 Mechanical ventilation is a patient care modality that artificially provides a means of breathing for patients in respiratory failure. Respiratory failure can happen under a variety of circumstances, including conditions that involve the heart, lungs, brain, spinal cord, or muscles involving ventilation. Under normal conditions, we use a large muscle in the torso called the diaphragm to generate negative pressure within the chest and lungs to draw fresh air into our body. When fresh air is introduced into our air sacs called alveoli, oxygen diffuses into our bloodstream to be delivered to our muscles and other tissues and organs in the body. At the same time, a byproduct of metabolism called carbon dioxide is diffused back into the air sacs to be exhaled out of the lungs before the next breath. This cyclical motion and exchange of gas is the normal process we associate with spontaneous breathing. Mechanical ventilation uses positive pressure to force air and oxygen into a patient's lungs. Positive pressure ventilators require an artificial airway such as an endotracheal or tracheostomy tube. No matter the type, size, or operation of a mechanical ventilator or manual resuscitation bag, certain variables apply in all circumstances. This module will provide an overview of key concepts for the modules to follow. Mode of Ventilation The mode of ventilation determines the manner in which breaths are delivered to the patient. Modes can be classified into two categories, those that provide complete ventilatory support and those that provide partial ventilatory support. The care provider determines the mode of ventilation primarily by the patient's underlying condition and ability to breathe spontaneously. Careful monitoring should take place after all ventilator changes, especially when switching ventilation modes. Respiratory Rate The respiratory rate is the frequency of respirations expressed per minute. The care provider determines the respiratory rate primarily based on the patient's age and degree of lung disease. Normal respiratory rates for healthy adults range from 12 to 18 breaths per minute. A rate of 12 breaths per minute is equal to one breath every five seconds. Sometimes respiratory rate is abbreviated with RR or F for frequency. Title Volume Title volume is the amount of air that is delivered with each breath. The care provider determines the title volume primarily based on age, height, and degree of lung disease. Normal title volumes for healthy adults are about 5 to 10 milliliters per kilogram of ideal body weight. Thus a 65 kilogram or 143 pound person would have a spontaneous title volume of 325 milliliters. Careful consideration must be taken to determine mechanical title volumes in the face of lung disease to avoid detrimental complications. Title volume is sometimes abbreviated with VT. Inspiratory Time Inspiratory time is the time in seconds required to achieve one full inspiratory title breath. The inspiratory time is selected by the care provider to achieve comfortable synchronization between the patient and ventilator. It is also determined by maintaining an appropriate inspiratory to expiratory time ratio, I to E. Normal inspiratory time in adults is 0.75 to 1.25 seconds. I to E ratio The I to E ratio in many cases is a result of the set respiratory rate and the set inspiratory time and usually cannot be changed independently. Normal I to E is at least 1 to 2. I to E ratios of 1 to 3 or even greater are acceptable. I to E ratios of less than 1 to 1.5 should only be used in highly specialized circumstances to avoid ill effects of mechanical ventilation. I to E ratios are often calculated on mechanical ventilators, but not all ventilators provide this function. The calculation is a bit complex, but generally higher respiratory rates require faster inspiratory times to achieve an appropriate I to E ratio. Airway Pressure Airway pressure is a result of positive pressure being delivered to the lungs artificially via ventilator or resuscitation bag. This value is monitored on a ventilator by a manometer. Spontaneous breathing individuals have very low to zero positive pressure in the lungs. However, when positive pressure is introduced to the lungs via mechanical ventilator or manual resuscitator, airway pressures rise proportionally with tidal volume. The degree of positive pressure rise with each unit of volume is determined by the patient's lung compliance. Lung compliance changes greatly with lung disease and should be monitored closely in volume ventilation. In general, lung pressures above 35 cm of water are considered to be unsafe and should be avoided. Lung Compliance Lung compliance is a calculated value that is expressed in units of volume per unit of pressure. It may be thought of as the opposite of stiffness, i.e. as compliance decreases, lungs become stiffer. This calculated value changes greatly with lung characteristics. It may not be necessary to know exact values of lung compliance as long as peak airway pressures are monitored closely. Ventilator Sensitivity Ventilator sensitivity is a value set on ventilators that allows patients to tell the machine that a breath is required. To lower the value of sensitivity, the easier it is for patients to automatically trigger a breath. If not automatically determined by the ventilator, sensitivity is set as low as possible but not so low as to allow the ventilator to trigger automatically. Usually the set value for an adult is 2-3 cm of water below the set end expiratory pressure or peak. If there is no end expiratory pressure, then sensitivity may be a negative value. Ventilator sensitivity may also be known as trigger, breathing effort, or simply sensitivity. Peep Peep is an acronym that stands for positive and expiratory pressure. Peep is pressure left in the lungs even after exhalation of a tidal breath has occurred. Peep is used to help patients get higher values of oxygen to the blood from the lungs. This value is determined by the care provider based on the patients need to provide higher levels of oxygen. Generally, Peep is set between 0 and as high as 20 cm of water in special circumstances. FiO2 FiO2 is the fraction of inspired oxygen concentration delivered by the ventilator. Room air that we normally breathe is 21% oxygen. Supplemental oxygen can and should be delivered when warranted by the patient. Supplemental oxygen should also be given during procedures such as sectioning. The care provider determines the amount of oxygen based on clinical information such as pulse oximetry and arterial oxygen values. Though hazards of supplemental oxygen do exist, the benefits far outweigh the risks. Supplemental oxygen should always be given when the patient's status is in question. High pressure alarm limit The high pressure alarm limit is a setting on the ventilator that designates the highest possible pressure that will be delivered by the ventilator. An airway pressure that reaches this set value will result in an alarm as well as premature stop of inspiratory tidal volume. This setting acts as a safety mechanism to prevent patients from experiencing airway pressures that are too high. Airway pressures that are too high could cause serious injury. As discussed earlier, airway pressures above 35 cm of water should be avoided. However, intermittent breaths with higher pressures can be acceptable as long as it resolves in a short period of time. In general, the high pressure setting should be set 10-15 cm of water above the average monitored peak airway pressure. The low pressure alarm limit is an alarm setting on the ventilator that activates when inspiratory pressure does not exceed the set value. This value is generally set 5-10 cm of water above end expiratory pressure. In most cases, activation of this alarm is a result of disconnected circuitry and prompt action may be required.