Correspondence
Correction
Acute Respiratory Distress Syndrome
N Engl J Med 1995; 332:1648-1650June 15, 1995
- Article
To the Editor:
With respect to the review article on the acute respiratory distress syndrome (ARDS) by Kollef and Schuster (Jan. 5 issue),1 only two randomized studies of extracorporeal gas exchange have been published,2,3 and neither could establish that this technique improved the outcome in patients with severe ARDS. However, even in the recently published study by Morris et al.,3 the equipment used was not the most up to date. Since hemorrhage was a major complication of extracorporeal gas exchange with high-dose heparin,2,3 the introduction of covalently heparinized systems in conjunction with percutaneously inserted cannulas was a major step forward, leading to a marked reduction in blood loss.4
In our opinion, the present challenge is to identify the patients who are likely to benefit most from extracorporeal gas exchange. One such group appears to be patients whose underlying disease is curable and who fulfill the “fast entry” criteria of the U.S. Extracorporeal Membrane Oxygenation Study.2 Eleven of the 21 patients we have treated with extracorporeal gas exchange were recruited according to these fast-entry criteria, and all survived.
Since extracorporeal gas exchange is complicated and expensive, we agree with Kollef and Schuster that it cannot be a routine treatment option for patients with ARDS. However, when performed in an appropriate clinical setting, it may improve survival for selected patients.
4 ReferencesMathias Haller, M.D.
Gustav Schelling, M.D.
Josef Briegel, M.D.
Klinikum Grosshadern, 81366 Munich, Germany1
Kollef MH ,Schuster DP . The acute respiratory distress syndrome. N Engl J Med 1995;332:27-37
Full Text | Web of Science | Medline2
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
CrossRef | Web of Science | Medline3
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
Web of Science | Medline4
Knoch M ,Kollen B ,Dietrich G ,Muller 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
Web of Science | Medline
To the Editor:
Kollef and Schuster recommend inverse-ratio ventilation in patients with ARDS and “persistent hypoxemia or peak airway pressures >40–45 cm of water.” However, two recent controlled studies1,2 comparing pressure-controlled inverse-ratio ventilation with conventional volume-controlled ventilation at the same tidal volume and total positive end-expiratory pressure have questioned the value of this approach in patients with ARDS. Indeed, the decreased peak airway pressure was associated with unchanged plateau pressure, a much more relevant indicator of barotrauma than peak airway pressure.3 Thus, as recently stated by Shanholtz and Brower,4 inverse-ratio ventilation “remains of unproven value in the management of ARDS.”
4 ReferencesAlain Mercat, M.D.
Jean-Louis Teboul, M.D.
Christian Richard, M.D.
Hôpital de Bicêtre, 94275 Kremlin-Bicêtre, France1
Mercat A ,Graini 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
CrossRef | Web of Science | Medline2
Lessard MR ,Guerot 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
CrossRef | Web of Science | Medline3
ACCP Consensus Conference: mechanical ventilation. Chest 1993;104:1833-1859
CrossRef | Web of Science | Medline4
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
Web of Science | Medline
To the Editor:
In their review article, Drs. Kollef and Schuster did not mention the measurement of respiratory-system mechanics, specifically pressure–volume relations. Measuring pressure–volume curves in patients with the acute respiratory distress syndrome who are receiving total ventilatory support is useful for several reasons.1-3
First, this method can detect a lower inflection zone, which has a concave shape, at the beginning of the inspiratory limb of the pressure–volume curve. This zone represents lung areas that are potentially recruitable when a positive end-expiratory pressure 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–volume curve. This indicates that total lung capacity has been reached and may consequently help in determining a nonharmful level of tidal volume by avoiding overinflation. Third, it can be used to evaluate the time course of the disease.
The importance and clinical usefulness of pressure–volume curves were recently emphasized at a consensus conference on mechanical ventilation.3 Moreover, no sophisticated equipment is required to fashion pressure–volume curves in order to optimize ventilator settings, since these curves can be easily plotted at the bedside with the data obtained directly from the ventilator.4
4 ReferencesLluis Blanch, M.D., Ph.D.
Rafael Fernández, M.D., Ph.D.
Hospital of Sabadell, 08208 Sabadell, SpainJordi Mancebo, M.D.
Hospital de la Santa Creu i Sant Pau, 08025 Barcelona, Spain1
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
CrossRef | Web of Science | Medline2
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
Web of Science | Medline3
Slutsky AS . Consensus conference on mechanical ventilation -- January 28-30, 1993 at Northbrook, Illinois. Part 1. Intensive Care Med 1994;20:64-79
CrossRef | Web of Science | Medline4
Fernandez 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 | Web of Science | Medline
To the Editor:
In their otherwise excellent review, Kollef and Schuster misquote an article1 on mechanical controlled hypoventilation of which I was a coauthor. Indeed, my colleague and I never proposed correcting hypercapnic acidosis with sodium bicarbonate, even in the presence of low pH. There are several reasons for this.2 First, as has been clearly demonstrated by experimental studies, tolerance to respiratory acidosis is remarkably good. Second, were alkaline therapy required, sodium bicarbonate would not be ideal, since a large part of infused bicarbonate may be rapidly lost in urine3; the sensitivity of pH to bicarbonate concentrations varies inversely with the partial pressure of carbon dioxide, as shown by the Henderson–Hasselbalch equation; and the infusion of bicarbonate generates carbon dioxide from the buffering of protons, and this product cannot be immediately excreted under conditions of deliberate hypoventilation.
In such circumstances, the administration of sodium bicarbonate would still increase the partial pressure of arterial carbon dioxide without correcting extracellular pH.4 It could decrease intracellular pH even further by rapidly diffusing the newly generated carbon dioxide across cell membranes. In severe respiratory acidosis, Carbicarb (an equimolar mixture of sodium bicarbonate and sodium carbonate and a buffering agent that does not generate carbon dioxide) might be a more effective buffer, since it does not produce carbon dioxide and may even consume it. This potential advantage in cases of permissive hypercapnia has to be confirmed in clinical practice.
4 ReferencesClaude H. Perret, M.D.
University Hospital, 1011 Lausanne, Switzerland1
Darioli R ,Perret C . Mechanical controlled hypoventilation in status asthmaticus. Am Rev Respir Dis 1984;129:385-387
Web of Science | Medline2
Feihl F ,Perret C . Permissive hypercapnia: how permissive should we be? Am J Respir Crit Care Med 1994;150:1722-1737
Web of Science | Medline3
Adrogue 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
CrossRef | Web of Science | Medline4
Arieff AI . Indications for use of bicarbonate in patients with metabolic acidosis. Br J Anaesth 1991;67:165-177
CrossRef | Web of Science | Medline
To the Editor:
Kollef and Schuster's excellent review of ARDS does not mention the use of induced hypothermia in this condition. This technique has theoretical potential,1 and though there have been only a few, favorable reports, mostly anecdotal, a recent controlled trial2 suggests it deserves more extensive study. Their review also does not address the vital distinction between hypoxemia and hypoxia in the context of oxygen balance in this condition.3 The failure to make this distinction has led to numerous enthusiastic but unsubstantiated claims for positive end-expiratory pressure (PEEP).
Kollef and Schuster suggest that its chief purpose is to increase lung volume and keep alveoli open — hardly of major importance in the context of multiple-organ dysfunction. As a recent study4 shows, cardiac status rather than respiratory therapy is the key factor in surviving ARDS, and deterioration of cardiac status may be irreversible.
4 ReferencesAlan Gilston, F.F.A.R.C.S.
20 Hocroft Ave., London NW2 2EH, United Kingdom1
Gilston A . Oxygen dynamics and induced hypothermia in sepsis. Resuscitation 1994;28:65-70
CrossRef | Web of Science | Medline2
Villar J ,Slutsky AS . Effects of induced hypothermia in patients with septic adult respiratory distress syndrome. Resuscitation 1993;26:183-192
CrossRef | Web of Science | Medline3
Gilston A . PEEP and oxygen balance: where are the emperor's clothes? Intensive Crit Care Dig 1990;9:7-134
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
CrossRef | Web of Science | Medline
Author/Editor Response
The authors reply:
To the Editor: In general, the letters we received reflect the continued pressure to refine and improve supportive therapy for patients with ARDS. Thus, as we said at the beginning of our article, as our understanding of ARDS and its optimal clinical management continues to evolve, our recommendations for treatment will need to be modified.
We appreciate Dr. Gilston's bringing to our attention the possibility of using mild hypothermia during ARDS. Routine application would depend on the results of larger prospective trials. As to the use of PEEP, we noted that tissue oxygen delivery may be preferable as an end point for monitoring the effects of PEEP to gas-exchange variables alone. Likewise, the recommendation of Blanch et al. to use bedside measurements of pressure–volume curves to help titrate PEEP and lung volume makes eminent physiologic sense. However, the advantage of using one strategy over any other has not been determined.
We agree with Dr. Mercat and colleagues that inverse-ratio ventilation is of unproved value in ARDS. Our qualified recommendation certainly indicates that we do not yet have great confidence in this mode of ventilatory support. Nevertheless, it does remain a viable alternative for patients with unacceptable arterial oxygenation or excessive airway pressures, especially in the context of an overall strategy to avoid overdistention of diseased and even normal lung units.
As we noted in our article, various forms of extracorporeal support for patients with ARDS are more popular in Europe than in the United States, despite the lack of supportive evidence that the outcome is favorably affected. Dr. Haller and colleagues may be right that newer forms of technology may be associated with improved outcomes, especially in selected patients. The challenge, of course, will be to define and document exactly which patients to select.
We did not mean to misquote Drs. Darioli and Perret. Our reference to their article1 was to the preceding sentence, which referred to the fact that patients usually tolerate gradual increases in the partial pressure of arterial carbon dioxide without adverse effects.
Finally, we wish to correct the omission of one reference. Footnotes in Table 2 and Table 3 mistakenly referred to a reference by Kollef et al.2 The correct reference should have been to Schuster and Kollef.3
3 ReferencesDaniel P. Schuster, M.D.
Marin H. Kollef, M.D.
Washington University School of Medicine, St. Louis, MO 63110-10931
Darioli R ,Perret C . Mechanical controlled hypoventilation in status asthmaticus. Am Rev Respir Dis 1984;129:385-387
Web of Science | Medline2
Kollef MH, Wragge T, Pasque C. Determinants of mortality and multiorgan dysfunction in cardiac surgery patients requiring prolonged mechanical ventilation. Chest (in press).
3
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).
