "sat probe". Measuring the oxygen saturation using a saturation probe is called "pulse oximetry". An oxygen saturation measurement that is obtained from a pulse oximeter probe is called an SpO2 reading.

The clip reflects an infrared light beam into the blood vessels of the finger. As the light reflects off the hemoglobin molecules in the blood, the waveform color will change according to the amount of oxygen present. The oxygen saturation will be displayed on the bedside monitor.

Oxygen saturation is normally close to 100%. Oxygen saturations >92% are generally acceptable. We may accept lower values in certain patients.

If an increase in the oxygen concentration fails to improve the patients blood oxygen concentration, we will try to reopen the gas exchange areas by using CPAP or PEEP.

We adjust the ventilator to maintain acceptable blood gases.

Minute Volume, also called Minute Ventilation, is the total amount of air that is moved in and out of the lungs per minute. It is measured by the ventilator. Carbon dioxide is cleared during exhalation (when the minute volume is produced). If the amount of carbon dioxide in the patient's blood increases, we need to adjust the ventilator to move more air in and out per minute (increase the minute volume). The simplest way to do this for a patient who is receiving breaths from the ventilator is to increase the breathing rate (the AC or SIMV).

Tidal Volume is the size, measured in cc's of air, of each breath.



Image 1: Ventilator







Last Reviewed: October 23, 2014

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Last Updated October 23, 2014 | © 2007, LHSC, London Ontario Canada