Although mechanical ventilation (MV) is necessary to improve ventilatory support in respiratory failure, it is generally known that this procedure markedly increases the incidence of pulmonary infection and consequently the morbidity and mortality of patients. Mucociliary clearance has been reported to be impaired in patients under MV and this is probably an important underlying mechanism in the pathogenesis of pulmonary infection in these patients [1]. Mucociliary clearance has a pivotal role in the protection of the respiratory tract against inhaled noxious agents that are trapped in the blanket of mucus and transported towards the pharynx by ciliary beating or coughing. The efficiency of the mucociliary system depends not only on the integrity of the epithelium and on ciliary activity but also on the amount of mucus, the depth of the periciliary layer and the viscoelastic properties of mucus [2].

Airway epithelium is an absorptive and secretory type of epithelium [3]; the transepithelial movement of electrolytes generates osmotic gradients that are responsible for the secretion or absorption of water. Pulmonary epithelial ion transport systems are important in the modulation of the ionic content and volume of periciliary fluid, which in turn modulates the physical properties and transportability of mucus. Small changes in the depth of periciliary fluid could greatly alter the efficiency of interaction between mucus and cilia [4]. Diuretics with an action on ionic channels present in the airway epithelium can alter ionic movement and change the physical properties and transportability of mucus. For instance, inhaling amiloride, a diuretic with action on the apical Na⁺ channel, has been reported to increase mucociliary clearance and alter the physical properties of mucus in patients with cystic fibrosis [5,6,7,8].

Intravenous (IV) furosemide is frequently used in patients under MV with the aim of equilibrating a cumulative positive fluid balance. However, the possible effects of IV furosemide on respiratory mucus are largely ignored. Furosemide is a potent diuretic that acts by inhibiting the NaK(Cl)₂ co-transporter in the ascending limb of the loop of Henle. Besides its renal action, furosemide can also affect epithelial ion transport in the airway. Earlier studies demonstrated that furosemide inhibits the NaK(Cl)₂ co-transporter in canine airway epithelium [9] and also decreases intracellular Cl^- activity in cultured human airway epithelium [10]. The effects of inhaled furosemide have also been investigated. Inhaled furosemide prevents exercise-induced bronchoconstriction in asthmatic patients [11]. Hasani et al. [12] reported that inhaled furosemide had no effects on mucociliary clearance in humans. However, the primary site of furosemide action is the basolateral membrane of the airway, where it inhibits the NaK(Cl)₂ co-transporter. Therefore the effects of the drug on the respiratory epithelium might depend on the route of administration. The aim of the present study was to investigate the effects of IV furosemide on the transportability and rheological properties of mucus in patients under MV.

Demographic and MV parameters are described in Tables 1 and 2. The time lag between the initiation of MV and the study was 9 ± 6 and 9 ± 6 days for the furosemide and control groups, respectively (P = 0.9). In the furosemide group, two patients were using the heat and moisture exchanger (HME), and 10 were using the heated humidifier. In the control group, six patients were using the HME and eight were using the heated humidifier.

The results of mucus transportability in the frog palate (MCT) and cough (CC) are presented in Figs 1 and 2, respectively. MCT decreased significantly after furosemide administration and did not recover to baseline values by 4 hours (P = 0.0001). In contrast, MCT remained constant in the control group (Fig. 1). There was a trend that did not reach statistical significance for a decrease in CC in the furosemide group (Fig. 2).

The results of the remaining parameters, contact angle, log G* and tan δ measured at 1 and 100 radians/s, are presented in Table 3. There were no significant differences between groups.

To our knowledge this is the first study to investigate the effects of IV furosemide on mucus transportability in vitro and the physical properties of mucus from patients under MV. Our results suggest that IV furosemide might acutely impair MCT for up to 4 hours after administration.

The mucociliary escalator of the lungs is an important protective transport system by means of which inhaled particles and microorganisms are removed from the tracheobronchial system. Lung mucociliary clearance is influenced by several factors, including the integrity of the ciliated epithelium and the thickness and physical properties of the periciliary or mucous layer [12]. Under normal circumstances, active ion transport in the respiratory epithelium is important in the production and regulation of the volume and composition of the respiratory tract secretion, which in turn is important for adequate mucociliary interaction [22]. Pharmacological interference in ionic transport is caused by a new class of drugs that can change MCT. For instance, inhalation of amiloride increases MCT in patients with cystic fibrosis by inhibiting the active absorption of salt and water from airway surfaces [23,24].

The effects of furosemide on the respiratory epithelium have attracted interest in the decade since Bianco et al. [11] reported that inhaled furosemide prevents exercise-induced bronchoconstriction in asthmatic patients. The mechanism of this protective effect remains to be established. The effects of inhaled furosemide on mucociliary clearance have been investigated and the results are controversial. Hasani et al. [12] reported that nebulized furosemide does not affect mucociliary clearance measured with a radioaerosol technique in healthy and asthmatic subjects. It must be stressed that the primary site of furosemide action is the basolateral membrane of the airway, where it inhibits the NaK(Cl)₂ co-transporter. Inhaled furosemide might therefore not reach the basolateral membrane of airway epithelial cells in vivo [11,25]. In fact, experimental studies have demonstrated that, in contrast with the serosal application of furosemide, mucosal application has no effect on co-transporter function [26]. Winters and Yeates [27] have reported an increase in lung mucociliary clearance in vivo after the inhalation of aerosolized furosemide and the IV administration of furosemide in dogs and baboons. However, in this study the properties and in vitro transportability of mucus were not determined.

In our study we observed a decrease in MCT after furosemide administration that did not recover to the baseline by 4 hours. Furosemide inhibits the NaK(Cl)₂ co-transporter, which is one of the physiological mechanisms involved in the respiratory hydration of mucus; its inhibition could therefore interfere in the rheological properties of mucus [4,28]. The ionic concentration of Na⁺ and Cl^2- in mucus can also influence the rheology and transportability of mucus independently of its total water content [6,29]. In addition, diuresis might lead to systemic dehydration and impairment of mucociliary clearance [30,31]. In our study, furosemide administration was a clinical decision based on cumulative positive fluid balance and determined by the medical staff. Interestingly, the furosemide and control groups had similar fluid balance in the 24 hours before the onset of the study. As expected, furosemide promoted increased diuresis. It must be stressed that in our study the patients were not monitored invasively. Fluid balance, diureses and haemodynamic status can give only gross estimates of fluid balance. In summary, from this study it is not possible to determine the mechanism involved in the effects of furosemide on MCT.

The mode of humidification was not uniform between the groups. Nakagawa et al. [13] have recently compared the effects of two systems of humidification (HME with a Pall BB 100 F, and a heated humidifier) on respiratory mucus and its transportability in patients under MV. The effects were evaluated for up to 72 hours of MV. They observed a decrease in CC in the HME group only after 72 hours of MV. Because the present study was limited to an intervention in a short period (4 hours), baseline clinical conditions, including age, MV parameters and the mode of humidification, probably did not influence the results. Indeed, our control group showed no time-dependent changes in all parameters studied. Infection also affects respiratory mucous and epithelium. However, the occurrence of pulmonary infection was similar in both groups (10 patients in the control group and 9 in the furosemide group), suggesting that this factor did not influence our results.

In our study, impairment in MCT was not matched with significant changes in other physical properties of mucus. It is possible that MCT is a more sensitive method for detecting mucociliary impairment. Because our study involved a relatively small number of patients, we cannot discard a type 2 error to explain the absence of furosemide effect on other mucus parameters. An alternative explanation is that furosemide has direct effects on the ciliary beating frequency of the frog palate.

In conclusion, our preliminary results support the hypothesis that IV furosemide might acutely impair mucociliary clearance. In patients with respiratory failure and MV, many factors can potentially impair MCT, such as ventilation with a high concentration of oxygen, the activation of inflammatory mediator systems, colonization by bacteria, suction-induced lesions of the mucous membrane, infections and drugs [1]. The mechanisms and the clinical relevance of our findings remain to be established.

None declared.

CA = contact angle; CC = cough clearance; HME = heat and moisture exchanger; IV = intravenous; MCT = mucociliary transport; MV = mechanical ventilation.