Intensive Care Med (1992) 18:190-192

IntensiveCare Medicine 9 Springer-Verlag 1992

Meeting report Round table conference on ventilatory failure, Brussels, Belgium, March 16-18, 1991" J.J. Marini and Ch. Roussos St. Paul Ramsey Medical Center, 640 Jackson Street, St. Pau[, MN 55101-2595, USA

In recent years, there has been explosive growth in our understanding of ventilatory failure (VF), stimulating new approaches based on such advances. The European Society of Intensive Care Medicine sponsored a round table conference on VF prior to the 1lth Scientific Symposium on Intensive care and Emergency Medicine, held in Brussels, on March 16-18, 1991. This 2 1/2 day meeting was organized locally by Jean-Louis Vincent and cochaired by John Marini (Minneapolis/St. Paul, USA) and Charis Roussos (Athens). Participants were major contributors to the basic and clinical science of this discipline. What follows is a brief synopsis of that conference. The meeting began with an overview, delivered by John Marini in the absence of Dudley Rochester. Perhaps the most common approach to VF is to consider the ventilatory requirement and the ability to fulfill that requirement on a continuing basis. VF might be defined as the inability to maintain stable and metabolically appropriate values for PaCO2 and pH without intolerable dyspnea or external assistance. VF can result from an excessive workload, insufficient central drive, or a defective pump. The first segment of the conference was devoted to discussing selected aspects of ventilatory demand. Fran9ois Lemaire described the subcomponents of the breathing workload, i.e., those factors which influence pressure and volume requirements. Dr. Lemaire stressed the relative importance of dynamic hyperinflation and reviewed those factors that contribute to the ventilatory work that accompanies ventilator assisted breathing, an area that has received considerable attention in recent years. Roberto Rodriguez-Roisin followed by focusing on the extrapulmonary and intrapu!monary determinants of pulmonary gas exchange. Dr. Rodriguez-Roisin emphasized the complex interplay among the pulmonary (V/Q matching, shunt, diffusion) and the extra-pulmonary (FiO2, alveolar ventilation, cardiac output, VOz hemo-

* The proceedings of this conference have been published in the series

Update in Intensive and Emergency, (Number 15, Ventilatory Failure, 453 pages, Springer-Verlag, Berlin, 1991)

globin, temperature, pH, Ps0, VCOz) factors modulating arterial and tissue concentrations of respiratory gases. In the second segment attention turned to the response of the pump to ventilatory demand. In the first of two talks related to the failure of neural outflow, Josef Milic-Emili described the control of ventilation and the response of the normal and compromised pump to internal and external loading. Relatively little is known about the factors which influence the choices among various stress-compensating mechanisms made by different individuals responding to similar stress. Jean-William Fitting then addressed the techniques available to assess adequacy of neural stimulation in the laboratory and at the bedside. Although the quantified activity of respiratory muscles provides the most direct index of central neural drive, the difficulty of signal recording and processing prevents its widespread clinical application. The P0.~ and the assessment of sternocleidomastoid activity, however, hold significant promise. The third segment was devoted to selected aspects of muscle weakness and fatigue. To begin, Marc Decramer described the role of the extra-diaphragmatic musculature in breathing, emphasizing the particular importance of the parasternals, sternocleidomastoids, and expiratory muscles in stressful circumstances encountered by weak or critically ill patients. Next, Rolf Hubmayr discussed the dynamics of breathing and coordination, noting that the response to loading is presumably designed to minimize the force or power output needed to accomplish ventilation. Because the compensatory response itself modifies the effective load experienced by the respiratory musculature, indices of fatigue must take into account the dynamic characteristics and response potential of the respiratory system operating under similar loads. Steven Kelsen then addressed the evaluation of respiratory muscle strength, reviewing the causes of weakness, strength adaptations, and assessment techniques. In response to weakness, numerous adaptive adjustments may occur in fiber composition, thoracoabdominal configuration, and sarcomere number to help restore mechanical

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advantage. Stimulating the drive to breathe before airway occlusion, using semi-voluntary, stereotyped maneuvers (such as the sniff Pro), and bypassing the central controller itself (electrophrenic "twitch" Pdi) present promising options for assessing strength in poorly cooperative patients. Charis Roussos discussed neuromuscular fatigue. A decline in force generation by the diaphragm may be partially explained by reduced central motor output. It is not known whether this is due primarily to "central" failure or to an adaptation of the CNS to signals emanating from the fatiguing muscle, thereby averting catastrophic energy depletion. COz retention could reflect a homeostatically effective pattern, not a manifestation of ongoing "chronic fatigue". Bartolome Celli opened our sessions on specific disorders by discussing respiratory muscle training in COPD. Limited but convincing evidence suggests that training improves endurance and function, but not pulmonary mechanics. Dysfunctional breathing patterns often emerge during unsupported use of the arms, and endurance can be noticeably improved by a program targeting arm exercise. The value (or potential harm) of muscle retraining during recovery from VF has not been scientifically proven. Michel Aubier reviewed recent data on pharmacotherapy of the respiratory muscles. Muscle function can be pharmacologically modulated by altering excitation-contraction coupling or the energy supplied to the muscles. Dr. Aubier discussed the data supporting the use of digitalis, aminophylline, and dopamine. The impact of electrolyte imbalance was nicely illustrated in a review of work addressing CO2 retention, acidosis and hypophosphatemia. The importance of the microenvironment was echoed in the subsequent presentation by Susan Pingleton regarding nutrition and VF. Adverse effects of malnutrition upon thoracopulmonary function include altered pulmonary defense mechanisms, decreased ventilatory drive, and impaired muscle contractility. Dr. Pingleton succinctly reviewed an extensive literature addressing the influence of semi-starvation, weight loss, and re-nutrition on ventilatory pump function. Andrea Rossi called attention to the prevalence of dynamic hyperinflation and to its powerful, many-faceted contribution to the VF that accompanies acute airflow obstruction. He emphasized that intrinsic (or auto) PEEP, the correlate of dynamic hyperinflation, can complicate cardiopulmonary monitoring, increase the work of breathing, disrupt the coordination between patient and ventilator, and exert adverse hemodynamic effects. Disorders of the chest wall were described by Dennis McCool. Dr McCool reviewed recent progress in our understanding of three disorders representing this type of ventilatory problem: diaphragmatic paralysis, kyphoscoliosis, and flail chest. Prevention of nocturnal hypoventilation with intermittent ventilatory support appears to be an effective measure for patients with severe problems. In the subsequent presentation, Marc Estenne noted that neuromyopathic VF is not due simply to underlying weakness. Rather, stiffening of the lungs and rib cage,

distortion of the chest wall and inefficient patterns of ventilation markedly increase the functional load. Sleepinduced hypoventilation may precipitate decreased chemosensitivity during wakefulness, and impaired cardiovascular performance may limit energy supply to compromised and overloaded musculature. Claude Gaultier discussed VF in infants and children. Newborns are predisposed to such problems by narrow caliber airways and by a ventilatory pump poorly designed to cope with increased workloads. The newborn utilizes several breathing strategies to contend with its ventilatory disadvantages, including accessory muscle recruitment and "laryngeal braking". These are nullified, however, during by REM, the most prevalent sleep state post-partum. John Marini initiated the sessions devoted to therapy of VF by discussing the targets, hazards, and options of controlled ventilation. Because the therapeutic goals of the ventilatory process determine the need to apply potentially injurious airway pressure, it is essential to identify the minimum effective levels of PaO2, PaCO2, and pH compatible with satisfactory organ function. There is a good rationale for re-targeting therapeutic objectives from the outset of treatment, gradually exchanging "abnormal" blood gases values for safe airway pressures. To avert barotrauma while assuring adequate gas exchange, maximal, minimal, and mean airway pressures must all be monitored and controlled. The clinician may need extended (if not inverse) ratio ventilation, as well as permissive hypercapnia or adjunctive CO2 elimination. Arthur Slutsky next described techniques of unvalved, constant flow ventilation (CFV). In some experimental animals, effective ventilation can be achieved solely by injecting a high continuous flow of fresh gas into the trachea or main bronchi, with the benefits of mechanical simplicity and less exposure of the lung to pressure. Although the precise mechanism for gas transport remains elusive, improved gas mixing and collateral ventilation are likely. Variants of CFV may prove useful adjuncts to low pressure conventional ventilation. Luciano Gattinoni addressed the option of extracorporeal CO2 removal (ECCOzR) for acute lung injury. Here a fraction of the venous blood returning to the right heart first diverts to a membrane that extracts CO2 and oxygenates the diverted flow. Exposure of the lung to high pressure can be obviated, perhaps allowing healing without accentuating tissue injury. Despite the apparent value of ECCOzR for well selected patients managed by experienced teams, ECCOzR remains an experimental technique whose ultimate worth vis-a-vis modified conventional therapy must be judged after current problems with bleeding complications are overcome. Extra-pulmonary, intracorporeal gas exchange was then addressed by J.D. Mortensen, developer of the IVOX catheter. IVOX, an elongated catheter comprised of thousands of small hollow fibers, is a membrane oxygenator and CO2 removal device that rests within the vena cava. As vacuum draws fresh gas through the catheter, Oz and CO2 exchange occur across the heparin-impregnated, gas permeable walls of each fiber. If IVOX proves as safe and effective in its ongoing clinical trials as

192 it has during animal testing, it may eventually provide a supplement to conventional ventilation, or even obviate the need for intubation in selected clinical settings. Proportional assist ventilation (PAV) was described by its originator, Magdy Younes. When a PAV-aided patient makes a spontaneous breathing effort, the machine senses the inspired volume and flow rate, boosting the applied airway pressure proportionately. The ventilator essentially becomes an auxiliary respiratory muscle, while the patient remains in control of the breathing rhythm and flow waveform. In theory, PAV should prove both safe and comfortable; however, a great deal of work is required before the true indications and limits of this attractive new possibility are understood. Laurent Brochard addressed the applications and limitations of pressure support (PS). Although the value of PS in overcoming endotracheal tube resistance and in weaning is uncontested, several characteristics of PS waveshape and duty cycle remain actively investigated. Comfort during PS undoubtedly depends upon the support level in relation to total pressure requirement, the rate of pressure buildup, and the off-trigger criterion. In a recent European multi-center trial of weaning techniques, PS compared favorably to the tested SIMV or TPiece alternatives. Stewart Gottfried examined the value of PEEP in acute VF consequent to airflow obstruction. When exhalation is flow limited during tidal breathing, the addition of PEEP or CPAP counterbalances auto-PEEP, with minimal (but usually perceptible) increase in resting lung volume. Although the muscles remain disadvantaged by hyperinflation, the workload is markedly diminished and triggering effort declines. The addition of CPAP has been reported to improve the synchrony between patient and machine, and to improve coordination of the respiratory muscles. Dr. Gottfried highlighted the role for CPAP in averting intubation and its value during weaning. Pressure release ventilation and bi-PAP were discussed by Jean-Jaques Rouby. In both modes, CPAP is applied and periodically released, thereby providing ventilatory assistance. Although both modes can be applied in paralyzed subjects, they were designed to provide a clinician-variable amount of ventilatory assistance during spontaneous breathing. Already helpful in the outpatient setting for intermittent non-invasive ventilation, these options also prove useful for acutely ill patients in whom partial ventilatory assistance is required but high peak airway pressures must be avoided. Only a minority of mechanically ventilated patients require gradual transition to spontaneous breathing. Drawing from data of the recently completed European cooperative weaning trial, Salvador Benito examined the question of how best to identify those patients requiring continued machine support. Most patients ventilated for COPD and a minority of those with neurological disorders had difficulty with rapid machine withdrawal. Those requiring weaning had undergone a longer period of prior mechanical ventilation, and evidenced a rapid, shallow breathing pattern.

To close the conference, Margaret Branthwaite reviewed recent advances in non-invasive ventilatory support (NVS). There are three common clinical circumstances when the balance between demand and capacity is likely to be finely poised: (1) acute exacerbations of chronic lung disease; (2) difficult weaning from ventilatory support; (3) sleep related disturbances in alveolar ventilation. Sleep occasionally interferes with airway patency, and during the REM phase attenuates or abolished activity of the extradiaphragmatic musculature, often causing hypoventilation, hypoxemia, and (perhaps) depressed chemosensitivity that carries over to the waking period. Improved sleep quality, improved daytime blood gases, and increased activity levels have been demonstrat9ed in a variety of outpatient groups receiving NVS. In the acutely ill, NVS has enormous potential to avoid intubation or facilitate post-extubation management.

Summary It was possible to reach agreement on several important issues relating to VE First, the phenomenon of CO2 retention may have both pathophysiologic and compensatory components. There is increased awareness of the nature, intensity, and significance of the cross-talk between the ventilatory control center and the pump itself, as expressed in breathing pattern and indices of ventilatory drive. We are learning to interpret that information more effectively to assess functional reserve. Second, knowledge concerning the relative importance of various muscle groups is still incomplete, and the impact of disease on muscle function, lung mechanics, and ventilatory control is not fully understood. Dynamic hyperinflation and sleep disturbances provide two clear examples of conditions whose wide-ranging influence on drive, workload, and muscle function was, until quite recently, under appreciated. Finally, there was a general consensus that our therapeutic approaches to VF should be modified to reflect improved understanding of the pathogenesis of CO2 retention and iatrogenic lung injury. In the acute setting, measures to limit alveolar distention, such as controlling airway pressure, revising blood gas targets, and/or using adjunctive methods for blood gas exchange may avoid barotraumatic edema and rupture. The potential for non-invasive ventilation to avert intubation, facilitate ventilator withdrawal, and help patients with chronic VF to achieve compensation without machine dependence is now being actively investigated. This two day conference proved a stimulating forum for interchange of ideas regarding the state of the field, and allowed many opportunities for scientific interaction, both during outside the formal program. The participants are indebted to the European Society of Intensive Care Medicine for its sponsorship, and to the local organizing committee and secretariat for coordinating a professionally valuable and memorable educational event.

Round table conference on ventilatory failure, Brussels, Belgium, March 16-18, 1991.

It was possible to reach agreement on several important issues relating to VF. First, the phenomenon of CO2 retention may have both pathophysiologic a...
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