“The Use of Positive End-Expiratory Pressure in Mechanical Ventilation”
Section snippets
History
Positive pressure ventilation has been used for centuries. Paracelsus introduced a mechanical device to support respiration in resuscitation efforts [4]. By inserting the nozzle of a bellows into the nostrils of apneic patients, he attempted to inflate their lungs. Two centuries later, this method was refined by John Hunter, who used a double-chambered bellows of his own design to ventilate experimental animals [4], [5]. His device induced both positive pressure ventilation and negative
What is positive end-expiratory pressure?
Clinicians are concerned about maintaining the adequacy of two important lung volumes:
- 1.
The tidal volume of each breath and the resting lung volume in between breaths
- 2.
The pressure generated during active inspiration either by the ventilator or the patient will determine the tidal volume (mediated, of course, through compliance). The resting lung volume is determined by the resting transpulmonary pressure [14].
In normal people (with normal lung compliance), the vectorial difference between airway
Extrinsic positive end-expiratory pressure
Inflating a balloon is difficult at first, and then it suddenly gets easier once some volume is inside. As full inflation is reached, it again may become difficult as the limits of the balloon's compliance are reached. If one lets go, the balloon recoils (elastance) and collapses. Alveoli, in many ways, are similar. If they start fully collapsed, they are difficult to inflate at first. Once there is some volume, it becomes easier; this point of change in compliance is referred to as critical
Intrinsic positive end-expiratory pressure
Most patients who are on assisted mechanical ventilation have their own inspiratory effort triggering the ventilator [18]. A threshold for airway pressure is set to trigger the ventilator. To reach this starting point, the patient must initiate an inspiratory effort. The breaths that do not reach this starting point on the ventilator have greater tidal volumes and shorter expiratory times than do breaths that trigger the ventilator.
Each time a person takes a breath, the airway pressure remains
Physiological positive end-expiratory pressure
The benefits of using a small amount PEEP (3 to 5 cm H2O), almost considered physiologic, have been applied to patients who have obstructive airway disease and hyperinflation in an attempt to decrease incomplete exhalation [21].
Indications for the use of positive end-expiratory pressure
Despite decades of clinical debate, PEEP is indicated for patients who have acute lung injury and ARDS. The use of PEEP will improve pulmonary shunting, helping the respiratory muscles to decrease the work of breathing [21]. The addition of PEEP also helps to decrease the amount of infiltrated–atelectatic tissues [18]. PEEP also is used to overcome intrinsic PEEP in patients who have COPD, bronchomalacia, or who have other causes of expiratory flow limitation with hyperinflation [25], [26].
Effects of positive end-expiratory pressure on surface tension
The pulmonary surfactant is a complex of highly active phospholipids, neutral lipids, and four specific surfactant proteins (A, B, C and D) that synthesized in alveolar type II pneumocytes and secreted into the alveolar space [28], [29]. Surfactant lies as a monolayer at the air–liquid interface. This reduces the lung surface tension and prevents alveolar collapse [28].
Ashbaugh and colleagues [1] were the first to demonstrate that surfactant abnormalities had a major role in the pathophysiology
Positive end-expiratory pressure contraindications
As noted previously, the use of PEEP increases intrathoracic pressure, which can decrease venous return into the chest or specifically restrict cardiac filling, both of which may result in reduced cardiac output and hypotension. These hemodynamic changes are worse in hypovolemic patients [34].
When PEEP is added to patients with intracranial abnormalities (ie, intracranial hypertension), it may decrease cerebral perfusion by decreasing mean arterial pressure [35]. Some authors have expressed
Initiation and titration of positive end-expiratory pressure
For most clinicians, an initial setting of PEEP at 5 cm H2O is acceptable, titrating up or down by 2 or 3 cm H2O. It is recommended that after each adjustment of PEEP a complete assessment of pulmonary function, pressure–volume relationships, oxygenation, and hemodynamics—is performed [14]. The goal of titration for ideal PEEP will be defined as the level of PEEP that allows the lowest FiO2 while maintaining adequate oxygenation and avoiding adverse effects [8].
Monitoring positive end-expiratory pressure
This includes monitoring pulmonary function with assessment of gas exchange either by pulse oximetry or arterial blood gases, overseeing plateau pressures and inspiratory:expiratory (I:E) ratios, and looking for changes on lung pressure–volume relationships (Fig. 1).
To monitor the hemodynamic effects of PEEP, it is useful to asses for changes in heart rate, blood pressure, and other parameters of organ perfusion such as urine output. If it is also feasible, monitor changes in cardiac output and
Outpatient use of positive end-expiratory pressure
In patients who have expiratory flow limitation with hyperinflation (eg, COPD, bronchomalacia), long-term oxygen therapy and the application of PEEP can be useful [39]. The use of home mechanical ventilation has a positive impact in patients who are using it. Furthermore, the use of noninvasive ventilator techniques has proved a significant improvement in oxygenation therapy [40].
The positive end-expiratory pressure controversy
The question of whether the use of PEEP actually will reduce mortality rates or number of ventilator use days remains an issue after 40 years of debate. The use of low tidal volumes (which is one of the two important lung volumes that should be balanced in mechanical ventilation) rather than traditional tidal volumes has been proved to decrease the mortality rate and decrease the ventilator use days in patients who have acute lung injury and ARDS [41]. PEEP in and of itself, however, never has
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