Cervical spinal cord hemisection impacts sigh and the respiratory reset in male rats

Abstract

Cervical spinal cord injury impacts ventilatory and non-ventilatory functions of the diaphragm muscle (DIAm) and contributes to clinical morbidity and mortality in the afflicted population. Periodically, integrated brainstem neural circuit activity drives the DIAm to generate a markedly augmented effort or sigh-which plays an important role in preventing atelectasis and thus maintaining lung function. Across species, the general pattern of DIAm efforts during a normal sigh is variable in amplitude and the extent of post-sigh "apnea" (i.e., the post-sigh inter-breath interval). This post-sigh inter-breath interval acts as a respiratory reset, following the interruption of regular respiratory rhythm by sigh. We examined the impact of upper cervical (C₂ ) spinal cord hemisection (C₂ SH) on the transdiaphragmatic pressure (P_di ) generated during sighs and the post-sigh respiratory reset in rats. Sighs were identified in P_di traces by their characteristic biphasic pattern. We found that C₂ SH results in a reduction of P_di during both eupnea and sighs, and a decrease in the immediate post-sigh breath interval. These results are consistent with partial removal of descending excitatory synaptic inputs to phrenic motor neurons that results from C₂ SH. Following cervical spinal cord injury, a reduction in the amplitude of P_di during sighs may compromise the maintenance of normal lung function.

Keywords: diaphragm; respiration; spinal cord; transdiaphragmatic pressure; ventilation.

FIGURE 1

(a) Representative tracing of P _di during eupnea, sigh, and the immediate post‐sigh breath in PRE‐SHAM, POST‐SHAM, PRE‐C₂SH, and POST‐C₂SH conditions. (b) Scatterplot of P _di (cm H₂O) showing reduction in the POST‐C₂SH (orange) compared to the PRE‐C₂SH (green) condition during eupnea, sigh, and the post‐sigh breath, with no changes between PRE‐SHAM and POST‐SHAM with sigh (except during POST‐C₂SH) having greater P _di than all other groups (behavior: p < 0.0001; type of surgery: p = 0.0002; and time: p < 0.0001). (c) Scatterplot of normalized P _di (P _di%PRE) shows reductions in the POST‐C₂SH compared to PRE‐C₂SH condition during eupnea, sigh, and the post‐sigh breath, with no changes between PRE‐SHAM and POST‐SHAM (behavior: p < 0.0001; type of surgery: p = 0.0002; and time: p < 0.0001). All tests are paired three‐way ANOVAs with Bonferroni post‐tests, n = 8 for all groups. Each scatter point represents the mean value of each rat, * denotes significant differences within behavior between PRE‐C₂SH and POST‐C₂SH groups for P _di during eupnea (p = 0.012), sigh (p < 0.0001) and the post‐sigh breath (p = 0.029) and for normalized P _di%PRE during eupnea (p = 0.011), sigh (p = 0.0001), and the post‐sigh breath (p = 0.010).

FIGURE 2

(a) Scatterplot of breath duration (s) showing increased breath duration of sigh compared to eupnea and the immediate post‐sigh breath, regardless of injury (behavior: p < 0.0001; type of surgery: p = 0.787; and time: p = 0.227). (b) Scatterplot of breath durations normalized within rats to PRE‐injury eupnea values shows extended breath durations during sigh compared to eupnea and the post‐sigh breath, regardless of injury (behavior: p < 0.0001; type of surgery: p = 0.469; and time: p = 0.270). All tests are paired three‐way ANOVAs with Bonferroni post‐tests, n = 8 for all groups. Each scatter point represents the mean value of each rat.

FIGURE 3

(a) Scatterplot of inter‐breath intervals (s) showing increased inter‐breath intervals (IBE) of the post‐sigh breath compared to all other behaviors, regardless of injury (behavior: p < 0.0001; type of surgery: p = 0.659; and time: p = 0.610). (b) Scatterplot of IBE normalized within rats to PRE eupnea values shows longer inter‐breath intervals in the post‐sigh breath regardless of injury (behavior: p < 0.0001; type of surgery: p = 0.924; and time: p = 0.661). All tests are paired three‐way ANOVAs, n = 8 for all groups. Each scatter point represents the mean value of each rat.

FIGURE 4

(a) Scatterplot showing the % of all sighs exhibiting “stereotypical” twice eupnea P _di shows reduction in the % of sighs with canonical P _di amplitudes in POST‐C₂SH compared to PRE‐C₂SH rats (p = 0.0078, Wilcoxon matched‐pairs signed‐rank test). (b) Scatterplot showing the % of all sighs exhibiting “stereotypical” twice eupnea inter‐breath intervals (i.e., post‐sigh respiratory reset) of the post‐sigh breath shows no change with injury in the % of sighs with canonical post‐sigh respiratory reset in PRE‐ and POST‐C₂SH groups (p = 0.373, paired t‐test). Each scatter point represents the mean value of each rat, n = 8 for all groups, * denotes significant differences (p = 0.0078) within behavior between PRE‐ and POST‐C₂SH groups.

FIGURE 5

(a) Scatterplot of all individual sigh P _di (cm H₂O), showing reduced amplitudes in the POST‐C₂SH compared to PRE‐C₂SH timepoints (p < 0.0001). (b) Scatterplot of all individual inter‐breath intervals (s) between sigh and the post‐sigh breath, showing no difference between PRE‐ and POST‐C₂SH timepoints (p = 0.979). In all plots, green symbols represent “stereotypical” sighs of >twice eupneic P _di and post‐sigh respiratory reset, purple symbols represent sighs satisfying “stereotypical” P _di criteria, yellow symbols represent sighs satisfying inter‐breath interval criteria and red symbols represent sighs where neither criterion was satisfied. All tests are unpaired t‐tests, * denotes p < 0.0001, n = 36 (PRE‐C₂SH) and n = 24 (POST‐C₂SH).