To the Editor: With respect to the review article on the acuterespiratory distress syndrome (ARDS) by Kollef and Schuster(Jan. 5 issue),1 only two randomized studies of extracorporealgas exchange have been published,2,3 and neither could establishthat this technique improved the outcome in patients with severeARDS. However, even in the recently published study by Morriset al.,3 the equipment used was not the most up to date. Sincehemorrhage was a major complication of extracorporeal gas exchangewith high-dose heparin,2,3 the introduction of covalently heparinizedsystems in conjunction with percutaneously inserted cannulaswas a major step forward, leading to a marked reduction in bloodloss.4
In our opinion, the present challenge is to identify the patientswho are likely to benefit most from extracorporeal gas exchange.One such group appears to be patients whose underlying diseaseis curable and who fulfill the "fast entry" criteria of theU.S. Extracorporeal Membrane Oxygenation Study.2 Eleven of the21 patients we have treated with extracorporeal gas exchangewere recruited according to these fast-entry criteria, and allsurvived.
Since extracorporeal gas exchange is complicated and expensive,we agree with Kollef and Schuster that it cannot be a routinetreatment option for patients with ARDS. However, when performedin an appropriate clinical setting, it may improve survivalfor selected patients.
Mathias Haller, M.D. Gustav Schelling, M.D. Josef Briegel, M.D. Klinikum Grosshadern 81366 Munich, Germany
References
Kollef MH, Schuster DP. The acute respiratory distress syndrome. N Engl J Med 1995;332:27-37. [Free Full Text]
Zapol WM, Snider MT, Hill JD, et al. Extracorporeal membrane oxygenation in severe acute respiratory failure: a randomized prospective study. JAMA 1979;242:2193-2196. [Abstract]
Morris AH, Wallace CJ, Menlove RL, et al. Randomized clinical trial of pressure-controlled inverse ratio ventilation and extracorporeal CO2 removal for adult respiratory distress syndrome. Am J Respir Crit Care Med 1994;149:295-305. [Abstract]
Knoch M, Köllen B, Dietrich G, Müller E, Mottaghy K, Lennartz H. Progress in veno-venous long-term bypass techniques for the treatment of ARDS -- controlled clinical trial with the heparin-coated bypass circuit. Int J Artif Organs 1992;15:103-108. [Medline]
To the Editor: Kollef and Schuster recommend inverse-ratio ventilationin patients with ARDS and "persistent hypoxemia or peak airwaypressures >40–45 cm of water." However, two recentcontrolled studies1,2 comparing pressure-controlled inverse-ratioventilation with conventional volume-controlled ventilationat the same tidal volume and total positive end-expiratory pressurehave questioned the value of this approach in patients withARDS. Indeed, the decreased peak airway pressure was associatedwith unchanged plateau pressure, a much more relevant indicatorof barotrauma than peak airway pressure.3 Thus, as recentlystated by Shanholtz and Brower,4 inverse-ratio ventilation "remainsof unproven value in the management of ARDS."
Alain Mercat, M.D. Jean-Louis Teboul, M.D. Christian Richard,M.D. Hôpital de Bicêtre 94275 Kremlin-Bicêtre,France
References
Mercat A, Graïni L, Teboul JL, Lenique F, Richard C. Cardiorespiratory effects of pressure-controlled ventilation with and without inverse ratio in the adult respiratory distress syndrome. Chest 1993;104:871-875. [Free Full Text]
Lessard MR, Guérot E, Lorino H, Lemaire F, Brochard L. Effects of pressure-controlled with different I:E ratios versus volume-controlled ventilation on respiratory mechanics, gas exchange, and hemodynamics in patients with adult respiratory distress syndrome. Anesthesiology 1994;80:983-991. [Medline]
ACCP Consensus Conference: mechanical ventilation. Chest 1993;104:1833-1859. [Free Full Text]
Shanholtz C, Brower R. Should inverse ratio ventilation be used in adult respiratory distress syndrome? Am J Respir Crit Care Med 1994;149:1354-1358. [Abstract]
To the Editor: In their review article, Drs. Kollef and Schusterdid not mention the measurement of respiratory-system mechanics,specifically pressure–volume relations. Measuring pressure–volumecurves in patients with the acute respiratory distress syndromewho are receiving total ventilatory support is useful for severalreasons.1,2,3
First, this method can detect a lower inflection zone, whichhas a concave shape, at the beginning of the inspiratory limbof the pressure–volume curve. This zone represents lungareas that are potentially recruitable when a positive end-expiratorypressure level close to the end-inflection point is used. Second,it can detect an upper inflection zone, which has a convex shape,at the end of the inspiratory limb of the pressure–volumecurve. This indicates that total lung capacity has been reachedand may consequently help in determining a nonharmful levelof tidal volume by avoiding overinflation. Third, it can beused to evaluate the time course of the disease.
The importance and clinical usefulness of pressure–volumecurves were recently emphasized at a consensus conference onmechanical ventilation.3 Moreover, no sophisticated equipmentis required to fashion pressure–volume curves in orderto optimize ventilator settings, since these curves can be easilyplotted at the bedside with the data obtained directly fromthe ventilator.4
Lluis Blanch, M.D., Ph.D. Rafael Fernández, M.D., Ph.D. Hospital of Sabadell 08208 Sabadell, Spain
Jordi Mancebo, M.D. Hospital de la Santa Creu i Sant Pau 08025Barcelona, Spain
References
Matamis D, Lemaire F, Harf A, Brun-Buisson C, Ansquer JC, Atlan G. Total respiratory pressure-volume curves in the adult respiratory distress syndrome. Chest 1984;86:58-66. [Free Full Text]
Ranieri VM, Giuliani R, Fiore T, Dambrosio M, Milic-Emili J. Volume-pressure curve of the respiratory system predicts effects of PEEP in ARDS: "occlusion" versus "constant flow" technique. Am J Respir Crit Care Med 1994;149:19-27. [Abstract]
Slutsky AS. Consensus conference on mechanical ventilation -- January 28-30, 1993 at Northbrook, Illinois. Part 1. Intensive Care Med 1994;20:64-79. [CrossRef][Medline]
Fernández R, Blanch L, Artigas A. Inflation static pressure-volume curves of the total respiratory system determined without any instrumentation other than the mechanical ventilator. Intensive Care Med 1993;19:33-38. [CrossRef][Medline]
To the Editor: In their otherwise excellent review, Kollef andSchuster misquote an article1 on mechanical controlled hypoventilationof which I was a coauthor. Indeed, my colleague and I neverproposed correcting hypercapnic acidosis with sodium bicarbonate,even in the presence of low pH. There are several reasons forthis.2 First, as has been clearly demonstrated by experimentalstudies, tolerance to respiratory acidosis is remarkably good.Second, were alkaline therapy required, sodium bicarbonate wouldnot be ideal, since a large part of infused bicarbonate maybe rapidly lost in urine3; the sensitivity of pH to bicarbonateconcentrations varies inversely with the partial pressure ofcarbon dioxide, as shown by the Henderson–Hasselbalchequation; and the infusion of bicarbonate generates carbon dioxidefrom the buffering of protons, and this product cannot be immediatelyexcreted under conditions of deliberate hypoventilation.
In such circumstances, the administration of sodium bicarbonatewould still increase the partial pressure of arterial carbondioxide without correcting extracellular pH.4 It could decreaseintracellular pH even further by rapidly diffusing the newlygenerated carbon dioxide across cell membranes. In severe respiratoryacidosis, Carbicarb (an equimolar mixture of sodium bicarbonateand sodium carbonate and a buffering agent that does not generatecarbon dioxide) might be a more effective buffer, since it doesnot produce carbon dioxide and may even consume it. This potentialadvantage in cases of permissive hypercapnia has to be confirmedin clinical practice.
Claude H. Perret, M.D. University Hospital 1011 Lausanne, Switzerland
References
Darioli R, Perret C. Mechanical controlled hypoventilation in status asthmaticus. Am Rev Respir Dis 1984;129:385-387. [Medline]
Feihl F, Perret C. Permissive hypercapnia: how permissive should we be? Am J Respir Crit Care Med 1994;150:1722-1737. [Medline]
Adrogué HJ, Brensilver J, Cohen JJ, Madias NE. Influence of steady-state alterations in acid-base equilibrium on the fate of administered bicarbonate in the dog. J Clin Invest 1983;71:867-883.
Arieff AI. Indications for use of bicarbonate in patients with metabolic acidosis. Br J Anaesth 1991;67:165-177. [Free Full Text]
To the Editor: Kollef and Schuster's excellent review of ARDSdoes not mention the use of induced hypothermia in this condition.This technique has theoretical potential,1 and though therehave been only a few, favorable reports, mostly anecdotal, arecent controlled trial2 suggests it deserves more extensivestudy. Their review also does not address the vital distinctionbetween hypoxemia and hypoxia in the context of oxygen balancein this condition.3 The failure to make this distinction hasled to numerous enthusiastic but unsubstantiated claims forpositive end-expiratory pressure (PEEP).
Kollef and Schuster suggest that its chief purpose is to increaselung volume and keep alveoli open — hardly of major importancein the context of multiple-organ dysfunction. As a recent study4shows, cardiac status rather than respiratory therapy is thekey factor in surviving ARDS, and deterioration of cardiac statusmay be irreversible.
Alan Gilston, F.F.A.R.C.S. 20 Hocroft Ave. London NW2 2EH, UnitedKingdom
References
Gilston A. Oxygen dynamics and induced hypothermia in sepsis. Resuscitation 1994;28:65-70. [Medline]
Villar J, Slutsky AS. Effects of induced hypothermia in patients with septic adult respiratory distress syndrome. Resuscitation 1993;26:183-192. [CrossRef][Medline]
Gilston A. PEEP and oxygen balance: where are the emperor's clothes? Intensive Crit Care Dig 1990;9:7-13.
Steltzer H, Krafft P, Fridrich P, Hiesmayr M, Hammerle AF. Right ventricular function and oxygen transport patterns in patients with acute respiratory distress syndrome. Anaesthesia 1994;49:1039-1045. [Medline]
The authors reply:
To the Editor: In general, the letters we received reflect thecontinued pressure to refine and improve supportive therapyfor patients with ARDS. Thus, as we said at the beginning ofour article, as our understanding of ARDS and its optimal clinicalmanagement continues to evolve, our recommendations for treatmentwill need to be modified.
We appreciate Dr. Gilston's bringing to our attention the possibilityof using mild hypothermia during ARDS. Routine application woulddepend on the results of larger prospective trials. As to theuse of PEEP, we noted that tissue oxygen delivery may be preferableas an end point for monitoring the effects of PEEP to gas-exchangevariables alone. Likewise, the recommendation of Blanch et al.to use bedside measurements of pressure–volume curvesto help titrate PEEP and lung volume makes eminent physiologicsense. However, the advantage of using one strategy over anyother has not been determined.
We agree with Dr. Mercat and colleagues that inverse-ratio ventilationis of unproved value in ARDS. Our qualified recommendation certainlyindicates that we do not yet have great confidence in this modeof ventilatory support. Nevertheless, it does remain a viablealternative for patients with unacceptable arterial oxygenationor excessive airway pressures, especially in the context ofan overall strategy to avoid overdistention of diseased andeven normal lung units.
As we noted in our article, various forms of extracorporealsupport for patients with ARDS are more popular in Europe thanin the United States, despite the lack of supportive evidencethat the outcome is favorably affected. Dr. Haller and colleaguesmay be right that newer forms of technology may be associatedwith improved outcomes, especially in selected patients. Thechallenge, of course, will be to define and document exactlywhich patients to select.
We did not mean to misquote Drs. Darioli and Perret. Our referenceto their article1 was to the preceding sentence, which referredto the fact that patients usually tolerate gradual increasesin the partial pressure of arterial carbon dioxide without adverseeffects.
Finally, we wish to correct the omission of one reference. Footnotesin Table 2 and Table 3 mistakenly referred to a reference byKollef et al.2 The correct reference should have been to Schusterand Kollef.3
Daniel P. Schuster, M.D. Marin H. Kollef, M.D. Washington UniversitySchool of Medicine St. Louis, MO 63110-1093
References
Darioli R, Perret C. Mechanical controlled hypoventilation in status asthmaticus. Am Rev Respir Dis 1984;129:385-387.
Kollef MH, Wragge T, Pasque C. Determinants of mortality and multiorgan dysfunction in cardiac surgery patients requiring prolonged mechanical ventilation. Chest (in press).
Schuster DP, Kollef MH. Acute respiratory distress syndrome. In: Rippe JM, Irwin RS, Fink MP, Cerra FB, eds. Intensive care medicine. 3rd ed. Boston: Little, Brown (in press).
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