Quantifying hypoxia-induced chemoreceptor sensitivity in the awake rodent

Abstract

We evaluated several methods for characterizing hypoxic chemosensitivity in the conscious rat. Adult Sprague-Dawley rats (n = 30) were exposed to normobaric hypoxia [inspired oxygen fraction (Fio2) 0.15, 0.12, and 0.09]. We measured ventilation (V̇e; barometric plethysmography), arterial oxygen saturation (SpO2; pulse oximeter), and oxygen consumption and carbon dioxide production (V̇o2 and V̇co2; analysis of expired air). Linear regression analysis was used to define stimulus-response relationships. Testing was performed on 2 days to assess day-to-day reproducibility. Exposure to graded, steady-state hypoxia caused progressive reductions in SpO2 that were, for any given Fio2, quite variable (SpO2 range, 20-30%) among individuals. Hypoxia produced progressive increases in V̇e caused by increases in both tidal volume (VT) and breathing frequency. Hypoxia also increased the VT:inspiratory time (Ti) ratio, an indicator of central respiratory "drive." Hypoxia caused consistent, progressive declines in V̇o2, V̇co2, and core temperature (>20% at the lowest SpO2). We propose that optimal quantification of carotid chemoreceptor hypoxic sensitivity in the unanesthetized rodent should employ SpO2 [a surrogate for arterial Po2 (PaO2 )] as the stimulus variable and the ventilatory equivalent for V̇co2 (V̇e/V̇co2) and/or mean inspiratory flow rate (VT/Ti) normalized for V̇co2 as the response variables. Both metrics take into account not only the important influence of a falling metabolic rate, but also SpO2, which represents the hypoxic stimulus at the carotid body. Because of the somewhat curvilinear nature of these responses, exposure to multiple levels of graded hypoxia provides the most complete characterization of hypoxic chemosensitivity.

Keywords: chemoreflex; hypoxic ventilatory response; metabolic rate.

Fig. 1.

A, top: minute ventilation (V̇e) increased in a curvilinear fashion during graded reductions in inspired oxygen fraction (Fio₂). B: this increase was contributed to by increases in both breathing frequency (f_B) and tidal volume (VT). A, bottom: falling Fio₂ also elicited increases in VT/inspiratory time (T_i), an index of “central respiratory drive,” which was caused by both increased VT and reduced T_i. Values shown are means ± SE (n = 30; each rat's responses on day 1 and day 2 were averaged before generating group means).

Fig. 2.

A: means (circles and dashed line) and individual values (diamonds) depicting decreases in arterial oxygen saturation (SpO₂) during graded reductions in Fio₂ in all 60 plethysmograph tests, i.e., 2 tests per rat. Note the variability in SpO₂ (carotid body stimulus level) for any given Fio₂. B: means and individual values showing increases in V̇e/oxygen consumption (V̇o₂) during graded reductions in Fio₂ (n = 60; 2 tests per rat). Interindividual variation in the ventilatory response (V̇e/V̇o₂ for any given Fio₂) would be expected to result in widely ranging alveolar Po₂, which may explain, at least in part, variation in SpO₂.

Fig. 3.

Hypoxia-induced increases in V̇e (a) and VT/T_i (B) plotted vs. SpO₂. Values are means ± SE (n = 30; see above).

Fig. 4.

Individual responses and group mean values (circles and heavy lines) for V̇o₂ (A) and carbon dioxide production (V̇co₂) (B) during exposure to graded hypoxia (n = 30; see above).

Fig. 5.

Core temperature is plotted every 15 s to illustrate the time course of responses to graded reductions in Fio₂. Temperature fell within 3–5 min of the transition to a lower Fio₂. Data shown are mean values (solid line) and SE (dashed lines) (n = 6).

Fig. 6.

Acute hypoxia-induced increases in V̇e (A) and central respiratory “drive” (B) are shown in relation to V̇co₂ as Fio₂ is progressively reduced from 0.21 to 0.09. Hyperventilation is evident, as V̇e increases in a curvilinear fashion despite the falling V̇co₂ (see text for further explanation). Values are means ± SE (n = 30; see above).

Fig. 7.

Ventilatory equivalent for V̇co₂ (A) and mean inspiratory flow rate (VT/T_i) normalized for V̇co₂ (B) plotted vs. SpO₂. We propose that these are the most relevant estimates of peripheral chemoreceptor hypoxic sensitivity in the awake rodent. Values are means ± SE (n = 30; see above).

Fig. 8.

Day-to-day reproducibility of the slopes of the relationships between ventilatory equivalent for V̇co₂ (A) and mean inspiratory flow rate (VT/T_i) normalized for V̇co₂ (B) vs. SpO₂ in 30 rats. The dashed line is the line of identity. CV, coefficient of variation; p, P value associated with the paired t-test comparing day 1 mean with day 2 mean.