Contribution of peripheral chemoreception to the depression of the hypoxic ventilatory response during halothane anesthesia in cats

The Effects of Volatile Anesthetics on Renal Sympathetic and Phrenic Nerve Activity during Acute Intermittent Hypoxia in Rats

Coordinated activation of sympathetic and respiratory nervous systems is crucial in responses to noxious stimuli such as intermittent hypoxia. Acute intermittent hypoxia (AIH) is a valuable model for studying obstructive sleep apnea (OSA) pathophysiology, and stimulation of breathing during AIH is known to elicit long-term changes in respiratory and sympathetic functions. The aim of this study was to record the renal sympathetic nerve activity (RSNA) and phrenic nerve activity (PNA) during the AIH protocol in rats exposed to monoanesthesia with sevoflurane or isoflurane. Adult male Sprague-Dawley rats (n = 24; weight: 280-360 g) were selected and randomly divided into three groups: two experimental groups (sevoflurane group, n = 6; isoflurane group, n = 6) and a control group (urethane group, n = 12). The AIH protocol was identical in all studied groups and consisted in delivering five 3 min-long hypoxic episodes (fraction of inspired oxygen, FiO2 = 0.09), separated by 3 min recovery intervals at FiO2 = 0.5. Volatile anesthetics, isoflurane and sevoflurane, blunted the RSNA response to AIH in comparison to urethane anesthesia. Additionally, the PNA response to acute intermittent hypoxia was preserved, indicating that the respiratory system might be more robust than the sympathetic system response during exposure to acute intermittent hypoxia.

Volatile anaesthetic depression of the carotid body chemoreflex-mediated ventilatory response to hypoxia: directions for future research

In assessing whether volatile anaesthetics directly depress the carotid body response to hypoxia it is necessary to combine in meta-analysis studies of when it is "functionally isolated" (e.g., recordings are made from its afferent nerve). Key articles were retrieved (full papers in English) and subjected to quantitative analysis to yield an aggregate estimate of effect. Results from articles that did not use such methodology were assessed separately from this quantitative approach, to see what could be learned also from a nonquantitative overview. Just 7 articles met the inclusion criteria for hypoxia and just 6 articles for hypercapnia. Within these articles, the anaesthetic (mean dose 0.75, standard deviation (SD) 0.40 minimum alveolar concentration, MAC) statistically significantly depressed carotid body hypoxic response by 24% (P = 0.041), but a similar dose (mean 0.81 (0.42) MAC) did not affect the hypercapnic response. The articles not included in the quantitative analysis (31 articles), assessed qualitatively, also indicated that anaesthetics depress carotid body function. This conclusion helps direct future research on the anaesthetic effects on putative cellular/molecular processes that underlie the transduction of hypoxia in the carotid body.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Respiratory function during anesthesia: effects on gas exchange

Anaesthesia causes a respiratory impairment, whether the patient is breathing spontaneously or is ventilated mechanically. This impairment impedes the matching of alveolar ventilation and perfusion and thus the oxygenation of arterial blood. A triggering factor is loss of muscle tone that causes a fall in the resting lung volume, functional residual capacity. This fall promotes airway closure and gas adsorption, leading eventually to alveolar collapse, that is, atelectasis. The higher the oxygen concentration, the faster will the gas be adsorbed and the aleveoli collapse. Preoxygenation is a major cause of atelectasis and continuing use of high oxygen concentration maintains or increases the lung collapse, that typically is 10% or more of the lung tissue. It can exceed 25% to 40%. Perfusion of the atelectasis causes shunt and cyclic airway closure causes regions with low ventilation/perfusion ratios, that add to impaired oxygenation. Ventilation with positive end-expiratory pressure reduces the atelectasis but oxygenation need not improve, because of shift of blood flow down the lung to any remaining atelectatic tissue. Inflation of the lung to an airway pressure of 40 cmH2O recruits almost all collapsed lung and the lung remains open if ventilation is with moderate oxygen concentration (< 40%) but recollapses within a few minutes if ventilation is with 100% oxygen. Severe obesity increases the lung collapse and obstructive lung disease and one-lung anesthesia increase the mismatch of ventilation and perfusion. CO2 pneumoperitoneum increases atelectasis formation but not shunt, likely explained by enhanced hypoxic pulmonary vasoconstriction by CO2. Atelectasis may persist in the postoperative period and contribute to pneumonia.

Anesthetics and control of breathing

An important side effect of general anesthetics is respiratory depression. Anesthetics have multiple membrane targets of which ionotropic receptors such as gamma-aminobutyric acid-A (GABA(A)), glycine, N-methyl-D-aspartate and nicotinic acetylcholinergic (nACh) receptors are important members. GABA, glutamate and ACh are crucial neurotransmitters in the respiratory neuronal network, and the ability of anesthetics to modulate their release and interact with their receptors implies complex effects on respiration. Metabotropic receptors and intracellular proteins are other important targets for anesthetics suggesting complex effects on intracellular signaling pathways. Here we briefly overview the effects of general anesthetics on protein targets as far as these are relevant for respiratory control. Subsequently, we describe some methods with which the overall effect of anesthetics on the control of breathing can be measured, as well as some promising in vivo approaches to study their synaptic effects. Finally, we summarize the most important respiratory effects of volatile anesthetics in humans and animals and those of some intravenous anesthetics in animals.

Effects of volatile anesthetics on carotid body response to hypoxia in animals

This study was a systematic review of the anesthetic effect on carotid body response to hypoxia. We undertook a systematic literature search (electronic plus manual) for full-paper articles in English that used methodologies enabling any anesthetic effect to be located to the carotid body. We found just 7 articles that met our inclusion criteria, incorporating 16 separate studies. Anesthetic (mean dose +/- SD 0.70 +/- 0.33 MAC) significantly depressed carotid body response by 24% (p = 0.041). There were no differences between individual agents (halothane, enflurane, isoflurane) and no influence of the use of neuromuscular blockade or of species (although the data were sufficiently sparse to interpret such sub-group analysis with caution).

Postoperative pulmonary complications

PURPOSE OF REVIEW

Postoperative pulmonary complications, including pneumonia, bronchospasm, respiratory failure and prolonged mechanical ventilation, occur commonly and are a significant source of morbidity and mortality. This review will discuss the etiology of postoperative pulmonary complications and the interventions that reduce their risk.

RECENT FINDINGS

General anesthesia and surgery produce changes in the respiratory system and are responsible, along with underlying conditions, for postoperative pulmonary complications. Risk factors include upper abdominal or thoracic surgery, cigarette smoking, chronic respiratory disease, emergency surgery, anesthetic time of 180 min or more, age greater than 70 years, renal failure, poor nutritional status, and significant intraoperative blood loss. The inhibition of phrenic nerve output results in postoperative diaphragmatic dysfunction. Sleep-disordered breathing occurs after surgery even in patients without obstructive sleep apnea, but patients with obstructive sleep apnea may have a worsening of their disease after surgery. A clear advantage of one anesthetic technique over another in reducing postoperative pulmonary complications has not been demonstrated. Conflicting results have been obtained regarding the value of epidural analgesia in preventing postoperative pulmonary complications. Incentive spirometry decreases rates of postoperative pulmonary complications and hospital lengths of stay.

SUMMARY

Understanding risk factors for the development of postoperative pulmonary complications allows targeted interventions aimed at reducing their frequency and severity. Further research is needed to define the role of regional analgesic and anesthetic techniques in reducing postoperative pulmonary complications, and also to define the nature of risk factors and develop better predictive models of patients at risk of developing postoperative pulmonary complications.

Preoperative assessment: pulmonary

Understanding the risk factors for the development of PPCs allows targeted interventions aimed at reducing the frequency and severity of PPCs. The broad categories of what increases the likelihood of developing a PPC are understood but specific understanding of how individual risk factors act to cause PPCs is lacking,and there is little information regarding the interaction or synergy between risk factors. Further research is needed to define the nature of risk factors and develop better predictive models of patients at risk for developing PPCs. It is clear that anesthetic agents produce significant changes in the respiratory system but further information is needed to define how such changes contribute, if at all, to the subsequent development of PPCs. The ongoing controversy regarding the value of regional analgesia or anesthetic techniques, especially epidural analgesia and anesthesia, in reducing or preventing PPCs requires well-done randomized clinical trials. Further research is also needed in the area of postoperative care such as interventions in patients with OSA or the use of inventive spirometric techniques.

An oxygen-, acid- and anaesthetic-sensitive TASK-like background potassium channel in rat arterial chemoreceptor cells

The biophysical and pharmacological properties of an oxygen-sensitive background K+ current in rat carotid body type-I cells were investigated and compared with those of recently cloned two pore domain K+ channels. Under symmetrical K+ conditions the oxygen-sensitive whole cell K+ current had a linear dependence on voltage indicating a lack of intrinsic voltage sensitivity. Single channel recordings identified a K+ channel, open at resting membrane potentials, that was inhibited by hypoxia. This channel had a single channel conductance of 14 pS, flickery kinetics and showed little voltage sensitivity except at extreme positive potentials. Oxygen-sensitive current was inhibited by 10 mM barium (57% inhibition), 200 microM zinc (53% inhibition), 200 microM bupivacaine (55% inhibition) and 1 mM quinidine (105 % inhibition). The general anaesthetic halothane (1.5%) increased the oxygen-sensitive K+ current (by 176%). Halothane (3 mM) also stimulated single channel activity in inside-out patches (by 240%). Chloroform had no effect on background K+ channel activity. Acidosis (pH 6.4) inhibited the oxygen-sensitive background K+ current (by 56%) and depolarised type-I cells. The pharmacological and biophysical properties of the background K+ channel are, therefore, analogous to those of the cloned channel TASK-1. Using in situ hybridisation TASK-1 mRNA was found to be expressed in type-I cells. We conclude that the oxygen- and acid-sensitive background K+ channel of carotid body type-I cells is likely to be an endogenous TASK-1-like channel.