Respiratory mechanics and morphometric changes during anesthesia with ketamine in normal rats

Comparison of the respiratory effects of commonly utilized general anaesthesia regimes in male Sprague-Dawley rats

Background: Respiratory parameters in experimental animals are often characterised under general anaesthesia. However, anaesthesia regimes may alter the functional and mechanical properties of the respiratory system. While most anaesthesia regimes have been shown to affect the respiratory system, the effects of general anaesthesia protocols commonly used in animal models on lung function have not been systematically compared. Methods: The present study comprised 40 male Sprague-Dawley rats divided into five groups (N = 8 in each) according to anaesthesia regime applied: intravenous (iv) Na-pentobarbital, intraperitoneal (ip) ketamine-xylazine, iv propofol-fentanyl, inhaled sevoflurane, and ip urethane. All drugs were administered at commonly used doses. End-expiratory lung volume (EELV), airway resistance (Raw) and tissue mechanics were measured in addition to arterial blood gas parameters during mechanical ventilation while maintaining positive end-expiratory pressure (PEEP) values of 0, 3, and 6 cm H2O. Respiratory mechanics were also measured during iv methacholine (MCh) challenges to assess bronchial responsiveness. Results: While PEEP influenced baseline respiratory mechanics, EELV and blood gas parameters (p < 0.001), no between-group differences were observed (p > 0.10). Conversely, significantly lower doses of MCh were required to achieve the same elevation in Raw under ketamine-xylazine anaesthesia compared to the other groups. Conclusion: In the most frequent rodent model of respiratory disorders, no differences in baseline respiratory mechanics or function were observed between commonly used anaesthesia regimes. Bronchial hyperresponsiveness in response to ketamine-xylazine anaesthesia should be considered when designing experiments using this regime. The findings of the present study indicate commonly used anaesthetic regimes allow fair comparison of respiratory mechanics in experimental animals undergoing any of the examined anaesthesia protocols.

Paced breathing and phrenic nerve responses evoked by epidural stimulation following complete high cervical spinal cord injury in rats

Spinal cord injury (SCI) at the level of cervical segments often results in life-threatening respiratory complications and requires long-term mechanical ventilator assistance. Thus restoring diaphragm activity and regaining voluntary control of breathing are the primary clinical goals for patients with respiratory dysfunction following cervical SCI. Epidural stimulation (EDS) is a promising strategy that has been explored extensively for nonrespiratory functions and to a limited extent within the respiratory system. The goal of the present study is to assess the potential for EDS at the location of the phrenic nucleus (C₃-C₅) innervating the diaphragm: the main inspiratory muscle following complete C₁ cervical transection. To avoid the suppressive effect of anesthesia, all experiments were performed in decerebrate, C₁ cervical transection, unanesthetized, nonparalyzed ( n = 13) and paralyzed ( n = 7) animals. Our results show that C₄ segment was the most responsive to EDS and required the lowest threshold of current intensity, affecting tracheal pressure and phrenic nerve responses. High-frequency (200-300 Hz) EDS applied over C₄ segment (C₄-EDS) was able to maintain breathing with normal end-tidal CO₂ level and raise blood pressure. In addition, 100-300 Hz of C₄-EDS showed time- and frequency-dependent changes (short-term facilitation) of evoked phrenic nerve responses that may serve as a target mechanism for pacing of phrenic motor circuits. The present work provides the first report of successful EDS at the level of phrenic nucleus in a complete SCI animal model and offers insight into the potential therapeutic application in patients with high cervical SCI. NEW & NOTEWORTHY The present work offers the first demonstration of successful life-supporting breathing paced by epidural stimulation (EDS) at the level of the phrenic nucleus, following a complete spinal cord injury in unanesthetized, decerebrate rats. Moreover, our experiments showed time- and frequency-dependent changes of evoked phrenic nerve activity during EDS that may serve as a target mechanism for pacing spinal phrenic motor networks.

Impact of respiratory pattern on lung mechanics and interstitial proteoglycans in spontaneously breathing anaesthetized healthy rats

AIM

The aim of this study was to investigate the effect of different pattern of spontaneous breathing on the respiratory mechanics and on the integrity of the pulmonary extracellular matrix.

METHODS

Experiments were performed on adult healthy rats in which different spontaneously breathing pattern was elicited through administration of two commonly used anaesthetic mixtures: pentobarbital/urethane (P/U) and ketamine/medetomidine (K/M). The animals (five per group) were randomized and left to spontaneously breath for 10 min (P/U-sham; K/M-sham) or for 4h (P/U-4h; K/M-4h), targeting the anaesthesia level to obtain a tidal volume of about 8 mL kg(-1) body wt. At the end of the experiment, lung matrix integrity was assessed through determination of the glycosaminoglycans (GAGs) content in the lung parenchyma.

RESULTS

Compared with K/M, anaesthesia with P/U cocktail induced: (1) a higher respiratory rate and minute ventilation attained with lower P(a) CO(2) ; (2) a higher pressure-time-product and work of breathing per minute; (3) a lower static lung compliance; (4) an increased activation of lung tissue metalloproteases; and (5) greater extraction of pulmonary interstitial GAGs.

CONCLUSIONS

This study suggests that the breathing pattern induced by the different anaesthetic regimen may damage the pulmonary interstitium even during spontaneous breathing at physiological tidal volumes.