Overall, the same 3-factor / 7-indicator measurement model fit the Study Day 1 and Day 0 data and the COPD data very satisfactorily and substantially better than any alternative model. All 3 factors had strong internal consistency reliability regardless of diagnosis or study day. To our knowledge, this is the first time that a specific measurement model of dyspnea sensory quality has been validated in an independent sample using CFA methods (which supports the internal validity of the 3-factor measurement model), and it also is the first time a model derived in patients of one diagnosis has been validated in patients of another diagnosis (which supports its external validity). External validity of the measurement model is further supported by replication of the WORK-EFFORT, SMOTHERING-AIR HUNGER, and TIGHTNESS factors in an independent sample41,42,45
among patients in a different geographic region than the original study. In addition, robustness of fit of the 3-factor model over successive days characterized by very substantial change in ratings supports its construct validity.
We considered whether coexisting HF and COPD might be a possible explanation for the model fitting data from patients of either diagnosis. However, we believe this is unlikely because less than 10% of our HF sample had a diagnosis of COPD, and, in all cases, we confirmed that HF was the diagnosis driving the admission. In the earlier study, only 15% of participants had a history of HF in addition to COPD, but unless they were being treated with corticosteroids or beta-agonists (neither of which is indicated in HF), patients with HF were excluded.
As with any correlational method, CFA does not establish a causal basis for observed associations. Distinct sensory-perceptual mechanisms underlying effort perception, tightness, and air hunger are supported by substantial experimental and neuroimaging evidence.1,6,7,9,10,12,14,18-21,24,27,49-59
However, it would not be valid to infer that there are only three sensory quality factors or that only these three matter to patients with acute or exacerbated cardiopulmonary disease. The findings of this study also do not rule out the possibility that there may be other sensory quality factors that may be more diagnosis-specific. Rather, the 3-factor / 7-descriptor model represents sensory quality factors that can be reliably measured with relatively few items, at least for patients with exacerbated HF or COPD. Further research would be required to determine how well this measurement model fits other diagnoses.
A number of earlier studies focused on the possibility that certain combinations of sensory descriptor clusters might better characterize a particular diagnosis versus others and therefore might be useful in differential diagnosis.2,5,8,11
However, at least one study concluded that such clusters of descriptors may not be sufficiently specific for that purpose.25
Regardless of diagnosis, the number of afferent pathways and subcortical to cortical brain regions involved in processing and awareness of respiratory sensations is finite.60-65
Which stimuli and which pathways predominate at any given time may differ between diagnoses or even within diagnoses as clinical condition changes, but there is no compelling evidence that afferent or higher central nervous system pathways differ by diagnosis in patients with cardiopulmonary disease.66-71
Although few, if any, participants in the present study or the earlier COPD study spontaneously reported “air hunger” in open-ended statements prior to seeing the descriptor questionnaire, many patients in both samples endorsed the statement “I feel
] a hunger for air”
on the questionnaire, and ratings for that item correlated more strongly and consistently with ratings for smothering
than with the other descriptors. Numerous physiological investigations have focused on the sensation of “air hunger” in relation to increases in CO2
-mediated reflex inspiratory drive,6,7,18,49-52,54,59
or, more generally, when inspiratory drive exceeds a constrained ventilatory capacity.1,15,21,53,56,57
Few participants in either the HF or the COPD study had arterial blood gas measurements, and, of those who did, only a small number had elevated Pco2
. Therefore, it seems likely that the confluence of ratings of air hunger
, and suffocating
in both samples was due to factors other than increased Pco2
Limitations of the present study include convenience sampling, a preponderance of male participants, and a relatively small sample size for CFA.45
In addition, there was a lack of direct correspondence between study day and hospital day, and, in consequence, a variable recall interval for the day 0 ratings.
Convenience sampling is difficult to avoid in a clinical study with prospective data collection in an acute care setting. Recruitment was necessarily contingent on clinical condition and confirmation that HF was, indeed, a major focus of inpatient care. In addition, participation necessarily was limited to those who were willing to participate in an observational study that did not offer any prospect of direct clinical benefit. Although our sample was disproportionately male, there were no statistically significant differences by sex between those excluded versus eligible, nor, among the eligible, between those agreeing versus those declining to participate. Excluding non-English speakers was a limitation, but, even so, nearly 40% of the sample was Hispanic.
The possible impact of the relatively small sample and non-normally distributed data on statistical analyses was mitigated by using a robust estimator appropriate for small samples in which multivariate normality cannot be assumed. We preferred this approach to statistical transformations of variables because transformation is generally ineffective when scores are constrained to a narrow range,72,73
and the model was robust to substantial differences in item scores and factor variances as well as the reversal in the direction of skewness from Study Day 0 to Study Day 1).
Despite the lack of one-to-one correspondence between study day and hospital day, the recall interval, though not standardized, was relatively short. In the original COPD study, test-retest reliability of recall descriptor ratings during an ED visit was strong.23
In the present study, the changes in descriptor means and in the skewness of ratings between Day 0 and Day 1 reflect expected characteristics of score distributions in relation to aggressive, inpatient treatment over a relatively short interval.
Lastly, it is important to note that dyspnea encompasses multiple sensory qualities that vary not only in intensity,29
but in their unpleasantness,74
and in emotional responses (eg, overall distress and judgments about the seriousness and significance of what is felt).27,37,74-76
Thus, validation of a measurement model of dyspnea sensory quality in conjunction with ratings of descriptor intensity does not, by itself, capture the emotional dimension of the symptom. Research to address that limitation is ongoing.
Even with these limitations, this study demonstrates several strengths beyond using CFA. Confirmation of a diagnosis of HF among participants was essentially 100%. Any exclusions based on judgments that HF was not a major focus of inpatient management were made prospectively, based on clinical information available during rounds and through discussion with members of the cardiology services of the 2 facilities. There were no post hoc exclusions. Moreover, we did not exclude on the basis of presumed mechanism of HF (systolic vs. diastolic dysfunction) or functional consequence (eg, reduced vs. preserved ejection fraction), because HF is a heterogeneous condition, and ejection fraction and the underlying pathophysiology of HF may not always be ascertainable at the time of admission.
Our results have several notable clinical and research implications. First, nearly all currently available dyspnea measures have been developed and tested primarily in ambulatory settings.29,77,78
Therefore, their construct validity is uncertain with respect to patients in an acute care setting. Secondly, many such measures are diagnosis-specific, but in the context of an emergency department visit or hospital admission, the underlying diagnosis is not always immediately known, and chronic pulmonary and cardiac conditions frequently coexist. Our findings suggest that diagnosis-specific measures may not be necessary to reliably assess changes in dyspnea in response to treatment of exacerbated chronic cardiopulmonary disease in acute care settings.