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University Hospital, Newark, NJ
Correspondence to: John R. Bach, MD, FCCP, Physical Medicine and Rehabilitation, University Hospital, B-403, 150 Bergen St, Newark, NJ 07103-2406; e-mail: bachjr{at}umdnj.edu
To the Editor:
I read with interest the article by Dr. Miske et al1 on the use of the mechanical in-exsufflator in pediatric patients with neuromuscular disease and impaired cough. The article demonstrates how this device can be used in conjunction with noninvasive ventilation to avert respiratory failure and the need to resort to tracheotomy. Additional points can be made, however. While the authors reported that daily accustomization to the device resulted in more effective use during chest infections, they also pointed out that 8% of the patients did not want to use it routinely, thinking that the device was ineffective. Firstly, the authors did not distinguish between the use of the CoughAssist (J.H. Emerson; Cambridge, MA) for deep lung expansion from its use for assisted coughing. The patients undoubtedly found the device ineffective when they were well because they had no need to cough out secretions at that time. They should have been told of the importance of the daily use of air stacking (reference 6)1 or deep (positive pressure) insufflation therapy for regular lung expansion. While we recommend deep lung expansion for all patients with restrictive lung conditions, we have never found any need to use exsufflation except when cough assistance is needed. Perhaps if the patients had understood that regular lung expansion is important to maintain pulmonary "elasticity" and that the accompanying exsufflation is only needed for assisted coughing, these 8% would have been more compliant. Secondly, we have always found it extremely important to institute abdominal thrusts during the exsufflation cycling of the machine to maximize cough flows. Whether it is used via tracheostomy or noninvasive interface, it is much more effective with the thrusts for obvious reasons.
Patients initially experiencing chest pain with its use are usually experiencing intercostal muscle stretch. This usually occurs on the convex side of scoliotic curves. These patients need to begin insufflation therapy at lower pressures and build up gradually. Another point is that these authors used maximum expiratory pressure to gauge expiratory muscle weakness, whereas we use assisted peak cough flows. It must be kept in mind that maximum expiratory pressure can be normal but will not generate effective cough flows unless the airway is patent. This is not always the case for these patients with vocal cord weakness and upper airway stridor. Thus, cough flow measurements are preferable in general. We have also found that the use of oximetry as feedback to the patient is very effective in indicating effective airway secretion clearance. Thus, besides making it "easier to breathe," immediate increases in vital capacity and oxygen saturation also signal clinical efficacy. Finally, it is not true that quality-of-life studies have not been performed in this population.234
References
Children’s Hospital of Philadelphia, Philadelphia, PA
Correspondence to: Laura J. Miske, MSN, Children’s Hospital of Philadelphia, 34th St and Civic Center Blvd, 5th Floor Wood Building, Philadelphia, PA 19104
To the Editor:
Drs. Kang and Bach1 raise several interesting methodologic issues regarding use of the mechanical in-exsufflator (MI-E) in patients with neuromuscular disease. As noted, the use of air stacking has been shown to preserve inspiratory capacity in adults with neuromuscular weakness. The degree to which such a maneuver affects lung or chest wall mechanics in young children with neuromuscular disease, however, is not known. Unlike adults with neuromuscular weakness who have abnormally stiff chest walls,23 children with neuromuscular weakness studied up to 4 years of age demonstrate abnormally high chest wall compliance.4 Those authors speculated that relative immobility and reduced exposure to normal mechanical stress during growth and development of chest wall structures contributes to the abnormally high compliance. The effects of passive deep lung inflation, therefore, could have important effects on both maturation and function of the chest wall and lung in children with neuromuscular weakness, and should be carefully studied. We sought to demonstrate that the use of the MI-E was safe, well tolerated, and effective in preventing pulmonary complications in pediatric patients with neuromuscular disease. We did not assess changes in lung or chest wall mechanics resulting from MI-E use in this group of patients as part of our study, but agree that similar studies would be useful. Additionally, 86% of our patients were receiving assisted positive pressure ventilation during the observation period of our report. Thus, any effect of air stacking with the MI-E would have been difficult to distinguish from the respiratory system stretch that patients received all or part of the day from their mechanical ventilatory support.
We agree that abdominal thrusts, used in concert with the MI-E, will increase expiratory flow rates. Little information is available, however, as to the youngest age at which such a maneuver is considered safe. The potential added benefit of performing abdominal thrusts in very young children during MI-E use must be balanced against the known risk of causing trauma to abdominal organs associated with the maneuver.56
We also agree that a preferred method of detecting the risk for inadequate airway clearance is the use of peak cough flow measurements. A subject’s ability to compress the airway to generate supramaximal flow transients, which result in increased velocity of flow and more efficient secretion removal, however, has been associated with measurements of maximal expiratory pressure.56 Critical values of cough peak flow established by Drs. Bach and Saporito7 to determine risk for inadequate airway clearance were obtained in adult patients with neuromuscular weakness. Few data on normal cough peak flow rates in children have been published.89 We have found that cough peak flows < 270 L/min develop in healthy children < 6 years of age with no underlying pulmonary or neuromuscular disease.10 This is the cutoff below which use of cough assistance has been recommended for adults with neuromuscular weakness.11 Until the pediatric range of normal values and the effects of maturation on cough peak flows are better described, it seems reasonable to rely on the maximal expiratory pressure, which has been more fully characterized in children to detect those at risk for inadequate airway clearance. Finally, we appreciate Drs. Kang and Bach pointing out some of the more recent quality-of-life studies that have been published in this group of patients.
References
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