Ventilation has been described as the volume of air that is moved into and out of the lungs and airways over a period of time. Physiologically, lung volumes are subdivided into the categories of either dynamic lung volumes or static lung volumes. Clinically, dynamic lung volumes are used in the diagnosis and management of obstructive lung disease. These dynamic lung volumes are related to the rate of airflow. Static lung volumes, however, are important both in restrictive ventilatory defects and in obstructive lung disease. Static lung volumes are further broken down into standard lung volumes (tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume) and standard capacities (inspiratory capacity, functional residual capacity, vital capacity, and total lung capacity).
Tidal volume has been described as the volume of air that is transported into and out of the lungs with each respiratory cycle. Tidal volume is clinically important not only for ventilator settings in the critically ill patient, but can also be clinically important during spontaneous respiration. Tidal volume in a healthy adult male is approximately 500 mL and in a healthy adult female is approximately 400 mL, but can also be altered to fit physiological needs.
Tidal volume is extremely important to consider during mechanical ventilation, in order to insure adequate ventilation of the patient, without causing trauma to the lungs. Originally, patients undergoing mechanical ventilation were ventilated with tidal volumes of 10 mL/kg or greater because research had shown that large tidal volumes reduced hypoxemia, reduced airway closure, and increased functional residual capacity. However, volutrauma wasn’t described until 1974, when Webb and Tierney demonstrated the development of pulmonary edema in rats after exposure to high inflation pressures. Volutrauma, caused by ventilation with large tidal volumes, has been shown to cause increased lung permeability, pulmonary edema, an alteration of surfactant, and the production of cytokines that injure the lungs themselves. Lung injury, such as acute respiratory distress syndrome, can be caused by ventilating with very large tidal volumes in normal lungs, though also with ventilation with small tidal volumes in injured lungs. Due to continuing research in lung protective mechanical ventilation, tidal volumes of 6 mL/kg of predicted body weight have been accepted.
The pulmonary system is the primary organ system that is involved with taking a tidal breath. Functionally, the respiratory tract consists of the conducting airways, which extend from the nose down to the terminal bronchioles, and also the gas-exchanging airways, which extend from the respiratory bronchioles to the alveoli within the lungs.
Tidal volume is essentially every breath that you take. However, tidal volume plays a crucial role in calculating a patient's minute ventilation. Minute ventilation is the volume of gas exchanged by a patient's lungs per minute and is expressed as: minute ventilation = tidal volume(respiratory rate). During exercise, minute ventilation increases due to physiological demands for increased oxygenation causing an increase in both tidal volume and respiratory rate.
Air moves into the out of the lungs by way of movement of the diaphragm and the chest. When a person inspires, the diaphragm descends at the same time that the rib cage moves up and out. The lungs are connected to the chest wall via the pleura, so, therefore, expansion of the chest wall during inspiration also expands the lungs. As the lungs expand physically, this creates a sub-atmospheric intra-alveolar pressure, which then draws air into the alveoli. This occurs since air moves down its pressure gradient from an area of higher pressure to an area of lower pressure, in accordance with Boyle’s Law.
Expiration is usually a passive force due to the elastic recoil of the lungs and chest wall when the inspiratory muscles cease their contraction. However, gas flow during expiration can be increased by active contraction of the expiratory muscles, although the maximum expiratory flow rate is limited by airway compression.
Spirometry is the measurement of lung capacities and volumes during forced inspiration and expiration in order to determine how fast the lungs can be filled and emptied. It involves a patient taking one normal tidal breath followed by a maximum inhalation, a maximum exhalation, and then another normal tidal breath.
During spirometry testing, the patient first breathes quietly. This normal, quiet breathing involves inspiration and expiration of a tidal volume. Normal tidal volume is approximately 500 mL, and includes the volume of air that fills the alveoli in the lungs, as well as the volume of air that fills the airways. The patient then takes a maximal inspiratory breath, and a maximal expiratory breath. By doing so, more data can be collected during the spirometry testing. The additional volume that is inspired, above the tidal volume, is called the inspiratory reserve volume and this volume can measure approximately 3000 mL. The additional volume that is expired, below the tidal volume, is called the expiratory reserve volume and this volume can measure approximately 1200 mL.
Mechanical ventilation can sometimes contribute to worsening of lung injury, though mechanical ventilation is also sometimes necessary for patient survival in many different circumstances. In acute respiratory distress syndrome, or ARDS, for example, the volume of the aerated lung is much less than the volume of the aerated lung in a normal healthy lung, due to the edema and atelectasis caused by the lung injury. Therefore, ventilation of a patient with the normal expected tidal volume, may cause hyperinflation of the healthy aerated portion of the lungs. This, in turn, can cause an increase in pressure and lung injury. This injury results from disruption of the alveolar epithelium and capillary endothelium, as well as from the caused inflammatory response of the lungs which causes the release of cytokines.
Tidal volume amounts should be considered when ventilating a patient with a bag valve mask (BVM). It is important to differentiate between the pediatric patient and the adult patient, so that neither over-inflating nor under-inflation occurs during ventilation. The normal tidal volume in adults is approximately 500 mL, or 7 mL/kg for the average adult, and the normal tidal volume in pediatric patients is approximately ten mL/kg, though it varies with growth.
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