Surfactant replacement therapy in surfactant-deficient rabbits: early effects on lung function and biochemical aspects

Utility of deadspace and capnometry measurements in determination of surfactant efficacy in surfactant-depleted lungs

OBJECTIVE

To investigate if bronchoalveolar lavage leads to increased alveolar and physiologic deadspace or a deadspace/ tidal volume ratio and if surfactant replacement restores the lung to its prelavage condition.

DESIGN

Prospective, animal cohort study.

SETTING

An animal laboratory in a university medical center.

SUBJECTS

Seven adult rabbits receiving artificial ventilation.

METHODS

Our single-breath CO2 analysis station contained the following equipment: pneumotachometer Ventrak 1550, a mainstream capnometer Capnogard 1265, a signal processor, and computer software. Repeated bronchoalveolar lavage was performed in seven adult rabbits to simulate acute respiratory distress syndrome. Surfactant therapy was administered after bronchoalveolar lavage induced a 20% reduction in baseline arterial PO2. The calculated parameters of alveolar and physiologic deadspace and the deadspace/tidal volume ratio were derived from the single-breath CO2 plot by Ventrak 1550 in combination with the Capnogard 1265. The arterial end-tidal Pco2 difference, the alveolar-arterial PO2 difference, and the arterial/alveolar PO2 ratio were obtained by capnography and arterial blood gas analysis. Measurements of these parameters were performed before bronchoalveolar lavage, during bronchoalveolar lavage, and after surfactant application.

MEASUREMENTS AND MAIN RESULTS

The alveolar and physiologic deadspace and the deadspace/tidal volume ratio were significantly higher in lavaged animals. After application of natural surfactant, these parameters were significantly reduced but the baseline values could not be reached. Bronchoalveolar lavage led to a significant fall in the arterial/alveolar PO2 ratio, which increased after surfactant therapy. There was a negative correlation between the arterial/alveolar PO2 ratio and the deadspace/tidal volume ratio. The alveolar and physiologic deadspace and the deadspace/tidal volume ratio correlated with the arterial end-tidal Pco2 difference. The best correlation was obtained between the arterial end-tidal Pco2 difference and the alveolar deadspace/tidal volume ratio (r = 0.98).

CONCLUSIONS

Bronchoalveolar lavage elevates the alveolar and physiologic deadspace and the deadspace/tidal volume ratios and is combined with a fall in the arterial/alveolar PO2 ratio. Surfactant treatment improves the gas exchange but does not restore the lung to its prebronchoalveolar lavage condition, which indicates that the exogenous surfactant affects only partly the recruitment of the atelectatic areas.

A comparison of the hemodynamic and respiratory effects of surfactant instillation during interrupted ventilation versus noninterrupted ventilation in rabbits with severe respiratory failure

The purpose of this study was to evaluate whether avoiding interruption of ventilation during surfactant instillation improves the effects on lung function and surfactant distribution and whether it prevents the adverse effects on blood pressure and cerebral blood flow. The study was performed using rabbits with severe respiratory failure induced by lung lavages. These rabbits were randomized to 99mTc-Nanocoll labeled surfactant instillation through a side lumen of the endotracheal tube without interrupting ventilation or instillation during a short interruption of ventilation. After surfactant instillation with interruption of ventilation, PaO2 rose from 8.7+/-1.3 to 24.9+/-6.4 kPa (mean+/-SEM). Without interruption, PaO2 rose from 8.4+/-0.8 to 32.4+/-4.3 kPa. PaCO2 decreased with interruption from 4.69+/-0.51 to 3.61+/-0.26 kPa and without interruption from 5.06+/-0.41 to 4.13+/-0.23 kPa. Dynamic and static compliance indices were not statistically different after both procedures. Surfactant distribution tended to be less nonuniform after instillation without interrupting ventilation. In contrast, avoidance of interruption of ventilation resulted in less uniform lobar distribution and less peripheral deposition of surfactant. By instillation with interruption, blood pressure increased quickly (28+/-6.6%), followed by a 22+/-5.3% decrease. Blood pressure increased quickly (16+/-4.2%), followed by a 40+/-10% decrease by surfactant instillation without interruption. Cerebral blood flow, measured by an ultrasonic transit time flow probe on the carotid artery, increased quickly (45+/-14%), followed by a 64+/-11% decrease with interruption, whereas it increased 15+/-4.9% (p = 0.06 versus with interruption) and decreased 61+/-13% without interruption of ventilation. Therefore, avoiding interruption of ventilation during surfactant instillation tends to prevent the potential adverse effects of a rapid rise in cerebral blood flow, and furthermore, tends to improve uniformity of surfactant distribution, whereas having no detrimental effect on respiratory function.

Surfactant nebulization versus instillation during high frequency ventilation in surfactant-deficient rabbits

Surfactant nebulization improves lung function at low alveolar doses of surfactant. However, efficiency of nebulization is low, and lung deposition seems to depend on lung aeration. High frequency ventilation (HFV) has been shown to improve lung aeration. We hypothesize that the combination of HFV and surfactant nebulization may benefit lung deposition of surfactant and consequently, lung function. The aim of this study was to compare the effect of surfactant nebulization versus instillation during HFV on lung function, surfactant distribution, and cerebral blood flow. Therefore, severe respiratory failure was induced by lung lavages in 18 rabbits. HFV was applied: frequency = 8 Hz, mean airway pressure = 12 cm H2O, amplitude = 100%, fraction of inspired O2 = 1.0. Technetium-99m-labeled surfactant (Alveofact, 100 mg/kg of BW) was nebulized or instilled (n = 6 each). Six other rabbits did not receive surfactant (control, HFV only). We found that after instillation partial arterial O2 tension increased from 7.0 kPa (95% confidence interval, 6.3-8.0 kPa) to 34 kPa (16-51 kPa), and during nebulization from 7.0 kPa (6.0-9.0 kPa) to 46 kPa (27-58 kPa). Partial arterial CO2 tension decreased after instillation from 6.1 kPa (5.3-7.1 kPa) to 4.8 kPa (3.9-5.6 kPa), and during nebulization, after an initial rise, it decreased from 6.3 kPa (5.3-7.4 kPa) to 4.9 kPa (4.4-5.6 kPa). Both treatments resulted in nonuniform distribution. Surfactant deposition after nebulization was 9.8%. Instillation resulted in a drop of mean arterial blood pressure of 17% (8-31%), and an even more pronounced drop in cerebral blood flow of 39% (18-57%). Nebulization did not affect blood pressure. Cerebral blood flow decreased with a maximum of 27% (10-37%). We conclude that surfactant nebulization during HFV improves lung function in rabbits with severe respiratory failure, without improving distribution, but with less effects on blood pressure and cerebral blood flow, when compared with surfactant instillation.

Surfactant replacement therapy for adult respiratory distress syndrome in children

Surfactant replacement therapy may have a role in the treatment of ARDS in children. The current studies suggest that rapid instillation of exogenous surfactant is more effective than slow tracheal instillation or aerosolized delivery. Studies suggest that exogenous surfactant given early in the development of ARDS is more effective than therapy provided late in the course of the disease. Natural surfactants appear to be more effective than artificial surfactants due to the presence of SP-B and SP-C, which prevent inhibition of the exogenous surfactant by the protein leakage into the alveolus that is characteristic of ARDS. Exogenous surfactant replacement therapy appears to be safe and well tolerated. A surfactant that can be delivered by aerosol would be useful since this is more easily tolerated by the patients, requires less surfactant, and would be more cost effective when compared with tracheal instillation. Aerosolized surfactant could be given to patients who have not yet required mechanical ventilation, thus potentially preventing the progression of the acute lung injury to respiratory failure. The recent failure of a large multi-center trial of aerosolized Exosurf for the treatment of sepsis-related ARDS72 may have been due to the failure of the delivery system as opposed to the surfactant used in the trial; therefore, further research into aerosol delivery systems is needed. There may be different responses to exogenous surfactant therapy by patients with ARDS of different etiologies, such as aspiration pneumonia, sepsis, or trauma. Well-planned placebo-controlled trials will be required to determine these differences. The data supporting the role of surfactant replacement for the treatment of ARDS in children is growing. However, before widespread use of surfactant is considered, a multi-center, placebo-controlled trial will be required to establish the safety and efficacy of surfactant replacement in such patients.

Artificial surfactant administration in an animal model of near drowning

OBJECTIVE

To study the use of exogenous surfactant in a rabbit fresh-water near-drowning model.

METHODS

In a randomized, placebo-controlled experiment, 21 rabbits were anesthetized, paralyzed, and subjected to near drowning with 6 mL/kg of distilled water. Vital signs, arterial blood gases, and pulmonary compliance were measured at predetermined intervals. The animals were randomized into two groups: a normal O2 group that had their pO2 levels adjusted to 75-150 torr using supplemental O2 as needed, and a high O2 group ventilated with an FIO2 = 100%. Ventilator settings were further adjusted to normalize pH and pCO2 levels prior to instillation of distilled water. The two groups were further divided and 15 minutes after simulated near drowning, the animals were either treated with surfactant (5 mL/kg instilled endotracheally) or not treated. Data were analyzed by repeated-measures analysis of variance.

RESULTS

Near drowning had no significant effect on heart rate or blood pressure. Acidosis was not changed by surfactant; hypoxia was worsened by surfactant: 158 +/- 54 torr before and 66 +/- 19 torr after administration (p < 0.01). Compliance did not change as a result of surfactant administration. Only the animals (n = 3) in the normal O2 plus surfactant group died during the 90-minute post-near-drowning period of the study.

CONCLUSIONS

Surfactant administration (5 mL/kg) in this animal model of freshwater near drowning offers no benefit over simple ventilation. Assessment of the merit of smaller volumes of surfactant should be investigated because administration of this solution may represent a second pulmonary insult, given current volume recommendations.

Ventilation-perfusion relationships in preterm infants after surfactant treatment

Arterial-alveolar partial pressure differences for oxygen, carbon dioxide, and nitrogen were measured before and after surfactant replacement therapy on 15 occasions in 14 ventilator-dependent preterm infants with hyaline membrane disease (HMD). Eight treatments resulted in a significant improvement in arterial partial pressure of oxygen (PaO2) 2 hr after treatment; 7 did not. Neither group showed any significant change in arterial-alveolar partial pressure differences for oxygen, nitrogen, and carbon dioxide. This observation suggests that if surfactant replacement therapy produces an improvement in PaO2 it does so by recruitment of atelectatic alveoli with a balanced ventilation/perfusion ratio rather than by redistribution of ventilation within already ventilated alveoli.

Surfactant and the adult respiratory distress syndrome

ARDS includes a complex series of events leading to alveolar damage, high permeability pulmonary edema, and respiratory failure. The endogenous pulmonary surfactant system is crucial to maintaining normal lung function, and only recently has it been appreciated that alterations in the surfactant system significantly contributed to the pathophysiology of the lung injury of patients with ARDS. Through a combination of analyzing BAL samples from patients with ARDS and extensive animal studies, there have been significant insights into the variety of surfactant abnormalities that can occur in injured lungs. These include altered surfactant composition and pool sizes, abnormal surfactant metabolism, and inactivation of alveolar surfactant by serum proteins present within the airspace. Positive effects of exogenous surfactant administration on acute lung injury have been reported. There is now a prospective, randomized clinical trial evaluating the efficacy of aerosolized exogenous surfactant in patients with ARDS. This trial has demonstrated improvements in gas exchange and a trend toward decreased mortality in response to the surfactant. Despite these encouraging results, there are multiple factors requiring further investigation in the development of optimal surfactant treatment strategies for patients with ARDS. Such factors include the development of optimal surfactant delivery techniques, determining the ideal time for surfactant administration during the course of injury, and the development of optimal exogenous surfactant preparations that will be used to treat these patients. With further clinical trials and continued research efforts, exogenous surfactant administration should play a useful role in the future therapeutic approach to patients with ARDS.

Inactivation of exogenous surfactant in experimental respiratory failure induced by hyperoxia

Adult guinea pigs were exposed to 100% oxygen until, after 54-85 h, they developed severe respiratory insufficiency. One subgroup of animals was ventilated artificially with 100% oxygen for an additional 60-960 min. When the PaO2 was less than 15 kPa or the PaCO2 greater than 20 kPa, 1 ml of porcine surfactant (phospholipid concentration 80 mg.ml-1) was instilled via the trachea. These animals were ventilated for one more hour and then sacrificed. Surfactant instillation did not improve the blood gases, nor the pulmonary pressure-volume characteristics. All hyperoxia-exposed guinea pigs showed prominent histologic lung lesions, including intraalveolar edema and desquamation of airway epithelium. Compared to normal guinea pigs the volume density of intraalveolar "gas" was decreased and that of intraalveolar fluid increased. The alveolar expansion pattern in histologic sections was not improved in the surfactant-treated animals, compared to hyperoxia-exposed guinea pigs studied immediately after death. In hyperoxia-exposed animals, about 1.5 ml of edema fluid was sampled from the airways. Evaluated with pulsating bubble, our surfactant preparation had a minimum surface tension (gamma min) close to zero. However, the gamma min values of edema fluid from surfactant-treated and nontreated guinea pigs were both about 20 mN.m-1. the edema fluid thus seemed to inhibit the essential physical properties of exogenous surfactant. This, together with the prominent lung lesions, may explain the failure of surfactant replacement therapy at a late stage of hyperoxia-induced respiratory failure.