WildLife Group of the SAVA
also inhibit the actions of vagal motorneurones which promote dilation of vocal folds ( Lalley 2003 ), thereby causing an increase in upper airway resistance ( Lalley 2008 ). In addition , opioids cause catatonia ( Haveman & Kuschinsky 1982 ), which results in generalised muscle rigidity and a decrease in the chest wall compliance ( Chen et al . 1996 ). They also increase the resistance to chest wall expansion by tonically activating expiratory neurones during inspiration ( Lalley 2003 ; Lalley 2008 ).
The respiratory depressant effects of the opioids are not limited to their effects on respiratory neurones in the central neuronal network . Opioids have a direct action on carotid bodies , which causes a decrease in chemoreceptor discharge and thus further depresses the response to hypoxia and hypercapnia ( Pattinson 2008 ; Santiago & Edelman 1985 ). Opioids also activate pulmonary J receptors which may cause opioid-induced apnoea ( Bowdle 1998 ; McKenzie 1993 ; Santiago & Edelman 1985 ; Shook et al . 1990 ). The combination of all these effects will lead to a decrease in respiratory pump action and cause hypoventilation with its associated pathophysiological effects . The effects of the opioids , on both the central and peripheral neurones , therefore play a major role in decreasing pulmonary ventilation . However , the respiratory depressant effects of the opioids are not limited to just the effects on ventilation ; opioids also have marked effects on the cardiovascular system that may cause a decrease in the perfusion both of the lungs and other organs thus decreasing oxygen delivery to tissues .
The effects of opioids on the cardiovascular system are complex and dependent on the opioid and dose used ( Bowdle 1998 ). However , the opioids used during chemical immobilization of wildlife generally cause systemic and pulmonary vasoconstriction ( Haigh 1990 ; Heard et al . 1990 ; Heard et al . 1996 ; Santiago & Edelman 1985 ) by either activating the sympathetic nervous system ( Hannon & Bossone 1991 ; Heard et al . 1990 ; Heard et al . 1996 ; Roquebert & Delgoulet 1988 ) or possibly by causing the release of histamine ( Hakim et al . 1992 ; Mather 1994 ). These vascular effects would result in a decrease in blood perfusion of body tissues , and would cause ventilation perfusion mismatching or shunting of blood in the lungs , and a decrease in oxygen diffusion across the alveolar membrane ( Nunn 1993 ). Few studies have assessed pulmonary blood shunting and oxygen diffusion during opioid immobilization of animals . Two of the studies that have done so , have shown that opioid immobilization causes a decrease in oxygen diffusion in immobilized rhinoceros ( Wenger et al . 2007 ) and elephant ( Still et al . 1996 ).
Although much is known about the neurological control of respiration , and the effects of opioids on this control , a better and more integrative understanding of the effects of the opioids on cardio-respiratory control and function is needed , especially if more effective treatments for opioidinduced respiratory depression are to be established , particularly during chemical immobilization .
References Ballanyi K , Lalley PM , Hoch B , Richter DW . 1997 . cAMPdependent reversal of opioid- and prostaglandin-mediated depression of the isolated respiratory network in newborn rats . J . Physiol . 504 : 127-134 . Bowdle TA . 1988 . Clinical pharmacology of antagonists of narcotic-induced respiratory depression . A brief review . Acute Care 12 : 70-76 . Bowdle TA . 1998 . Adverse effects of opioid agonists and agonist-antagonists in anaesthesia . Drug Safety 19 : 173-189 . Chen S , Maguire PA , Davies MF , Beatty MF , Loew GH . 1996 . Evidence for μ1-opioid receptor involvement in fentanylmediated respiratory depression . Eur . J . Pharmacol . 312 : 241-244 . Haigh JJ . 1990 . Opioids in zoological medicine . J . Zoo Wildl . Med . 21 : 391-413 . Haji A , Takeda R , Okazaki M . 2000 . Neuropharmacology of control of respiratory rhythm and pattern in mature mammals . Pharmacol . Therapeut . 86 : 277-304 . Hakim TS , Grunstein MM , Michel RP . 1992 . Opiate action in the pulmonary circulation . Pulm . Pharmacol . 5 : 159-165 . Hannon JP , Bossone CA . 1991 . Cardiovascular and pulmonary effects of morphine in conscious pigs . Am . J . Physiol . Reg .
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