House Dust Mites and Asthma

Over the last 20 years, the prevalence of asthma has increased. For instance, in Australian children aged 8-11, wheezing and bronchial hyperresponsiveness prevalence have doubled. Asthma has been associated with sensitization to house dust mites (HDM). The greatest reservoir of HDM is in carpets and mattresses. Improvements in symptoms, histamine-induced bronchial hyperactivity, and sensitivity to HDM challenge have been demonstrated when sensitive individuals are moved to antigen-free environs.

This study examined over a five-month period the HDM antigen in the home situations of 88 children with proven HDM sensitivity. Comparisons were made between carpeted and uncarpeted bedrooms, between various types of mattress covers, and other carpeted vs. non-carpeted areas of the homes.

Mattresses that were encased in impermeable covers had a 20-fold less concentration of HDM antigen than typical cotton, wool, or sheepskin covers. This absolute concentration is felt to be below the necessary threshold to induce HDM sensitivity. Concentrations of HDM antigen in house dust were at least five times greater in carpeted than non-carpeted rooms.

Hill et al conclude that excluding carpet and using impermeable mattress covers reduces absolute HDM antigen levels. This type of intervention could have long-term implication in development of asthma and atopic disease on a public health basis. Although topical powdered agents (acaracides) are useful to reduce HDM antigen, they are costly, require substantial effort to use, and are less effective than these simpler measures.

Hill D, et al. J Allergy Clin Immunol 1997;99:323-329.

Clinical Scenario: The rhythm in the figure was obtained from a middle-aged man with a history of chronic obstructive pulmonary disease (COPD). The computerized interpretation assessed this irregularly irregular rhythm as atrial fibrillation with premature ventricular contractions (PVCs). Do you agree?

Interpretation: Although the rhythm in the figure appears to be regular in some parts of the tracing, a slight irregularity is actually present throughout. By far, atrial fibrillation is the most common cause of an irregularly irregular rhythm. However, this is not the cause of irregularity in this case. Instead, definite atrial activity is present in this lead V1 monitoring lead. Close inspection of this atrial activity (i.e., "A Case for Bifocals") reveals ever-so-subtle (but definite) differences both in P wave morphology as well as in the PR interval before each QRS complex. The presence of multiple different P waves in association with an irregularly irregular rhythm identifies this arrhythmia as multifocal atrial tachycardia (MAT). A clue to this diagnosis was present in the history—in that this rhythm is most commonly seen in patients with longstanding pulmonary disease. The rhythm is easy to overlook because atrial activity is not always present in all leads. This emphasizes the importance of routinely obtaining multiple viewpoints (and ideally a full 12-lead ECG) whenever doubt exists as to the etiology of an arrhythmia. The different looking beats in this tracing (i.e., beats #2, 5, and 7) are not PVCs. On the contrary—note that each of these different looking beats is preceded by a P wave. This strongly suggests that the reason for the altered QRS appearance is that these beats are being conducted with aberration (presumably as a result of their early occurrence in the relative refractory period).