1 Kingston-upon-Thames, UK 2 London, UK
EditorWe read with interest the review article by Moloney and Griffiths1 on protective ventilation of patients with acute respiratory distress syndrome (ARDS). Recent insights into pulmonary mechanics have led to a re-evaluation of the role of positive end-expiratory pressure (PEEP) in ventilated ARDS patients. We felt that the authors did not highlight this issue correctly. In their review, the authors have implicated the recruitment of alveoli as a mechanism responsible for improved oxygenation with PEEP. They have also emphasized the need to set PEEP above the lower inflectory point (LIP) on the inspiratory phase of the pressurevolume curve.
The recent understanding of the role of PEEP and strategies for setting optimal PEEP are:
(i) Recruitment of collapsed alveoli occurs during inspiration.
(ii) De-recruitment or collapse of alveoli occurs during expiration.
(iii) PEEP is an expiratory phenomenon and it helps to prevent de-recruitment.
(iv) PEEP might indirectly cause recruitment by increasing end-inspiratory pressure.
(v) The lowest possible value of PEEP that prevents de-recruitment is the collapse pressure. This pressure may be measured on the expiratory limb of the static pressurevolume curve. These measurements are rarely done in clinical practice.
LIP may not necessarily reflect alveolar opening pressure but may be caused by other factors such as the method of measurement of pressurevolume curves.2
The European Society of Intensive Care Medicine3 has suggested the following methods for titrating PEEP:
(i) The best arterial oxygenation method. The theoretical basis is that when PEEP is decreased from a higher towards a lower value, arterial oxygenation (measured by arterial PaO2 or SaO2) will be reduced when de-recruitment occurs because of blood shunting away from the de-recruited lung regions. This method consists of a lung recruitment manoeuvre followed by setting of PEEP at a high level (1820 cm H2O) and then reducing the PEEP gradually (1 cm H2O every 23 min) until the arterial oxygenation decreases. This indicates the collapse pressure. Then a new recruitment manoeuvre is performed and PEEP is set about 1 cm H2O above the collapse pressure.
(ii) The best breath-by-breath compliance method. The theory behind this method is that when PEEP is reduced from a higher towards a lower level, breath-by-breath compliance may first increase to a maximum and then decrease again. The PEEP at which compliance starts to decrease is similar to the collapse pressure. Then a new recruitment manoeuvre is performed and PEEP is set about 1 cm H2O above the collapse pressure.
We feel that a decremental PEEP trial as described in these methods is more in line with current understanding of pulmonary mechanics in ARDS patients.
R. Baba
K. Paramesh
J. W. Zwaal
Kingston-upon-Thames, UK
EditorWe thank Drs Baba and colleagues for their interest in our article. Certainly the concept of optimal PEEP is extremely complex,4 and prescribing the best means of setting PEEP for a patient with acute lung injury was beyond the scope of our article. We did not advocate setting PEEP above the lower inflection point of a pressurevolume curve derived from the patients injured lungs. Rather, we described this manoeuvre as part of the open lung strategy,5 6 and attempted to outline some of the theoretical benefits and practical limitations of this approach. The two approaches suggested in the European Society of Intensive Care Medicine PACTS module on mechanical ventilation3 are both reasonable starting points in our opinion. However, given the lack of evidence guiding this decision, we would recommend continuous reassessment of the patients mechanical ventilation parameters according to their overall condition (lung mechanics, gas exchange and radiology).
M. Griffiths
E. Moloney
London, UK
References
1 Moloney ED, Griffiths MJD. Protective ventilation of patients with acute respiratory distress syndrome. Br J Anaesth 2004; 92: 26170
2 I de Chazal, RD Hubmayr. Novel aspects of pulmonary mechanics in intensive care. Br J Anaesth 2003; 91: 8191
3 Patient Centred Acute Care Training (PACT), European Society of Intensive Care
4 Rouby JJ, Lu Q, Goldstein I. Selecting the right level of positive end-expiratory pressure in patients with acute respiratory distress syndrome. Am J Respir Crit Care Med 2002; 165: 11826
5 Lachmann B. Open up the lung and keep the lung open. Intensive Care Med 1992; 18: 31921[ISI][Medline]
6 Amato MB, Barbas CS, Medeiros DM, et al. Beneficial effects of the open lung approach with low distending pressures in acute respiratory distress syndrome. A prospective randomized study on mechanical ventilation. Am J Respir Crit Care Med 1995; 152: 183546[Abstract]