Skip to Main Content

++

The main reasons for instituting mechanical ventilation are to decrease the work of breathing, support gas exchange, and buy time for other interventions to reverse the cause of respiratory failure.1 Mechanical ventilation can be applied in patients who are making or not making respiratory efforts, whereby assisted or controlled modes of support are used, respectively.1 In patients without respiratory efforts, the respiratory system represents a passive structure, and thus the ventilator is the only system that controls breathing. During assisted modes of ventilator support, the patient’s system of control of breathing is under the influence of the ventilator pump.24 In the latter instance, ventilatory output is the final expression of the interaction between the ventilator and the patient’s system of control of breathing. Thus, physicians who deal with ventilated patients should know the effects of mechanical ventilation on control of breathing, as well as their interaction. Ignorance of these issues may prevent the ventilator from achieving its goals and also lead to significant patient harm.

++

The respiratory control system consists of a motor arm, which executes the act of breathing, a control center located in the medulla, and a number of mechanisms that convey information to the control center.5,6 Based on information, the control center activates spinal motor neurons that subserve the respiratory muscles (inspiratory and expiratory); the intensity and rate of activity vary substantially between breaths and between individuals. The activity of spinal motor neurons is conveyed, via peripheral nerves, to respiratory muscles, which contract and generate pressure (Pmus). According to equation of motion, Pmus at time t during a breath is dissipated in overcoming the resistance (Rrs) and elastance (Ers) of the respiratory system (inertia is assumed to be negligible) as follows:

++

Image not available.

++

where ΔV(t) is instantaneous volume relative to passive functional residual capacity and Image not available.(t) is instantaneous flow. Equation (1) determines the volume–time profile and, depending on the frequency of respiratory muscle activation, ventilation. Volume–time profile affects Pmus via neuromechanical feedback; inputs generated from other sources (cortical inputs) may modify the function of control center. Ventilation, gas-exchange properties of the lung, and cardiac function determine arterial blood gases, termed arterial oxygen tension (PaO2) and arterial carbon dioxide tension (PaCO2), which, in turn, affect the activity of control center via peripheral and central chemoreceptors (chemical feedback). This system can be influenced at any level by diseases or therapeutic interventions.

++

During mechanical ventilation, the pressure provided by the ventilator (Paw) is incorporated into the system.3 Thus, the total pressure applied to respiratory system at time t [PTOT(t)] is the sum of Pmus(t) and Paw(t). As a result, the equation of motion is modified as follows:

++

Image not available.

++

The relationships of Equation (2) determine the volume–time profile during mechanical ventilation, which via neuromechanical, chemical, and behavioral feedback systems affects the Pmus ...

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.

Ok

About MyAccess

If your institution subscribes to this resource, and you don't have a MyAccess profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.

Subscription Options

AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more

$995 USD
Buy Now

Pay Per View: Timed Access to all of AccessAnesthesiology

24 Hour Subscription $34.95

Buy Now

48 Hour Subscription $54.95

Buy Now

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.