The affective dimension of laboratory dyspnea: air hunger is more unpleasant than work/effort

Abstract

Rationale: It is hypothesized that the affective dimension of dyspnea (unpleasantness, emotional response) is not strictly dependent on the intensity of dyspnea.

Objectives: We tested the hypothesis that the ratio of immediate unpleasantness (A(1)) to sensory intensity (SI) varies depending on the type of dyspnea.

Methods: Twelve healthy subjects experienced three stimuli: stimulus 1: maximal eucapnic voluntary hyperpnea against inspiratory resistance, requiring 15 times the work of resting breathing; stimulus 2: Pet(CO(2)) 6.1 mm Hg above resting with ventilation restricted to less than spontaneous breathing; stimulus 3: Pet(CO(2)) 7.7 mm Hg above resting with ventilation further restricted. After each trial, subjects rated SI, A(1), and qualities of dyspnea on the Multidimensional Dyspnea Profile (MDP), a comprehensive instrument tested here for the first time.

Measurements and main results: Stimulus 1 was always limited by subjects failing to meet a higher ventilation target; none signaled severe discomfort. This evoked work and effort sensations, with relatively low unpleasantness (mean A(1)/SI = 0.64). Stimulus 2, titrated to produce dyspnea ratings similar to those subjects gave during stimulus 1, evoked air hunger and produced significantly greater unpleasantness (mean A(1)/SI = 0.95). Stimulus 3, increased until air hunger was intolerable, evoked the highest intensity and unpleasantness ratings and high unpleasantness ratio (mean A(1)/SI = 1.09). When asked which they would prefer to repeat, all subjects chose stimulus 1.

Conclusions: (1) Maximal respiratory work is less unpleasant than moderately intense air hunger in this brief test; (2) unpleasantness of dyspnea can vary independently from perceived intensity, consistent with the prevailing model of pain; (3) separate dimensions of dyspnea can be measured with the MDP.

Figure 1.

Breathing circuit. During stimulus administration, subjects breathed via a tight-fitting facemask connected to a non-rebreathing valve system via a viral filter/rehumidifier (Airlife HEPA; Cardinal Health, McGaw Park, IL). Inspired gas was supplied from a 5-L rubber anesthesia bag; expired gas exited to the room. The subject's minute ventilation was set by the flow rate of gas into the bag (see stimulus descriptions in text). *During stimulus 1, an inspiratory resistance was imposed (14 cm H₂O at 1.0 L · s⁻¹). The resistor was not present during stimuli 2 and 3. We controlled gas flow to the bag and CO₂ concentration to meet the needs of each stimulus.

Figure 2.

Time traces of physiological data and online ratings (visual analog scale [VAS]) during rating focus periods for stimulus 1 (left) and stimulus 2 (right). Data are from subject BN28. Pao = pressure measured in the mask. Horizontal dashed line represents resting Pet_CO2 in this subject. Breaths 3, 4, and 6 in the right panel are examples of inadequate end-tidal samples that were dropped from analysis. Volume = separate inspiratory and expiratory volumes obtained by integrating flow from the two pneumotachometers, reset for each breath.

Figure 3.

Ratings of qualities of respiratory sensation (mean ± SE) during the rating focus periods for stimulus 1, normocapnic hyperpnea with inspiratory resistance; stimulus 2, hypercapnia with moderate restriction of ventilation; stimulus 3, hypercapnia with severe restriction of ventilation. SUFF = “I am smothering, suffocating”; W-E = “My breathing requires work or effort”; A-H = “I cannot get enough air. I feel hunger for air”; TIGHT = “My chest and lungs feel tight, constricted”; HEAVY = “I am breathing a lot; breathing rapidly, deeply or heavily”; CRUSH = “I feel a crushing, heavy sensation in my chest”. Scale maximum definition: “As intense as I can imagine”. Eight of the 12 subjects completed stimulus 3; all completed stimuli 1 and 2.

Figure 4.

Physiological variables during rating focus periods plus the preceding 20 seconds, as well as Pet_CO2 and f_R during resting breathing without mask. Values shown are the mean and SE for all subjects. Rates are normalized to 1 minute. Extensive variables (V̇e, WOB, Vt) were normalized to body weight. Several variables were multiplied by a scaling factor for the figure; factor is noted for each. f_R = breathing frequency (breaths · min⁻¹); Pet_CO2 = end-tidal Pco₂ (mm Hg); PTP = pressure time product (cm H₂O · s · min⁻¹; scaling factor, 0.04); V̇e = minute ventilation (ml · min⁻¹ · kg⁻¹; scaling factor, 0.1); Vt = expiratory tidal volume (ml · kg⁻¹); WOB = external work of breathing (joules · min⁻¹ · kg⁻¹; scaling factor, 25). Pet_CO2 for each time point was calculated based on published air hunger response dynamics (22).

Figure 5.

Sensory intensity (SI) and immediate unpleasantness (A₁) of each stimulus (mean ± SE; left panel). A 10 on the SI scale was defined as “maximum”; 10 on the A₁ scale was defined as “unbearable.” The relative unpleasantness (A₁/SI) ratios (right panel) were significantly greater for those stimuli evoking predominantly air hunger (stimuli 2 and 3). Although 4 of the 12 subjects were not tested with stimulus 3, A₁/SI for the 8 subjects tested was not significantly different from the group of 12 for the other stimuli (0.53 for stimulus 1 and 0.91 for stimulus 2).

Figure 6.

Ratings of A₂ (emotional/evaluative response: depression [DEPR], anxiety [ANX], frustration [FRUST], anger, and fear; 10 defined as “Most severe I can imagine”; mean ± SE).