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PEEP, Auto-PEEP, and Waterfalls

1989; Elsevier BV; Volume: 96; Issue: 3 Linguagem: Inglês

10.1378/chest.96.3.449

1931-3543

Martin J. Tobin, Robert F. Lodato,

Airway Management and Intubation Techniques

Normally, the lung volume at end-expiration generally approximates the relaxation volume of the respiratory system, ie, the lung volume determined by the static balance between the opposing elastic recoil of the lung and chest wall.1Vinegar A Sinnett EE Leith DE Dynamic mechanisms determine functional residual capacity in mice, Mus musculus..J Appl Physiol. 1979; 46: 867-871PubMed Google Scholar However, in patients with airflow limitation, the end-expiratory volume may no longer be determined by an equilibrium between static forces. Instead, end-expiratory lung volume may exceed predicted functional residual capacity (FRC), because the rate of lung emptying is slowed and expiration is interrupted by the next inspiratory effort before the patient has exhaled to the static relaxation volume.2Kimball WR Leith DE Robins AG Dynamic hyperinflation and ventilator dependence in chronic obstructive pulmonary disease.Am Rev Respir Dis. 1982; 126: 991-995PubMed Google Scholar This is termed dynamic hyperinflation, and the factors that determine its development include increased tidal volume, prolonged time constant (resistance × compliance) for emptying the respiratory system, and a shortened expiratory time.1Vinegar A Sinnett EE Leith DE Dynamic mechanisms determine functional residual capacity in mice, Mus musculus..J Appl Physiol. 1979; 46: 867-871PubMed Google Scholar Such hyperinflation has a number of adverse effects:3Tobin MJ Respiratory muscles in disease.Clinics Chest Med. 1988; 9: 263-285PubMed Google Scholar the respiratory muscles operate at an unfavorable position on their length-tension curve; elastic recoil of the chest wall is directed inwards, thereby causing an extra elastic load; and breathing takes place at the upper, less compliant portion of the pressure-volume curve of the lung. These factors cause a decrease in the efficiency of force generation by the respiratory muscles and an increase in the work of breathing. At normal end-expiration, the static recoil pressure of the respiratory system is zero, whereas a positive value is observed in patients with dynamic hyperinflation.24 This positive recoil pressure has been termed auto-PEEP (positive end-expiratory pressure) or intrinsic-PEEP.4Pepe PE Marini JJ Occult positive end-expiratory pressure in mechanically ventilated patients with airflow obstruction.Am Rev Respir Dis. 1982; 126: 166-170PubMed Google Scholar, 5Rossi A Gottfried SB Zocchi L Higgs BD Lennox S Calverly PMA et al.Measurement of static compliance of the total respiratory system in patients with acute respiratory failure during mechanical ventilation.Am Rev Respir Dis. 1985; 131: 672-677PubMed Google Scholar In patients receiving mechanical ventilation, it has been termed occult-PEEP, because, unlike externally applied PEEP, it is not registered on the ventilator pressure manometer, since the latter is open to atmosphere. If, however, the expiratory port of the ventilator circuit is occluded immediately before the onset of the next breath, the pressure in the lungs and ventilator circuit will equilibrate and the level of auto-PEEP will be displayed on the ventilator manometer.4Pepe PE Marini JJ Occult positive end-expiratory pressure in mechanically ventilated patients with airflow obstruction.Am Rev Respir Dis. 1982; 126: 166-170PubMed Google Scholar Alternatively, the level of auto-PEEP can be estimated from continuous recordings of airway pressure and flow during mechanical ventilation.5Rossi A Gottfried SB Zocchi L Higgs BD Lennox S Calverly PMA et al.Measurement of static compliance of the total respiratory system in patients with acute respiratory failure during mechanical ventilation.Am Rev Respir Dis. 1985; 131: 672-677PubMed Google Scholar Neither of these methods is practical for the detection of auto-PEEP during spontaneous breathing. However, dynamic hyperinflation during spontaneous breathing can be detected by monitoring the change in end-expiratory lung volume using devices that record motion of the chest wall, such as inductive plethysmography.6Tobin MJ Perez W Guenther SM Semmes BJ Mador MJ Allen SJ et al.The pattern of breathing during successful and unsuccessful trials of weaning from mechanical ventilation.Am Rev Respir Dis. 1986; 134: 1111-1118PubMed Google Scholar If thermally stable oscillators are employed, the baseline drift of a DC-coupled inductive Plethysmograph is small,7Hudgel DW Capehart M Johnson B Hill P Robertson D Accuracy of tidal volume, lung volume, and flow measurements by inductance vest in COPD patients.J Appl Physiol. 1984; 56: 1659-1665Crossref PubMed Scopus (15) Google Scholar, 8Dall'Ava-Santucci J Armaganidis A Brunet F Dhainaut JF Chelucci GL Monsallier JF et al.Causes of error of respiratory pressure-volume curves in paralyzed subjects.J Appl Physiol. 1988; 64: 42-49PubMed Google Scholar, 9Watson HL Poole DA Sackner MA Accuracy of respiratory inductive Plethysmographic cross-sectional areas.J Appl Physiol. 1988; 65: 306-308Crossref PubMed Scopus (40) Google Scholar and changes in end-expiratory lung volume provide a reliable measure of changes in FRC, provided there is no motion artifact.10Tobin MJ Jenouri G Birch S Lind B Gonzalez H Ahmed T et al.Effect of positive end-expiratory pressure on breathing patterns of normal subjects and intubated patients with respiratory failure.Crit Care Med. 1983; 11: 859-867Crossref PubMed Scopus (29) Google Scholar, 11Lennox S Mengeot PM Martin JG The contributions of rib cage and abdominal displacements to the hyperinflation of acute bronchospasm.Am Rev Respir Dis. 1985; 132: 679-684PubMed Google Scholar, 12Aronson RM Alex CG Onal E Lopata M Changes in end-expiratory lung volume during sleep in patients with occlusive apnea.J Appl Physiol. 1987; 63: 1642-1647Google Scholar, 13Begle RL Skatrud JB Dempsey JA Ventilatory compensation for changes in functional residual capacity during sleep.J Appl Physiol. 1987; 62: 1299-1306PubMed Google Scholar The increased work of breathing resulting from auto-PEEP can be decreased by therapeutic measures to reduce the level of auto-PEEP, including bronchodilator therapy, employment of a large bore endotracheal tube, decreasing the minute ventilation by controlling fever or pain, and minimizing the ratio of inspiratory time to expiratory time by increasing the inspiratory flow rate or using nondistensable tubing in the ventilator circuit.14Scott LR Benson MS Pierson DJ Effect of inspiratory flowrate and circuit compressible volume on auto-PEEP during mechanical ventilation.Respir Care. 1986; 31: 1075-1079Google Scholar It has also been suggested that this increased work of breathing can be decreased by the application of external PEEP.5Rossi A Gottfried SB Zocchi L Higgs BD Lennox S Calverly PMA et al.Measurement of static compliance of the total respiratory system in patients with acute respiratory failure during mechanical ventilation.Am Rev Respir Dis. 1985; 131: 672-677PubMed Google Scholar, 15Smith TC Marini JJ Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction.J Appl Physiol. 1988; 65: 1488-1499Crossref PubMed Scopus (352) Google Scholar, 16Gay PC Rodarte JC Hubmayer RD The effects of expiratory pressure on isovolume flow and dynamic hyperinflation in patients receiving mechanical ventilation.Am Rev Respir Dis. 1989; 139: 621-626Crossref PubMed Scopus (88) Google Scholar In understanding how this could occur, it is important to remember that inspiratory air flow can occur only when alveolar pressure is decreased below ambient pressure (assuming, for simplicity, a triggering sensitivity of 0 cm H2O if mechanically ventilated). During normal spontaneous inspiration, this is accomplished by only a small decrease in pleural pressure. In the presence of hyperinflation, however, to achieve a decrease in alveolar pressure below ambient pressure, a much greater decrease in pleural pressure is required; this greater decrease in pleural pressure is analogous to the further stretching of an already hyperextended spring. In this setting, if ambient pressure is elevated by the application of external PEEP, inspiration is more easily accomplished because alveolar pressure needs to be decreased only below the level of external PEEP (rather than below zero). Thus, we have the paradox whereby external PEEP, which is most commonly employed to induce hyperinflation in patients with diffuse micro-atelectasis, as in the adult respiratory distress syndrome, is being used to decrease the work of breathing induced by hyperinflation consequent to auto-PEEP! We suggest that this conundrum can be explained by referring to the analogy of a waterfall.17Permutt S Bromberger-Barnea B Bane HN Alveolar pressure, pulmonary venous pressure, and the vascular waterfall.Med Thorac. 1962; 19: 239-260PubMed Google Scholar, 18Pride NB Permutt S Riley RL Bromberger-Barnea B Determinants of maximal expiratory flow from the lungs.J Appl Physiol. 1967; 23: 646-662Crossref PubMed Scopus (251) Google Scholar The auto-PEEP that occurs in patients with airflow limitation is a direct result of critical closure of the airways.16Gay PC Rodarte JC Hubmayer RD The effects of expiratory pressure on isovolume flow and dynamic hyperinflation in patients receiving mechanical ventilation.Am Rev Respir Dis. 1989; 139: 621-626Crossref PubMed Scopus (88) Google Scholar (Note that some patients may have auto-PEEP in the absence of airflow limitation.16Gay PC Rodarte JC Hubmayer RD The effects of expiratory pressure on isovolume flow and dynamic hyperinflation in patients receiving mechanical ventilation.Am Rev Respir Dis. 1989; 139: 621-626Crossref PubMed Scopus (88) Google Scholar) The physiology of critical closure of the airways has been elegantly explained by the analogy of a waterfall,17Permutt S Bromberger-Barnea B Bane HN Alveolar pressure, pulmonary venous pressure, and the vascular waterfall.Med Thorac. 1962; 19: 239-260PubMed Google Scholar, 18Pride NB Permutt S Riley RL Bromberger-Barnea B Determinants of maximal expiratory flow from the lungs.J Appl Physiol. 1967; 23: 646-662Crossref PubMed Scopus (251) Google Scholar where the height of the waterfall reflects the magnitude of the critical closing pressure. One of the most characteristic and peculiar features of this phenomenon is the fact that elevating the pressure downstream (external PEEP) from the site of critical closure (the waterfall) has no influence on either the flow or the pressure upstream (auto-PEEP) from the site of critical closure (top portion of Figure). This situation exists up until the downstream pressure is elevated to a value equal to the critical closing pressure (middle section, Figure). However, once downstream pressure is elevated to a value above the critical closing pressure, the pressure upstream immediately increases and hyperinflation is exacerbated (bottom section in Figure). Thus, in the presence of auto-PEEP, external PEEP should not impede expiratory airflow or cause hyperinflation as long as its value is no higher than the critical closing pressure—we term this the critical level of external PEEP. Another consequence of the waterfall concept is that as external PEEP is increased from zero up until the critical level, there is a progressive apparent decrease in calculated expiratory resistance, (alveolar pressure—external PEEP)/flow; however, this is a spurious decrease because pressure downstream from the waterfall (external PEEP) is not the true backpressure for flow over the waterfall. Indeed, this phenomenon is supported by the data of Smith and Marini,15Smith TC Marini JJ Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction.J Appl Physiol. 1988; 65: 1488-1499Crossref PubMed Scopus (352) Google Scholar who suggest similar, but somewhat different models. Finally, taking the waterfall perspective in patients with airflow limitation presents a certain irony: the external PEEP appears to function much like pressure-support ventilation, in that it augments inspiration but is functionally absent in expiration. In this issue of Chest, Hoffman and colleagues (see page 613) report the use of inductive plethysmography to monitor the change in end-expiratory volume following the application of PEEP in patients displaying auto-PEEP. By noting the level of PEEP at which end-expiratory lung volume increased (what we term the critical level of external PEEP), they were able to obtain a remarkably close estimate of the patient's original level of auto-PEEP. An attractive feature of their technique is that it does not disturb expiration, unlike the occlusive technique, where foreshortening of expiratory time is unavoidable. This foreshortening of expiratory time will overestimate the critical level of external PEEP (height of the waterfall). This prediction of a small systematic difference between the two techniques is indeed supported by the data of Hoffman et al (see their Fig 2) and Gay et al.16Gay PC Rodarte JC Hubmayer RD The effects of expiratory pressure on isovolume flow and dynamic hyperinflation in patients receiving mechanical ventilation.Am Rev Respir Dis. 1989; 139: 621-626Crossref PubMed Scopus (88) Google Scholar While this and other reports15Smith TC Marini JJ Impact of PEEP on lung mechanics and work of breathing in severe airflow obstruction.J Appl Physiol. 1988; 65: 1488-1499Crossref PubMed Scopus (352) Google Scholar, 16Gay PC Rodarte JC Hubmayer RD The effects of expiratory pressure on isovolume flow and dynamic hyperinflation in patients receiving mechanical ventilation.Am Rev Respir Dis. 1989; 139: 621-626Crossref PubMed Scopus (88) Google Scholar cause us to reconsider the common clinical injunction against the use of PEEP in patients with airway obstruction, we need to remain cautious in its use, especially until we have more information regarding the risk of complications. Although external PEEP may help to decrease the work of breathing associated with auto-PEEP, it is important to note that it does nothing to relieve the accompanying hyperinflation, which has other detrimental effects.3Tobin MJ Respiratory muscles in disease.Clinics Chest Med. 1988; 9: 263-285PubMed Google Scholar Thus, it is important to continue with other therapeutic measures to reduce the level of auto-PEEP, as these have potential for decreasing not only the work of breathing, but also hyperinflation. Since the level of auto-PEEP may vary over time, the method of Hoffman et al offers promise, especially in spontaneously breathing patients, as a means of minimizing the level of auto-PEEP and determining if the level of external PEEP is still optimal. We suggest that this technique may also serve as a simple and convenient test of airflow limitation: in flow-limited patients, end-expiratory lung volume should remain unchanged until the critical level of external PEEP is exceeded, whereas in patients without flow limitation, a progressive increase in lung volume (and peak pressures) should occur, starting with even the smallest increments in external PEEP. A final attractive feature of their report is that it provides an example of how respiratory monitoring can be useful in the adjustment of therapy, since, all too often, monitoring-derived data are never used in clinical decision making.19Tobin MJ State of the Art: Respiratory monitoring in the intensive care unit.Am Rev Respir Dis. 1988; 138: 1625-1642Crossref PubMed Scopus (135) Google Scholar