Vent Mgmt #1: Basics
The goal of ventilatory support is to maintain appropriate O2 and CO2 in the blood while offloading the work of the respiratory muscles and minimizing iatrogenic lung damage. Understanding this principle will help guide your ventilator management. Many variables can be manipulated on the ventilator, but there are a few key variables that truly control oxygenation and ventilation. While there is not one ideal setting for every scenario, there are a few basic principles that cover the majority of ventilator management.
Basic Ventilator Settings
First, it is important to understand what the ventilator does. The ventilator can push air into patients. You can control how much air is pushed in (tidal volume), the number of breaths per minute (respiratory rate, RR), and the concentration of oxygen molecules in the air itself (fraction of inspired oxygen, FiO2). It's also possible to control how quickly air is pushed in (flow)- but we will get to that later. It is important to note: the ventilator does NOT generate pressure- it only monitors pressure to prevent damage from elevated pressures (barotrauma).
Breathing is controlled by three variables.
Trigger- this determines when a breath starts. Either time, flow, or pressure.
Time trigger is utilized when the patient is not generating any spontaneous breathing (ie mandatory breaths).
Flow and pressure triggers are utilized if the patient has spontaneous respiratory activity. When the patient attempts to inhale, there is a change in flow and/ or pressure. This is sensed by the ventilator, and a breath is delivered.
 Limit- this sets the maximum value a parameter can reach during a breath. For example, volume-limited indicates that a breath can't exceed a certain max mL and pressure-limited indicates that the pressure monitored by the machine can't exceed a certain max cm H2O. For a graphic representation, please refer to the image in the section on Limit Variables in Deranged Physiology. Limits impact the shape of the waveform.
Volume limited- flow ceases when the set/ target volume is delivered.
Pressure limited- a large portion of the TV delivered at the beginning of the breath until the set/ target pressure is reached and then the flow tapers, slowly delivering the remainder of the volume until the breath is time or flow cycled (see next)
Cycle- this determines the end of a breath.
Time cycled- inspiration ceases at the end of a set time duration. Used in mandatory breaths.
Flow cycled- inspiration ceases when flow drops below a certain level. Used in spontaneous breaths.
Volume and pressure are not currently used to cycle breaths.
The goals of mechanical ventilatory support are O2 delivery (oxygenation) and CO2 removal (ventilation). Effective oxygenation and ventilation are measured by an arterial blood gas- PaO2 indicates the partial pressure of O2 and PaCO2 indicates the partial pressure of CO2.
Oxygenation is a function of the concentration of O2 delivered to the patient (fraction of inspired O2, FiO2) and the surface available for O2 exchange. Positive pressure maintains open airways, which maintains the surface available for O2 exchange. Mean airway pressure (MAP) is the parameter that indicates the average pressure measured in the lungs throughout inspiration (inspiratory pressure) and expiration (positive end expiratory pressure, PEEP). Expiration is usually 2-3 times longer than inspiration, so MAP is often simplified to PEEP when trying to optimize oxygenation. However, increasing inspiratory time can improve MAP without adjusting PEEP.
Ventilation is controlled by minute ventilation (total volume of air exchanged every minute). Minute ventilation is respiratory rate multiplied by tidal volume. Therefore, respiratory rate (RR) and tidal volume (TV) are the two parameters that can optimize ventilation.
Lung-Protective Ventilation
Minimizing iatrogenic lung injury is also important when caring for patients receiving ventilatory support. Different types of trauma, including barotrauma (excess pressure), volutrauma (excess volume), and atelectrauma (repetitive opening and closing of alveoli), can damage lungs that are already diseased.
The risk of barotrauma can be minimized by monitoring airway pressures (peak and plateau pressures).
Volutrauma can be minimized by low tidal volume. Historically, larger tidal volumes were standard (10-12 mL/kg). Currently, the most commonly recommended volume is 6-8 mL/kg (there are some exceptions). Decreased TV leads to ↓minute ventilation and ↓CO2 clearance (↑PaCO2). This is the basic physiologic principle behind "permissive hypercapnia" during mechanical ventilation for ARDS.
Atelectrauma can be minimized by maintaining PEEP, which keeps alveoli open.
Additional References