Wavelet Analysis of Electrical Activities from Respiratory Muscles during Coughing and Sneezing in Anaesthetized Rabbits

Despite high behavioural similarity, some differences in the central neural control of the cough and sneeze reflexes have been suggested. The main aim of our study was to analyze and compare characteristics of electromyographic (EMG) activities of the respiratory muscles during these two behaviours. Data were taken from eight adult rabbits under pentobarbital anaesthesia. We compared diaphragm EMG activities in tracheobronchial cough, sneeze, and quiet breathing during inspiration. Electromyograms were read from the abdominal muscles during the expiratory phases of coughing and sneezing. Due to the non-stationary character of electromyographic signals, we used wavelet analysis to determine the time-frequency distribution of energy during the behaviours. Inspiratory durations of all above mentioned behaviours were similar. The maximum inspiratory power occurred later in sneeze than during quiet inspiration (P < 0.05). The total inspiratory power during sneeze was higher compared to that in cough (P < 0.05) and quiet inspiration (P < 0.01). Lower frequencies contributed to this increase significantly more in sneeze compared to cough (less than 287.5 Hz, P < 0.05; 287.5 Hz up to 575 Hz, P < 0.01). We found similar energy distribution for coughing and quiet inspiration. Its maximum occurred at lower frequency in quiet inspiration compared to sneezing (P < 0.01). The abdominal burst during cough was longer than that in sneezing (P < 0.001). Our results support the concept that both cough and eupnoeic inspiration are generated by similar neuronal structures. A non-specific mechanism producing expiratory activity during tracheobronchial cough and sneeze is suggested. Defensive respiratory reflexes, neuronal control of breathing and airway reflexes, multiresolution analysis, EMG Cough and sneeze are important defensive airway reflexes. They significantly contribute to physiological function of the respiratory system by active removal of irritants and noxious agents from the appropriate parts of the airways. Both reflexes differ from the eupnoea mainly in active and powerful expiratory expulsions, and also in accelerated deep preparatory inspirations (Korpas and Tomori 1979; Jakus et al. 2004). Despite the behavioural similarities, the cough and sneeze reflexes are different kinds of behaviours. Sneeze is considered a stereotype reflex while cough presents in a variety of forms (Korpas and Tomori 1979; Widdicombe 1995). The preparatory inspiration of sneeze is gradational with occasional interruptions that are atypical in coughing. The expiratory airflow in sneezing is directed mainly through the nose by an elevation of the tongue (Satoh et al. 1998). The cough expiration is solely through the mouth. The differences in central controls of cough and sneeze were indicated as well (Jakus et al. 2004). The motor patterns of breathing, coughing, the expiration reflex, and probably other responses result from the activity of multifunctional population of the neurons in a plastic neural network(s) located in the rostral ventrolateral medulla (Shannon et al. 1996; 1998; ACTA VET. BRNO 2009, 78: 387-397; doi:10.2754/avb200978030387 Address for correspondence: Ing. Juliana Knocikova Institute of Medical Biophysics Jessenius Faculty of Medicine Comenius University Mala Hora 4, 037 54 Martin, Slovak Republic Phone: +421 43 422 14 22 Fax: +421 43 413 14 26 E-mail: knocikova@chello.sk http://www.vfu.cz/acta-vet/actavet.htm Baekey et al. 2004). For example, it has been documented that the expiratory motor output in coughing is functionally related to the expiratory units of Botzinger complex (BOT) being situated in the rostral ventrolateral medulla (Bongianni et al. 1998). Central neuronal patterns are transmitted to spinal motoneurons, thus forming the common inspiratory and expiratory pre-motor and motor pathways of breathing, coughing, sneezing, and other motor behaviours (Miller et al 1995; Wallois et al. 1992; Shannon et al. 1996; Iscoe 1998; Bianchi et al. 1995). Inspiratory pre-motoneurons are located within the intermedial ventral respiratory group (VRG) and dorsal respiratory group (DRG) of the medulla, whereas the expiratory pre-motoneurons occupy mainly the caudal VRG of the medulla. Inspiratory and expiratory “pump” muscles are driven through the phrenic, intercostal, and lumbar nerves. Electrical signals propagating through the nerves and muscles carry information about the neuronal components generating the signals. Characteristics of their activities, including timing, intensity, frequency composition and other properties might be specific for individual behaviours. Power spectral analysis derived from the Fourier transformation is a commonly used method of frequency analysis (Ackerson et al. 1983; Barani et al. 2005). Typical characteristics of the power spectrum in respiratory motor output are the centroid frequency (Watchko et al. 1987) and high frequency oscillations (HFOs). HFOs detected from the inspiratory motor output during eupnoea are presumably generated within the respiratory central pattern generator CPG (Cohen et al. 1979, 1987). Variances of synchronization in respiratory outputs during transition between different motor patterns were explained by the respiratory network reconfiguration and alterations in the circuitry associated with the motor pools (Marchenko and Rogers 2006). Fourier transformation is suitable mainly for stationary signals while in contrast, respiratory motor outputs contain variable and markedly non-stationary signals especially during the airway reflexes. Hence, the wavelet transformation, which allows a multiresolution analysis, might be a more sensitive tool for analysis of non-stationary signals (Meyer 1993). The aim of our study was to analyze and compare the frequency composition of electromyographic (EMG) activity in the diaphragm and abdominal muscles during cough, sneeze, and quiet breathing. We hypothesized that wavelet analysis would expose significant differences in the frequency characteristics of inspiratory and expiratory outputs during these behaviours. Additionally, wavelet analysis may reveal specific neuronal components involved in the generation of inspiratory and expiratory activities during cough and sneeze. Materials and Methods Basic experimental procedures Experiments were performed on 8 adult rabbits (chinchilla) of both sexes (3.83 ± 0.52 kg). The EMG inspiratory activities of the diaphragm (DIA) and expiratory activities of abdominal muscles (ABD) were analyzed in 55 tracheobronchial (TB) coughs and 48 sneezes. Moreover, DIA activity was determined in 45 quiet inspirations. Anaesthesia was induced by a mixture of ketamine (Narkamon, Spofa; 25 mg/kg) and xylazine (Rometar, Spofa; 5 mg/kg) i.m. Subsequently, a cannula was introduced into the femoral vein and during next 2 h multiple small doses of pentobarbital i.v. (Vetbutal, Polfa) were used to replace the original anaesthesia (full dose of 30-40 mg/kg). Pentobarbital was then used to maintain a proper anaesthetic level for the remainder of the experiment. Atropine (0.15 mg/kg, i.v.) was given at the beginning of the experiment to reduce airway secretions along with hydrocortison (2 mg/kg i.v.) used to decrease a brain swelling later during the experiment. A plastic tube was interposed into the trachea and the animal was allowed to breathe spontaneously a gas mixture of 30–50% oxygen. Arterial blood pressure (BP) was measured through a cannula placed in femoral artery. The arterial BP, End-tidal CO2 concentration (ETCO2), respiratory rate (RR), and body temperature were continuously monitored (body temperature was maintained within 38–40 °C). Samples of arterial blood were taken periodically for blood gas analysis and the metabolic acidosis control. In order to detect the intrathoracic pressure changes a small balloon was inserted into the oesophagus (oesophageal pressure recording, EP). Animals were placed prone in a stereotaxic frame and the dorsal surface 388