Peripheral neural modulation of endotoxin-induced hyperventilation

Hyperoxia in critically ill patients with sepsis and septic shock: a systematic review

BACKGROUND

In septic patients, hyperoxia may help with its bactericidal effects, but it may cause systemic impairments. The role of hyperoxia and the appropriate oxygen target in these patients is unknown. The aim of this systematic review was to summarize the available literature.

METHODS

We conducted a systematic search screening PubMed and Cochrane Library. Studies on adult patients with sepsis or septic shock and admitted to ICU addressing the topic of hyperoxia were included and described.

RESULTS

We included 12 studies, for a total of 15.782 included patients. Five studies were randomized controlled trials (RCTs) or analyses from RCTs, three were prospective observational studies, and four were retrospective observational studies. The definition of hyperoxia was heterogeneous across the included studies. Mortality was the most frequent outcome: six studies showed an increased rate or risk of mortality with hyperoxia, three found no differences, and one a protective effect of hyperoxia. At the critical appraisal assessment stage, no major methodological flaws were detected, except for a single-center, pilot study, with a lack of adjustment for confounders and imbalance between the groups.

CONCLUSION

The optimum range of oxygen level able to minimize risks and provide benefits in patients with sepsis or septic shock seems still unknown. Clinical equipoise between hyperoxia and normoxia is uncertain as conflicting evidence exists. Further studies should aim at identifying the best range of oxygenation and its optimal duration, investigating how effects of different levels of oxygen may vary according to identified pathogens, source of infection, and prescribed antibiotics in critically ill patients with sepsis and septic shock.

Respiratory Drive in Patients with Sepsis and Septic Shock: Modulation by High-flow Nasal Cannula

BACKGROUND

Experimental and pilot clinical data suggest that spontaneously breathing patients with sepsis and septic shock may present increased respiratory drive and effort, even in the absence of pulmonary infection. The study hypothesis was that respiratory drive and effort may be increased in septic patients and correlated with extrapulmonary determinant and that high-flow nasal cannula may modulate drive and effort.

METHODS

Twenty-five nonintubated patients with extrapulmonary sepsis or septic shock were enrolled. Each patient underwent three consecutive steps: low-flow oxygen at baseline, high-flow nasal cannula, and then low-flow oxygen again. Arterial blood gases, esophageal pressure, and electrical impedance tomography data were recorded toward the end of each step. Respiratory effort was measured as the negative swing of esophageal pressure (ΔPes); drive was quantified as the change in esophageal pressure during the first 500 ms from start of inspiration (P0.5). Dynamic lung compliance was calculated as the tidal volume measured by electrical impedance tomography, divided by ΔPes. The results are presented as medians [25th to 75th percentile].

RESULTS

Thirteen patients (52%) were in septic shock. The Sequential Organ Failure Assessment score was 5 [4 to 9]. During low-flow oxygen at baseline, respiratory drive and effort were elevated and significantly correlated with arterial lactate (r = 0.46, P = 0.034) and inversely with dynamic lung compliance (r = -0.735, P < 0.001). Noninvasive support by high-flow nasal cannula induced a significant decrease of respiratory drive (P0.5: 6.0 [4.4 to 9.0] vs. 4.3 [3.5 to 6.6] vs. 6.6 [4.9 to 10.7] cm H2O, P < 0.001) and effort (ΔPes: 8.0 [6.0 to 11.5] vs. 5.5 [4.5 to 8.0] vs. 7.5 [6.0 to 12.6] cm H2O, P < 0.001). Oxygenation and arterial carbon dioxide levels remained stable during all study phases.

CONCLUSIONS

Patients with sepsis and septic shock of extrapulmonary origin present elevated respiratory drive and effort, which can be effectively reduced by high-flow nasal cannula.

EDITOR’S PERSPECTIVE

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Proportional modes of ventilation: technology to assist physiology

Proportional modes of ventilation assist the patient by adapting to his/her effort, which contrasts with all other modes. The two proportional modes are referred to as neurally adjusted ventilatory assist (NAVA) and proportional assist ventilation with load-adjustable gain factors (PAV+): they deliver inspiratory assist in proportion to the patient’s effort, and hence directly respond to changes in ventilatory needs. Due to their working principles, NAVA and PAV+ have the ability to provide self-adjusted lung and diaphragm-protective ventilation. As these proportional modes differ from ‘classical’ modes such as pressure support ventilation (PSV), setting the inspiratory assist level is often puzzling for clinicians at the bedside as it is not based on usual parameters such as tidal volumes and PaCO₂ targets. This paper provides an in-depth overview of the working principles of NAVA and PAV+ and the physiological differences with PSV. Understanding these differences is fundamental for applying any assisted mode at the bedside. We review different methods for setting inspiratory assist during NAVA and PAV+ , and (future) indices for monitoring of patient effort. Last, differences with automated modes are mentioned.

Pre-eclamptic Fetal Programming Alters Neuroinflammatory and Cardiovascular Consequences of Endotoxemia in Sex-Specific Manners

Pre-eclampsia (PE)-induced fetal programming predisposes offspring to health hazards in adult life. Here, we tested the hypothesis that pre-eclamptic fetal programming elicits sexually dimorphic inflammatory and cardiovascular complications to endotoxemia in adult rat offspring. PE was induced by oral administration of L-NAME (50 mg/kg per day for seven consecutive days) starting from day 14 of conception. Cardiovascular studies were performed in conscious adult male and female offspring preinstrumented with femoral indwelling catheters. Compared with non-PE male counterparts, intravenous administration of lipopolysaccharide (LPS, 5 mg/kg) to PE male offspring caused significantly greater 1) falls in blood pressure, 2) increases in heart rate, 3) rises in arterial dP/dtmax, a correlate of left ventricular contractility, and 4) decreases in time- and frequency-domain indices of heart rate variability (HRV). By contrast, the hypotensive and tachycardic actions of LPS in female offspring were independent of the pre-eclamptic state and no clear changes in HRV or dP/dtmax were noted. Measurement of arterial baroreflex activity by vasoactive method revealed no sex specificity in baroreflex dysfunction induced by LPS. Immunohistochemical studies showed increased protein expression of toll-like receptor 4 in heart as well as in brainstem neuronal pools of the nucleus of solitary tract and rostral ventrolateral medulla in endotoxic PE male, but not female, offspring. Enhanced myocardial, but not neuronal, expression of monocyte chemoattractant protein-1 was also demonstrated in LPS-treated male offspring. Together, pre-eclamptic fetal programming aggravates endotoxic manifestations of hypotension and autonomic dysfunction in male offspring via exacerbating myocardial and neuromedullary inflammatory pathways. SIGNIFICANCE STATEMENT: Current molecular and neuroanatomical evidence highlights a key role for pre-eclamptic fetal programming in offspring predisposition to health hazards induced by endotoxemia in adult life. Pre-eclampsia accentuates endotoxic manifestations of hypotension, tachycardia, and cardiac autonomic dysfunction in male offspring via exacerbating myocardial and central inflammatory pathways. The absence of such detrimental effects in female littermates suggests sexual dimorphism in the interaction of pre-eclamptic fetal programming with endotoxemia.

Clinical and experimental aspects of breathing modulation by inflammation

Neuroinflammation is produced by local or systemic alterations and mediated mainly by glia, affecting the activity of various neural circuits including those involved in breathing rhythm generation and control. Several pathological conditions, such as sudden infant death syndrome, obstructive sleep apnea and asthma exert an inflammatory influence on breathing-related circuits. Consequently breathing (both resting and ventilatory responses to physiological challenges), is affected; e.g., responses to hypoxia and hypercapnia are compromised. Moreover, inflammation can induce long-lasting changes in breathing and affect adaptive plasticity; e.g., hypoxic acclimatization or long-term facilitation. Mediators of the influences of inflammation on breathing are most likely proinflammatory molecules such as cytokines and prostaglandins. The focus of this review is to summarize the available information concerning the modulation of the breathing function by inflammation and the cellular and molecular aspects of this process. I will consider: 1) some clinical and experimental conditions in which inflammation influences breathing; 2) the variety of experimental approaches used to understand this inflammatory modulation; 3) the likely cellular and molecular mechanisms.

Correlation between transition percentage of minute volume (TMV%) and outcome of patients with acute respiratory failure

PURPOSE

We have previously shown in patients receiving adaptive support ventilation (ASV) that there existed a Transition %MinVol (TMV%) where the patient's work of breathing began to reduce. In this study, we tested the hypothesis that higher TMV% would be associated with poorer outcome in patients with acute respiratory failure.

MATERIALS AND METHODS

In this prospective observational study, we recruited patients with acute respiratory failure on ASV between December 2012 and September 2013 in a mixed ICU. The TMV% was determined by adjusting % MinVol until mandatory respiratory frequency was between 0 and 1breath/min. TMV% was measured on the first two days of mechanical ventilation.

RESULTS

A total of 337 patients (age: 70±16years) were recruited. In patients whose TMV% increased between Day 1 and Day 2, aOR for mortality was 7.0 (95%CI=2.7-18.3, p<0.001) compared to patients whose TMV% decreased. In patients whose TMV% was unchanged between Day 1 and Day2, aOR for mortality was 3.91 (95%CI=1.80-8.22, p<0.01).

CONCLUSIONS

An increase, or lack of decrease, of TMV% from Day 1 to Day 2 was associated with higher risk of in-hospital death.

The declined levels of inflammatory cytokines related with weaning rate during period of septic patients using ventilators

INTRODUCTION

Approximately 50% of patients with sepsis-induced acute lung injury and acute respiratory distress syndrome require mechanical ventilation. Patients with extended mechanical ventilator use routinely develop reinfections, which increases hospital stay, mortality, and health care cost. Some studies have pointed out inflammatory factors concentrations can affect ventilator weaning, but do not indicate changed inflammatory factors related to ventilator weaning during using ventilators.

OBJECTIVES

This study aimed to investigate during period of septic patients using ventilators, the inflammatory cytokines concentrations related with weaning rate.

METHODS

Blood was collected from 35 septic patients before and during ventilator use on days 1, 7, 14, and 21 (or weaning).

RESULTS

58.3% (N = 20) of septic patients with mechanical ventilators were weaned successfully within 21 days (ventilator weaned group, VW), 16.7% (N = 6) did not wean within 21 days (ventilator dependent group, VD), and 25% died (death group) in hospital. Before ventilator use, higher C-reactive protein (CRP), IL-6, and IL-8 levels were measured in the death group than in all other groups (P < .05). During ventilator use, CRP, IL-6, and IL-8 concentrations declined significantly in VW and VD patients (P < .05). In addition, IL-6 concentrations in the VW group were significantly lower than in the VD group at 14 and 21 days (P < .05).

CONCLUSION

The factors of ventilators weaning successfully such as disease control, nutritional status, and so on. The declined levels of serum inflammatory cytokines, especially IL-6, improved inflammation status might be one factor of successfully weaning during septic patients on ventilators.

Sepsis progression to multiple organ dysfunction in carotid chemo/baro-denervated rats treated with lipopolysaccharide

Sepsis progresses to multiple organ dysfunction (MOD) due to the uncontrolled release of inflammatory mediators. Carotid chemo/baro-receptors could play a protective role during sepsis. In anesthetized male rats, we measured cardiorespiratory variables and plasma TNF-α, glucocorticoids, epinephrine, and MOD marker levels 90min after lipopolysaccharide (LPS) administration in control (SHAM surgery) and bilateral carotid chemo/baro-denervated (BCN) rats. BCN prior to LPS blunted the tachypneic response and enhanced tachycardia and hypotension. BCN-LPS rats also showed blunted plasma glucocorticoid responses, boosted epinephrine and TNF-α responses, and earlier MOD onset with a lower survival time compared with SHAM-LPS rats. Consequently, the complete absence of carotid chemo/baro-sensory function modified the neural, endocrine and inflammatory responses to sepsis. Thus, carotid chemo/baro-receptors play a protective role in sepsis.

Peripheral neural detection of danger-associated and pathogen-associated molecular patterns

Objective:

Bidirectional links between the nervous and immune systems modulate inflammation. The cellular mechanisms underlying the detection of danger-associated molecular patterns and pathogen-associated molecular patterns by the nervous system are not well understood. We hypothesized that the carotid body, a tissue of neural crest origin, detect pathogen associated molecular patterns and danger associated molecular patterns via an inflammasome-dependent mechanism similar to that described in immune cells.

Design:

Randomized, controlled laboratory investigation.

Setting:

University laboratory.

Subjects:

C57Bl/6J mice; juvenile Sprague-Dawley rats, primary human neutrophils.

Interventions:

Rat carotid body chemosensitive cells, and human neutrophils, were treated with TLR agonists to activate inflammasome-dependent pathways. In mice, systemic inflammation was induced by the pathogen associated molecular pattern zymosan (intraperitoneal injection; 500 mg/kg). Isolated carotid body/carotid sinus nerve preparations were used to assess peripheral chemoafferent activity. Ventilation was measured by whole-body plethysmography.

Measurements and Main Results:

Chemosensitive carotid body glomus cells exhibited toll-like receptor (TLR-2 and TLR-4), NLRP1, and NLRP3 inflammasome immunoreactivities. Zymosan increased NLRP3 inflammasome and interleukin-1β expression in glomus cells (p < 0.01). Human neutrophils demonstrated similar LPS-induced changes in inflammasome expression. Carotid body glomus cells also expressed IL-1 receptor and responded to application of IL-1β with increases in intracellular [Ca²⁺]. Four hours after injection of zymosan carotid sinus nerve chemoafferent discharge assessed in vitro (i.e., in the absence of acidosis/circulating inflammatory mediators) was increased five-fold (p < 0.001). Accordingly, zymosan-induced systemic inflammation was accompanied by enhanced respiratory activity.

Conclusions:

In carotid body chemosensitive glomus cells, activation of toll-like receptors increases NLRP3 inflammasome expression, and enhances IL-1β production, which is capable of acting in an autocrine manner to enhance peripheral chemoreceptor drive.

Treadmill exercise preconditioning attenuates lung damage caused by systemic endotoxemia in type 1 diabetic rats

Endotoxemia induces a series of inflammatory responses that may result in lung injury. However, heat shock protein72 (HSP72) has the potential to protect the lungs from damage. The objective of this study was to determine whether prior exercise conditioning could increase the expression of HSP72 in the lungs and attenuate lung damage in diabetic rats receiving lipopolysaccharide (LPS). Streptozotocin was used to induce diabetes in adult male Wistar rats. Rats were randomly assigned to sedentary or exercise groups. Rats in the exercise condition ran on a treadmill 5 days/week, 30-60 min/day, with an intensity of 1.0 mile/hour over a 3-week period. Rats received an intravenous infusion of LPS after 24 hrs from the last training session. Elevated lavage tumor necrosis factor-alpha (TNF- α ) level in response to LPS was more marked in diabetic rats. HSP72 expression in lungs was significantly increased after exercise conditioning, but less pronounced in diabetic rats. After administration of LPS, exercised rats displayed higher survival rate as well as decreased lavage TNF- α level and lung edema in comparison to sedentary rats. Our findings suggest that exercise conditioning could attenuate the occurrence of inflammatory responses and lung damage, thereby reducing mortality rate in diabetic rats during endotoxemia.

Patterns of unexpected in-hospital deaths: a root cause analysis

BACKGROUND

Respiratory alarm monitoring and rapid response team alerts on hospital general floors are based on detection of simple numeric threshold breaches. Although some uncontrolled observation trials in select patient populations have been encouraging, randomized controlled trials suggest that this simplistic approach may not reduce the unexpected death rate in this complex environment. The purpose of this review is to examine the history and scientific basis for threshold alarms and to compare thresholds with the actual pathophysiologic patterns of evolving death which must be timely detected.

METHODS

The Pubmed database was searched for articles relating to methods for triggering rapid response teams and respiratory alarms and these were contrasted with the fundamental timed pathophysiologic patterns of death which evolve due to sepsis, congestive heart failure, pulmonary embolism, hypoventilation, narcotic overdose, and sleep apnea.

RESULTS

In contrast to the simplicity of the numeric threshold breach method of generating alerts, the actual patterns of evolving death are complex and do not share common features until near death. On hospital general floors, unexpected clinical instability leading to death often progresses along three distinct patterns which can be designated as Types I, II and III. Type I is a pattern comprised of hyperventilation compensated respiratory failure typical of congestive heart failure and sepsis. Here, early hyperventilation and respiratory alkalosis can conceal the onset of instability. Type II is the pattern of classic CO2 narcosis. Type III occurs only during sleep and is a pattern of ventilation and SPO2 cycling caused by instability of ventilation and/or upper airway control followed by precipitous and fatal oxygen desaturation if arousal failure is induced by narcotics and/or sedation.

CONCLUSION

The traditional threshold breach method of detecting instability on hospital wards was not scientifically derived; explaining the failure of threshold based monitoring and rapid response team activation in randomized trials. Furthermore, the thresholds themselves are arbitrary and capricious. There are three common fundamental pathophysiologic patterns of unexpected hospital death. These patterns are too complex for early detection by any unifying numeric threshold. New methods and technologies which detect and identify the actual patterns of evolving death should be investigated.

Lipopolysaccharide-induced carotid body inflammation in cats: functional manifestations, histopathology and involvement of tumour necrosis factor-alpha

In the absence of information on functional manifestations of carotid body (CB) inflammation, we studied an experimental model in which lipopolysaccharide (LPS) administration to pentobarbitone-anaesthetized cats was performed by topical application upon the CB surface or by intravenous infusion (endotoxaemia). The latter caused: (i) disorganization of CB glomoids, increased connective tissue, and rapid recruitment of polymorphonuclear cells into the vascular bed and parenchyma within 4 h; (ii) increased respiratory frequency and diminished ventilatory chemoreflex responses to brief hypoxia (breathing 100% N(2) for 10 s) and diminished ventilatory chemosensory drive (assessed by 100% O(2) tests) during normoxia and hypoxia; (iii) tachycardia, increased haematocrit and systemic hypotension in response to LPS i.v.; and (iv) increased basal frequency of carotid chemosensory discharges during normoxia, but no change in maximal chemoreceptor responses to brief hypoxic exposures. Lipopolysaccharide-induced tachypnoea was prevented by prior bilateral carotid neurotomy. Apoptosis was not observed in CBs from cats subjected to endotoxaemia. Searching for pro-inflammatory mediators, tumour necrosis factor-alpha (TNF-alpha) was localized by immunohistochemistry in glomus and endothelial cells; reverse transcriptase-polymerase chain reaction revealed that the CB expresses the mRNAs for both type-1 (TNF-R1) and type-2 TNF-alpha receptors (TNF-R2); Western blot confirmed a band of the size expected for TNF-R1; and histochemistry showed the presence of TNF-R1 in glomus cells and of TNF-R2 in endothelial cells. Experiments in vitro showed that the frequency of carotid nerve discharges recorded from CBs perfused and superfused under normoxic conditions was not significantly modified by TNF-alpha, but that the enhanced frequency of chemosensory discharges recorded along responses to hypoxic stimulation was transiently diminished in a dose-dependent manner by TNF-alpha injections. The results suggest that the CB may operate as a sensor for immune signals, that the CB exhibits histological features of acute inflammation induced by LPS, that TNF-alpha may participate in LPS-induced changes in chemosensory activity and that some pathophysiological reactions to high levels of LPS in the bloodstream may originate from changes in CB function.

The involvement of hydroxyl radical and cyclooxygenase metabolites in the activation of lung vagal sensory receptors by circulatory endotoxin in rats

Circulatory endotoxin can stimulate vagal pulmonary C fibers and rapidly adapting receptors (RARs) in rats, but the underlying mechanisms are not clear. We investigated the involvement of hydroxyl radicals and cyclooxygenase metabolites in the stimulation of C fibers and RARs by circulatory endotoxin (50 mg/kg) in 112 anesthetized, paralyzed, and artificially ventilated rats. In rats pretreated with the vehicle, endotoxin stimulated C fibers and RARs and caused a slight increase in total lung resistance (Rl) and a decrease in dynamic lung compliance. In rats pretreated with dimethylthiourea (a hydroxyl radical scavenger) alone, indomethacin (a cyclooxygenase inhibitor) alone, or a combination of the two, C-fiber and RAR responses [C fiber: change (Delta) = -62, -79, and -85%; RAR: Delta = -80, -84, and -84%, respectively] were reduced, and the Rl response was prevented. The suppressive effects of a combination of dimethylthiourea and indomethacin on the C-fiber and RAR responses were not superior to indomethacin alone. In rats pretreated with isoproterenol (a bronchodilator), the C-fiber response was not significantly affected (Delta = -26%), the RAR response was reduced (Delta = -88%), and the Rl response was prevented. None of these pretreatments affected the dynamic lung compliance response. These results suggest that 1) both hydroxyl radicals and cyclooxygenase metabolites are involved in the endotoxin-induced stimulation of C fibers and RARs, and 2) the involvement of these two metabolites in the C-fiber stimulation may be due to their chemical effects, whereas that in the RAR stimulation may be due to their bronchoconstrictive effects.

Breathing pattern variability: a weaning predictor in postoperative patients recovering from systemic inflammatory response syndrome

OBJECTIVE

To investigate whether breathing pattern variability can serve as a potential weaning predictor for postoperative patients recovering from systemic inflammatory response syndrome (SIRS).

DESIGN AND SETTING

A prospective measurement of retrospectively analyzed breathing pattern variability in a surgical intensive care unit.

PATIENTS

Seventy-eight mechanically ventilated SIRS patients who had undergone abdominal surgery were included when they were ready for weaning. They were divided into success (n=57) and failure (n=21) groups based upon their weaning outcome.

MEASUREMENTS AND RESULTS

Before weaning, tidal volume, total breath duration, inspiratory time, expiratory time, and peak inspiratory flow were continuously monitored for 30 min, while patients received 5 cmH2O pressure support weaning trial. After the patients successfully completed the trial, they were extubated. Successful weaning was defined as patients free from the ventilator for over 48 h, whereas a weaning failure was considered as reinstitution of mechanical ventilation within 48 h of extubation. The coefficient of variation and two values of standard deviation (SD1 and SD2; indicators of the dispersion of data points in the plot) obtained from the Poincaré plot of five respiratory parameters in the failure group were significantly lower than those in the success group. The area under the receiver operating characteristic curve of these variability indices was within the range of 0.73-0.80, indicating the accuracy of prediction.

CONCLUSIONS

Small breathing pattern variability is associated with a high incidence of weaning failure in postoperative patients recovering from SIRS, and this variability may potentially serve as a weaning predictor.

The effect of Escherichia coli endotoxin infusion on the ventilatory response to hypoxia in unanesthetized newborn piglets

To determine the effects of endotoxemia on the neonatal ventilatory response to hypoxia, 17 chronically instrumented and unanesthetized newborn piglets (</=7 d) were studied before and 30 min after the administration of Escherichia coli O55:B5 endotoxin (n = 8) or normal saline (n = 9). Minute ventilation, oxygen consumption, heart rate, arterial blood pressure, and blood gases were measured during normoxia and 10 min of hypoxia (fraction of inspired oxygen, 0.10). Basal ventilation was not modified by E. coli endotoxin infusion (mean +/- SE, 516 +/- 49 versus 539 +/- 56 mL/min/kg), but the ventilatory response to hypoxia was markedly attenuated at 1 min (955 +/- 57 versus 718 +/- 97 mL/min/kg, p < 0.002, saline versus endotoxin) and at 10 min (788 +/- 51 versus 624 +/- 66 mL/min/kg, p < 0.002). A larger decrease in oxygen consumption was observed during hypoxia and endotoxemia (6.3 +/- 2.8 versus 18.3 +/- 2.7%, p < 0.03, pre- versus post-endotoxin). A significant correlation was demonstrated between the changes in minute ventilation and oxygen consumption with hypoxia during endotoxemia (r = 0.9, p < 0.002). The ventilatory response to hypoxia was not modified by the saline infusion. These data show a significant attenuation in the ventilatory response to hypoxia during E. coli endotoxemia. This decrease in ventilation was associated with a significant decrease in the metabolic rate during hypoxia and endotoxemia.

Involvement of tachykinin NK(1) and NK(2) receptors in changes in lung mechanics and airway microvascular leakage during the early phase of endotoxemia in Guinea pigs

We investigated the role of tachykinins in airway neurogenic responses occurring in the early phase of endotoxemia. Forty-eight anesthetized guinea pigs were evenly divided into six groups pretreated with either saline vehicle, CP-96,345 (a tachykinin NK(1) receptor antagonist), SR-48,968 (a tachykinin NK(2) receptor antagonist) or CP-96,345 and SR-48,968 in combination. Animals then received an intravenous injection of either saline (the vehicle for endotoxin) or endotoxin (30 mg/kg). Total lung resistance (R(L)) and dynamic lung compliance (C(dyn)) were continuously measured before and 30 min after administration of saline or endotoxin. Airway microvascular leakage was assessed at the end of the observation period. Endotoxin significantly increased R(L) and decreased C(dyn) 10 min after intravenous endotoxin injection. Plasma extravasation significantly increased in the trachea, main bronchi and intrapulmonary airways with endotoxin administration. These changes in lung mechanics were abolished by SR-48,968, but were unaffected by CP-96,345. The plasma extravasation was largely attenuated by CP-96,345 and/or SR-48,968. We conclude that (1) endogenous tachykinins play an important role in producing changes in lung mechanics and airway microvascular leakage during the early phase of endotoxemia and (2) activation of tachykinin NK(2) receptors is responsible for the former response, while activation of both tachykinin NK(1) and NK(2) receptors is involved in the latter response.

Endotoxin induces respiratory failure and increases surfactant turnover and respiration independent of alveolocapillary injury in rats

Although endotoxin-induced acute lung injury is associated with inflammation, alveolocapillary injury, surfactant dysfunction, and altered lung mechanics, the precise sequence of these changes is polemic. We have studied the early pathogenesis of acute lung injury in spontaneously breathing anesthetized rats after intravenous infusion of Salmonella abortus equi endotoxin. The animals became hypoxic, and airway resistance, tissue resistance, lung elastance, and static compliance all deteriorated well before any change in alveolar neutrophils, macrophages, lung fluid (99mTc-labeled diethylenetriamine pentaacetic acid), or 125I-albumin flux, which were only appreciably increased at 8.5 hours. Lung elastance deteriorated before airway resistance, indicating that the compliance change was specific rather than caused by reduced lung volume. The subcellular and alveolar content of surfactant proteins A and B, cholesterol, disaturated phospholipids, and phospholipid classes remained normal in the face of a dramatic increase in the synthesis and turnover of 3H-disaturated phosphatidylcholine. Our findings indicate that the increase in surfactant disaturated phospholipid turnover reflects, at least in part, an approximately five-fold increase in "sigh frequency." We suggest that endotoxin has direct effects on tissue resistance and lung elastance independent of surfactant composition and that the initial respiratory failure results primarily from endotoxin-induced ventilation/perfusion mismatch independent of edema or alveolocapillary injury per se.

Activation of lung vagal sensory receptors by circulatory endotoxin in rats

Although endotoxin is known to induce various pulmonary responses that are linked to the function of lung vagal sensory receptors, its effects on these pulmonary receptors are still not clear. This study investigated the effects of circulatory endotoxin on the afferent activity of lung vagal sensory receptors in rats. We recorded afferent activity arising from vagal pulmonary C fibers (CFs), rapidly adapting receptors (RARs), tonic pulmonary stretch receptors (T-PSRs), and phasic pulmonary stretch receptors (P-PSRs) in 64 anesthetized, paralyzed, and artificially ventilated rats. Intravenous injection of endotoxin (50 mg/kg; lipopolysaccharide) stimulated 7 of the 8 CFs, 8 of the 8 RARs, and 4 of the 8 T-PSRs studied, while having no effect on the 8 P-PSRs tested. The stimulation started 3-16 min after endotoxin injection and lasted until the end of the 90-min observation period. The evoked discharge of either CFs or RARs was not in phase with the ventilatory cycle, whereas that of T-PSRs showed a respiratory modulation. Injection of a saline vehicle caused no significant change in the discharge of these receptors. Additionally, endotoxin significantly produced an increase in total lung resistance, and decreases in dynamic lung compliance and arterial blood pressure. Our results demonstrate that a majority of lung vagal sensory receptors are activated following intravenous injection of endotoxin, and support the notion that these pulmonary receptors may function as an important afferent system during endotoxemia.