What is mechanical ventilation
A mechanical ventilator is a machine that generates a controlled flow of gas into a patient’s airways. Oxygen and air are received from cylinders or wall outlets, the gas is pressure reduced and blended according to the prescribed inspired oxygen tension (FiO2), accumulated in a receptacle within the machine, and delivered to the patient using one of many available modes of ventilation.
The central premise of positive pressure ventilation is that gas flows along a pressure gradient between the upper airway and the alveoli. The magnitude, rate and duration of flow are determined by the operator. Flow is either volume targeted and pressure variable, or pressure limited and volume variable. The pattern of flow may be either sinusoidal (which is normal), decelerating or constant. Flow is controlled by an array of sensors and microprocessors. Conventionally, inspiration is active and expiration is passive (although modern ventilators have active exhalation valves).
There are two phases in the respiratory cycle, high lung volume and lower lung volume (inhalation and exhalation). Gas exchange occurs in both phases. Inhalation serves to replenish alveolar gas. Prolonging the duration of the higher volume cycle enhances oxygen uptake, while increasing intrathoracic pressure and reducing time available for CO 2 removal.
Failure to oxygenate is caused by reduced diffusing capacity and ventilation perfusion mismatch. This can often be overcome by restoring FRC by increasing baseline airway pressure using CPAP. If the problem is atelectasis due, for example, to mucus plugging or diaphragmatic splinting following abdominal surgery, or moderated amounts of pulmonary edema, CPAP, as delivered by facemask or endotracheal tube, may sufficiently restore pulmonary mechanics to avoid addition inspiratory support. CPAP is easy to apply: all that is required is a PEEP valve and a flow generator.
The flow generator is important as peak inspiratory flow in most patients is 30-60 liters per minute, and this flow rate is required to avoid a situation where the patient is attempting to breathe in against an expiratory (PEEP) valve. The magnitude of PEEP is determined by a spring loaded mechanism on the expiratory valve. When delivered through an endotracheal tube, CPAP can be administered by attaching a PEEP valve to the end of a T-piece, or through a “flow by circuit” within a mechanical ventilator.
For the majority of patients, however, the ability to generate minute ventilation or to inflate poorly compliant lungs, is inadequate. Inspiratory assistance using one of a multitude of modes of ventilation is required.
Ventilator screen of continuous positive airways pressure (CPAP). Note
the elevated baseline pressure and the sine wave pattern or inspiratory flow. The patient negatively deflects the baseline on inspiration, with a slight positive deflection in expiration. Tidal volumes vary from breath to breath.
The mechanics of inspiratory support are more complex than previously considered. It has been established that cyclical inflation and deflation injures lung parenchyma and worsens outcome (1). Large tidal volume ventilation, to “normalize” blood gases has been shown to worsen outcome in lung injury (2). presumably due to excessive pressure induced stretch injury of the parenchyma. Modern ventilation strategy involves attempting to achieve an adequate minute volume with the lowest possible airway pressure (as this relates to the degree of alveolar distension). The pressure that we are interested in minimizing is at the level of the alveolus, the plateau pressure.
The rate, pattern and duration of gas flow control the interplay between volume and pressure. In volume controlled modes, a desired tidal volume is delivered at a specific flow (peak flow) rate, using constant, decelerating or sinusoidal flow patterns: the airway pressure generated may be higher than is desirable. In pressure controlled modes, flow occurs until a preset peak pressure is met over a specified inspiratory period, the flow pattern is always decelerating: the tidal volume may be lower than that desired. Moreover, as pulmonary mechanics change, so too does the delivered tidal volume.
Ventilator “cycling” refers to the mechanism by which the phase of the breath switches from inspiration to expiration. Modes of ventilation are time cycled, volume cycled or flow cycled. Time cycling refers to the application of a set “controlled” breath rate. In “controlled ventilation” a number of mandatory breaths are delivered to the patient at a predetermined interval.
The respiratory rate may be controlled by the operator or the patient. The patient may breathe spontaneously, and with modern ventilators these breaths are supported either by delivering facsimiles of the controlled breaths synchronously with the patient’s effort or by allowing the patient more subjective control. Pressure support is a form of flow cycled ventilation in which the patient triggers the ventilator and a pressure limited flow of gas is delivered. The patient determines the duration of the breath and the tidal volume, which may vary from breath to breath.
(1) Marini JJ. New options for the ventilatory management of acute lung injury. New Horiz 1993; 1(4):489-503.
(2) Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000; 342(18):1301-1308.