Inhaled platelet-activating factor worsens gas exchange in mild asthma

Platelets in the immune response: Revisiting platelet-activating factor in anaphylaxis

Anaphylaxis is an acute, severe, life-threatening multisystem allergic reaction resulting from the sudden systemic release of biochemical mediators and chemotactic substances. Release of both preformed granule-associated mediators and newly generated lipid-derived mediators contributes to the amplification and prolongation of anaphylaxis. Platelet-activating factor (PAF) is a potent phospholipid-derived mediator the central role of which has been well established in experimental models of both immune-mediated and non-immune mediated anaphylaxis. It is produced and secreted by several types of cells, including mast cells, monocytes, tissue macrophages, platelets, eosinophils, endothelial cells, and neutrophils. PAF is implicated in platelet aggregation and activation through release of vasoactive amines in the inflammatory response, resulting in increased vascular permeability, circulatory collapse, decreased cardiac output, and various other biological effects. PAF is rapidly hydrolyzed and degraded to an inactive metabolite, lysoPAF, by the enzyme PAF acetylhydrolase, the activity of which has shown to correlate inversely with PAF levels and predispose to severe anaphylaxis. In addition to its role in anaphylaxis, PAF has also been implicated as a mediator in both allergic and nonallergic inflammatory diseases, including allergic rhinitis, sepsis, atherosclerotic disease, and malignancy, in which PAF signaling has an established role. The therapeutic role of PAF antagonism has been investigated for several diseases, with variable results thus far. Further investigation of its role in pathology and therapeutic modulation is highly anticipated because of the pressing need for more selective and targeted therapy for the management of severe anaphylaxis.

Blood gas tensions in adult asthma: a systematic review and meta-regression analysis

OBJECTIVE

The last half-century has seen substantial changes in asthma treatment and care. We investigated whether arterial blood gas parameters in acute and non-acute asthma have changed historically.

METHODS

We performed a systematic search of the literature for studies reporting P(aO2) , P(aCO2) and forced expiratory volume in 1 s, percentage of predicted (FEV1%). For each of the blood gas parameters, meta-regression analyses examined its association with four background variables: the publication year, mean FEV1%, mean age and female fraction in the respective studies.

RESULTS

After screening, we included 43 articles comprising 61 datasets published between 1967 and 2013. In studies of habitual-state asthma, mean P(aO2) was positively associated with the publication year (p = 0.001) and negatively with mean age (p < 0.01). Mean P(aCO2) showed a positive association with publication year (p = 0.001) and a negative association with female fraction (p < 0.05). In acute asthma studies, blood gas levels were unassociated with publication year and mean age, mean P(aO2) was positively associated with FEV1% (p < 0.05) whereas mean P(aCO2) showed a negative association with FEV1% (p < 0.05) for studies with mean FEV1% <40. In neither acute nor habitual-state studies was mean arterial pH associated with any of the predictor variables.

CONCLUSIONS

In studies of habitual-state asthma, mean reported P(aO2) and P(aCO2) levels were found to have increased since 1967. In acute asthma studies, mean P(aO2) and P(aCO2) were associated with mean FEV1% but not with either publication year or patient age.

Gas exchange in disease: asthma, chronic obstructive pulmonary disease, cystic fibrosis, and interstitial lung disease

Ventilation-perfusion (VA/Q) inequality is the underlying abnormality determining hypoxemia and hypercapnia in lung diseases. Hypoxemia in asthma is characterized by the presence of low VA/Q units, which persist despite improvement in airway function after an attack. This hypoxemia is generally attenuated by compensatory redistribution of blood flow mediated by hypoxic vasoconstriction and changes in cardiac output, however, mediator release and bronchodilator therapy may cause deterioration. Patients with chronic obstructive pulmonary disease have more complex patterns of VA/Q inequality, which appear more fixed, and changes in blood flow and ventilation have less benefit in improving gas exchange efficiency. The inability of ventilation to match increasing cardiac output limits exercise capacity as the disease progresses. Deteriorating hypoxemia during exacerbations reflects the falling mixed venous oxygen tension from increased respiratory muscle activity, which is not compensated by any redistribution of VA/Q ratios. Shunt is not a feature of any of these diseases. Patients with cystic fibrosis (CF) have no substantial shunt when managed according to modern treatment regimens. Interstitial lung diseases demonstrate impaired oxygen diffusion across the alveolar-capillary barrier, particularly during exercise, although VA/Q inequality still accounts for most of the gas exchange abnormality. Hypoxemia may limit exercise capacity in these diseases and in CF. Persistent hypercapnic respiratory failure is a feature of advancing chronic obstructive pulmonary disease and CF, closely associated with sleep disordered breathing, which is not a prominent feature of the other diseases. Better understanding of the mechanisms of hypercapnic respiratory failure, and of the detailed mechanisms controlling the distribution of ventilation and blood flow in the lung, are high priorities for future research.

The critically ill asthmatic--from ICU to discharge

Status asthmaticus (SA) is defined as an acute, severe asthma exacerbation that does not respond readily to initial intensive therapy, while near-fatal asthma (NFA) refers loosely to a status asthmaticus attack that progresses to respiratory failure. The in-hospital mortality rate for all asthmatics is between 1% to 5%, but for critically ill asthmatics that require intubation the mortality rate is between 10% to 25% primarily from anoxia and cardiopulmonary arrest. Timely evaluation and treatment in the clinic, emergency room, or ultimately the intensive care unit (ICU) can prevent the morbidity and mortality associated with respiratory failure. Fatal asthma occurs from cardiopulmonary arrest, cerebral anoxia, or a complication of treatments, e.g., barotraumas, and ventilator-associated pneumonia. Mortality is highest in African-Americans, Puerto Rican-Americans, Cuban-Americans, women, and persons aged ≥ 65 years. Critical care physicians or intensivists must be skilled in managing the critically ill asthmatics with respiratory failure and knowledgeable about the few but potentially serious complications associated with mechanical ventilation. Bronchodilator and anti-inflammatory medications remain the standard therapies for managing SA and NFA patients in the ICU. NFA patients on mechanical ventilation require modes that allow for prolonged expiratory time and reverse the dynamic hyperinflation associated with the attack. Several adjuncts to mechanical ventilation, including heliox, general anesthesia, and extra-corporeal carbon dioxide removal, can be used as life-saving measures in extreme cases. Coordination of discharge and follow-up care can safely reduce the length of hospital stay and prevent future attacks of status asthmaticus.

Platelet-activating factor antagonists: current status in asthma

Platelet-activating factor (PAF) is a potent lipid-derived mediator of inflammation that is considered to have a potential role in the pathogenesis of asthma. PAF is produced by many cells associated with asthmatic inflammation and has the ability to evoke some of the clinical hallmarks of asthma, such as bronchoconstriction, mucus production and airway hyperresponsiveness (AHR). In addition, PAF has profound chemoattractant properties for eosinophils and neutrophils and it promotes an increase in microvascular permeability and oedema formation within the airways. Nevertheless, the definitive role of PAF in asthma remains elusive. PAF is formed as a result of the action of phospholipase A(2) and acetyltransferase on membrane phospholipids and it is degraded by a PAF-specific acetylhydrolase. The biological effects of PAF are mediated by the activation of specific receptors expressed on effector cell surfaces, although intracellular signalling and paracrine actions have been described. In addition, at least part of the pulmonary effects of PAF could be related to the secondary release of leukotrienes. In the clinical setting, different ways of modifying the activity of PAF have been explored, in particular the inhibitory actions of PAF receptor antagonists. Both natural and synthetic PAF receptor antagonists have shown conflicting results. Although second generation PAF antagonists (apafant, UK-74505, SR-27417A) appear to have a good protective effect against the systemic and pulmonary actions of inhaled PAF, the protective effects of these compounds on allergen-induced responses and AHR are more modest. In the treatment of asthma, PAF receptor antagonists have failed to produce a significant impact in either acute asthma attacks or the maintenance therapy of chronic forms. Other pharmacological interventions of proven efficacy in asthma, such as salbutamol or 5-lipoxygenase antagonists, have shown some anti-PAF effects. Whether the overall negative results with PAF receptor antagonists indicate that extracellular PAF is not a relevant mediator of airway inflammation or that the compounds explored are not capable of blocking the paracrine actions of PAF remains speculative. A PAF synthase inhibitor could be valuable in the elucidation of the role of PAF and it might be a promising and useful complementary therapeutic tool in the future.

Repeat exercise normalizes the gas-exchange impairment induced by a previous exercise bout in asthmatic subjects

Twenty-one subjects with asthma underwent treadmill exercise to exhaustion at a workload that elicited approximately 90% of each subject's maximal O2 uptake (EX1). After EX1, 12 subjects experienced significant exercise-induced bronchospasm [(EIB+), %decrease in forced expiratory volume in 1.0 s = -24.0 +/- 11.5%; pulmonary resistance at rest vs. postexercise = 3.2 +/- 1.5 vs. 8.1 +/- 4.5 cmH2O.l(-1).s(-1)] and nine did not (EIB-). The alveolar-to-arterial Po2 difference (A-aDo2) was widened from rest (9.1 +/- 6.7 Torr) to 23.1 +/- 10.4 and 18.1 +/- 9.1 Torr at 35 min after EX1 in subjects with and without EIB, respectively (P < 0.05). Arterial Po2 (PaO2) was reduced in both groups during recovery (EIB+, -16.0 +/- -13.0 Torr vs. baseline; EIB-, -11.0 +/- 9.4 Torr vs. baseline, P < or = 0.05). Forty minutes after EX1, a second exercise bout was completed at maximal O2 uptake. During the second exercise bout, pulmonary resistance decreased to baseline levels in the EIB+ group and the A-aDo2 and PaO2 returned to match the values seen during EX1 in both groups. Sputum histamine (34.6 +/- 25.9 vs. 61.2 +/- 42.0 ng/ml, pre- vs. postexercise) and urinary 9alpha,11beta-prostaglandin F2 (74.5 +/- 38.6 vs. 164.6 +/- 84.2 ng/mmol creatinine, pre- vs. postexercise) were increased after exercise only in the EIB+ group (P < 0.05), and postexercise sputum histamine was significantly correlated with the exercise PaO2 and A-aDo2 in the EIB+ subjects. Thus exercise causes gas-exchange impairment during the postexercise period in asthmatic subjects independent of decreases in forced expiratory flow rates after the exercise; however, a subsequent exercise bout normalizes this impairment secondary in part to a fast acting, robust exercise-induced bronchodilatory response.

Acute asthma in adults: a review

All patients with asthma are at risk of having exacerbations. Hospitalizations and emergency department (ED) visits account for a large proportion of the health-care cost burden of asthma, and avoidance or proper management of acute asthma (AA) episodes represent an area with the potential for large reductions in health-care costs. The severity of exacerbations may range from mild to life threatening, and mortality is most often associated with failure to appreciate the severity of the exacerbation, resulting in inadequate emergency treatment and delay in referring to hospital. This review describes the epidemiology, costs, pathophysiology, mortality, and management of adult AA in the ED and in the ICU.

Pulmonary gas exchange and sputum cellular responses to inhaled leukotriene D(4) in asthma

Inhalational challenges with inflammatory mediators may provoke lung function disturbances similar to those shown in spontaneous acute asthma. Cysteinyl leukotrienes (CysLTs) have recently been established as mediators of bronchoconstriction in asthma but their effects on pulmonary gas exchange in asthma have not been assessed. We therefore investigated the effects of leukotriene D(4) (LTD(4)) challenge resulting in a significant decrease in FEV(1) (mean +/- SE, by 32 +/- 3%) in 13 nonsmoking, mild asthmatics. Respiratory system resistance (Rrs), and respiratory and inert gases were measured before and immediately after, and at 15 and 45 min after challenge. After bronchoprovocation, Rrs increased (by 106 +/- 12%), Pa(O(2)) decreased (by 25 +/- 4 mm Hg), and ventilation-perfusion distributions moderately to severely deteriorated, as shown by increases in the dispersions of pulmonary blood flow (Log SDQ, by 59 +/- 12%) and alveolar ventilation (Log SDV, by 65 +/- 20%) (p < 0.05 each). Sputum eosinophils (p < 0.05) and urinary LTE(4) (p < 0.005) increased after challenge. Despite the lack of mathematical correlations between spirometric and Rrs changes and gas exchange indices, the pattern of improvement of the functional variables after challenge ran in parallel. These findings support the evidence that CysLTs, in addition to being potent bronchoconstrictors, also provoke profound disturbances of pulmonary gas exchange in asthma.

The effects of 5-lipoxygenase inhibition by zileuton on platelet-activating-factor-induced pulmonary abnormalities in mild asthma

Platelet-activating factor (PAF) has been implicated in the pathogenesis of asthma. We investigated whether PAF-induced neutropenia and lung function disturbances are secondary to activation of the 5-lipoxygenase (5-LO) pathway with the consequent liberation of leukotrienes. The effect of a selective 5-LO inhibitor (zileuton) was examined in 10 mildly asthmatic patients (24 +/- 1 [mean +/- SE] yr; FEV1 = 94 +/- 4% predicted) before and after PAF inhalation, in a randomized, double-blind, placebo-controlled, crossover fashion. Patients were studied at baseline, 3 h after an oral single dose of zileuton (600 mg) or placebo, and then at 5 min, 15 min, and 45 min after PAF (18 microg) inhalation. Compared with vehicle, premedication with zileuton reduced both PAF-induced neutropenia at 5 min (by 43%) (p < 0.005) and the subsequent rebound neutrophilia at 15 min and 45 min (by 50% and 47%, respectively) (p < 0.025 each). In addition, at 5 min after PAF inhalation, zileuton attenuated increases in respiratory system resistance (Rrs) (by 39%) (p < 0.01) and in the alveolar-arterial PO2 difference (A-a)PO2 (by 40%) (p < 0.05), and the decrease in PaO2 (by 27%) (p < 0.005). The protective effect of zileuton against PAF-induced ventilation-perfusion (VA/Q) defects was shown by a parallel improvement (decrease) in an overall marker of VA/Q inequality (dispersion of retention minus excretion of inert gases corrected for dead space; DISP R-E.) (by 43%) 5 min after administration of PAF (p < 0.01). These findings indicate that PAF-induced systemic and pulmonary effects in patients with mild asthma are effectively mediated by the ongoing release of leukotrienes.

Roxithromycin, clarithromycin, and azithromycin attenuate the injurious effects of bioactive phospholipids on human respiratory epithelium in vitro

The effects of the bioactive phospholipids (PL), platelet-activating factor (PAF), lyso-PAF, and lysophosphatidylcholine (LPC) on the beat frequency and structural integrity of human ciliated respiratory epithelium were studied in vitro, in the presence or absence of polymorphonuclear leukocytes (PMNL), the antimicrobial agents, roxithromycin, clarithromycin, and azithromycin and the antioxidative enzymes catalase and superoxide dismutase (SOD). All three PL caused dose-dependent slowing of ciliary beat frequency (CBF) and epithelial damage (ED) at concentrations > or = 1 microgram/ml, which were unaffected by inclusion of the antimicrobial agents and antioxidative enzymes. When epithelial strips were exposed to the combination of PMNL and PL, there was significant potentiation of ciliary dysfunction and ED, which was ameliorated by pretreatment of the PMNL with the antimicrobial agents or by inclusion of catalase, but not SOD. These results demonstrate that LPC, PAF, and lyso-PAF cause epithelial damage by direct mechanisms which are oxidant-independent, as well as by indirect mechanisms involving phagocyte-derived reactive oxidants. Macrolides and azalide antimicrobial agents may have beneficial effects on airway inflammation in asthma and microbial infections by protecting ciliated epithelium against oxidative damage inflicted by PL-sensitized phagocytes.

Inhibition of PAF-induced gas exchange defects by beta-adrenergic agonists in mild asthma is not due to bronchodilation

Salbutamol inhibits neutropenia, increased airway resistance, and gas exchange abnormalities provoked by platelet-activating factor (PAF) challenge in normal persons. To further explore the intriguing dissociation between spirometric abnormalities and gas exchange defects shown in patients with asthma, we investigated whether the salbutamol-induced improvement in gas exchange disturbances after PAF is the result of bronchodilation by comparing this effect with that of ipratropium bromide. We hypothesized that ipratropium bromide, an anticholinergic agent without vascular effects, should block PAF-induced bronchoconstriction but not interfere with its systemic, neutropenic, and gas exchange effects. We studied eight nonsmokers with mild asthma (26 +/- 2.0 SE yr of age) who, prior to PAF challenge (18 micrograms), inhaled either ipratropium bromide (80 micrograms) or salbutamol (300 micrograms) in a randomized, double-blind, crossover fashion 1 wk apart. Peripheral blood neutrophils, respiratory system resistance (Rrs), arterial blood gases and ventilation-perfusion (VA/Q) inequalities were measured 5, 15, and 45 min after PAF. Compared with pretreatment with salbutamol, ipratropium bromide also blocked the increase of respiratory system resistance (Rrs) but did not prevent facial flushing and neutropenia (p < 0.03) at 5 min nor the decrease of PaO2 (p = 0.08 and 0.05), the increase of AaPO2 (p < 0.02 each), and the deterioration of VA/Q relationships (p < 0.05 each) at 5 and 15 min, respectively. This functional pattern was similar to that observed previously in normal subjects and in nonpremedicated asthmatic patients after PAF, with return to baseline values at 45 min. By contrast, salbutamol blocked PAF-induced increased Rrs, in addition to all the other PAF-induced abnormalities. These findings indicate that, in patients with mild asthma, salbutamol inhibits PAF-induced neutropenia and gas exchange abnormalities by mechanisms involving other than airway smooth muscle narrowing, possibly by acting on both the bronchial and pulmonary circulations.