Pressure-limited ventilation with permissive hypercapnia and minimum PEEP in saline-lavaged rabbits allows progressive improvement in oxygenation, but does not avoid ventilator-induced lung injury

In vivo support for the new concept of pulmonary blood flow-mediated CO2 gas excretion in the lungs

To further examine the validity of the proposed concept of pulmonary blood flow-dependent CO2 gas excretion in the lungs, we investigated the effects of intramediastinal balloon catheterization-, pulmonary artery catheterization-, or isoprenaline (ISP)-induced changes in pulmonary blood flow on the end-expiratory CO2 gas pressure (PeCO2), the maximal velocity of the pulmonary artery (Max Vp), systemic arterial pressure, and heart rate of anesthetized rabbits. We also evaluated the changes in the PeCO2 in clinical models of anemia or pulmonary embolism. An almost linear relationship was detected between the PeCO2 and Max Vp. In an experiment in which small pulmonary arteries were subjected to stenosis, the PeCO2 fell rapidly, and the speed of the reduction was dependent on the degree of stenosis. ISP produced significant increases in the PeCO2 of the anesthetized rabbits. Conversely, treatment with piceatannol or acetazolamide induced significant reductions in the PeCO2. Treatment with a cell surface F1/FO ATP synthase antibody caused significant reductions in the PeCO2 itself and the ISP-induced increase in the PeCO2. Neither the PeCO2 nor SAP was significantly influenced by marked anemia [%hematocrit (Ht), 70∼47%]. On the other hand, in the presence of less severe anemia (%Ht: 100∼70%) both the PeCO2 and SAP fell significantly when the rabbits' blood viscosity was decreased. The rabbits in which pulmonary embolisms were induced demonstrated significantly reduced PeCO2 values, which was compatible with the lowering of their Max Vp. In conclusion, we reaffirm the validity of the proposed concept of CO2 gas exchange in the lungs.

Repeated open endotracheal suctioning causes gradual desaturation but does not exacerbate lung injury compared to closed endotracheal suctioning in a rabbit model of ARDS

Background

Although endotracheal suctioning induces alveolar derecruitment during mechanical ventilation, it is not clear whether repeated endotracheal suctioning exacerbates lung injuries. The present study aimed to determine whether repeated open endotracheal suctioning (OS) exacerbates lung injury compared to closed endotracheal suctioning (CS) during mechanical ventilation in an animal model of acute respiratory distress syndrome (ARDS).

Methods

Briefly, thirty six Japanese white rabbits were initially ventilated in pressure-controlled mode with a constant tidal volume (6 mL/kg). Then, lung injury was induced by repeated saline lavage. The rabbits were divided into four groups, namely: a) OS; b) CS; c) control with ARDS only; d) and healthy control (HC) without ARDS. Animals in all the groups were then ventilated with positive end expiratory pressure (PEEP) at 10 cm H₂O. CS was performed using 6 French-closed suctioning catheters connected to endotracheal tube under the following conditions: a) a suctioning time and pressure of 10 sec and 140 mm Hg, respectively; and b) a suction depth of 2 cm (length of adapter) plus tracheal tube. OS was performed using the same conditions described for CS, except the ventilator was disconnected from the animals. Each endotracheal suctioning was performed at an interval of 30 min.

Results

PaO2/FIO2 (P/F) ratio for CS, control and HC groups remained at >400 for 6 hours, whereas that of the OS group progressively declined to 300 (p < 0.05), with each suctioning. However, no difference was observed either in lung injury score (histology) or in the expression pattern of inflammatory cytokines (tumor necrosis factor-α and interleukin-6) after 6 hours between the OS and CS groups in the circulatory as well as the pulmonary tissues.

Conclusions

Progressive arterial desaturation under conditions of repeated endotracheal suctioning is greater in OS than in CS time-dependently. However, OS does not exacerbate lung injury during mechanical ventilation when observed over a longer time span (6 hours) of repeated endotracheal suctioning, based on morphological and molecular analysis.

Improvement in ARDS experimental model installation: low mortality rate and maintenance of hemodynamic stability

INTRODUCTION

Many experimental models using lung lavage have been developed for the study of acute respiratory distress syndrome (ARDS). The original technique has been modified by many authors, resulting in difficulties with reproducibility. There is insufficient detail on the lung injury models used, including hemodynamic stability during animal preparation and drawbacks encountered such as mortality. The authors studied the effects of the pulmonary recruitment and the use of fixed tidal volume (Vt) or fixed inspiratory pressure in the experimental ARDS model installation.

METHODS

Adult rabbits were submitted to repeated lung lavages with 30 ml/kg warm saline until the ARDS definition (PaO₂/FiO₂ ≤ 100) was reached. The animals were divided into three groups, according to the technique used for mechanical ventilation: 1) fixed Vt of 10 ml/kg; 2) fixed inspiratory pressure (IP) with a tidal volume of 10 ml/kg prior to the first lung lavage; and 3) fixed Vt of 10 ml/kg with pulmonary recruitment before the first lavage.

RESULTS

The use of alveolar recruitment maneuvers, and the use of a fixed Vt or IP between the lung lavages did not change the number of lung lavages necessary to obtain the experimental model of ARDS or the hemodynamic stability of the animals during the procedure. A trend was observed toward an increased mortality rate with the recruitment maneuver and with the use of a fixed IP.

DISCUSSION

There were no differences between the three study groups, with no disadvantage in method of lung recruitment, either fixed tidal volume or fixed inspiratory pressure, regarding the number of lung lavages necessary to obtain the ARDS animal model. Furthermore, the three different procedures resulted in good hemodynamic stability of the animals, and low mortality rate.

Endogenous adenosine and secondary injury after chest trauma

BACKGROUND

No previous studies have examined actions of adenosine or related compounds after blunt chest trauma, but we have shown that the prototype adenosine-regulating agent, acadesine (aminoimidazole carboxamide ribonucleotide [AICAR]), has multiple favorable anti-inflammatory actions after other forms of trauma, ischemia, hemorrhage, and sepsis; and that a progressive inflammatory response in the contralateral (uninjured) lung after unilateral blunt chest trauma is caused (in part) by activation and sequestration of circulating leukocytes (white blood cells [WBCs]). Thus, we hypothesized that AICAR would ameliorate WBC-dependent, secondary pathophysiologic changes after blunt chest trauma.

METHODS

Mongrel pigs (28+/-1 kg, n = 21) were anesthetized, mechanically ventilated, and injured on the right chest (pulmonary contusion) with a captive bolt gun. Either AICAR (1 mg/kg + 0.2 mg/kg/min) or its saline vehicle were administered for a 12-hour period, beginning 15 minutes before injury.

RESULTS

Injury caused a three- to fourfold increase in bronchoalveolar lavage (BAL) WBC counts, 10- to 20-fold increases in BAL protein, and 200% increases in lung edema as measured by wet-dry ratio (all p < 0.05), in both the injured (right) and the noninjured (left) lungs. With AICAR versus saline, BAL WBC counts, lung myeloperoxidase levels, and systemic hemodynamics were similar. However, the increases in BAL protein were attenuated by 30% to 50% (p < 0.14, NS) and edema was reduced (p < 0.05) in both lungs. Furthermore, oxygenation, hypercapnia, acidosis (all p < 0.05), and survival were improved (9 of 10 vs. 4 of 11, p < 0.04).

CONCLUSION

Pretreatment with AICAR before experimental pulmonary contusion ameliorates the trauma-induced destruction of the alveolar capillary membrane, and attenuates the delayed secondary injury in the contralateral uninjured lung, by a mechanism that may be independent of leukocytes. Endogenous adenosine could have a role in the pathophysiologic response after blunt chest injury, with potential sites of action including the endothelium and alveolar macrophage. Adenosine-regulating agents may have therapeutic potential after blunt chest injury, but further studies are needed in clinically relevant models, with administration begun at the time of resuscitation.

Respiratory failure: current status of experimental therapies

A number of advances in the treatment of infants and children with respiratory failure have been investigated in the laboratory with translation to clinical practice. Investigators have recognized that application of high ventilating pressures and failure to apply adequate levels of positive end-expiratory pressure (PEEP) can inflict injury to the already failing lung. Other interventions such as prone positioning and application of new ventilating strategies such as proportional assist ventilation (PAV), inverse ratio ventilation (IRV), high frequency ventilation, liquid ventilation, and intratracheal pulmonary ventilation (ITPV), continue to be developed and explored. Administration of inhaled nitric oxide (iNO) may improve pulmonary physiology and gas exchange in patients with respiratory insufficiency. Finally, the technique of extracorporeal life support (ECLS) is being simplified and refined. This report summarizes the status of these advances and describes the basic science and clinical research that brought them to clinical application.

Prostanoids: early mediators in the secondary injury that develops after unilateral pulmonary contusion

BACKGROUND

We have previously shown a sequence of events after unilateral pulmonary contusion that suggests the release of blood-borne prostanoid mediators and that culminates in refractory bilateral pulmonary failure.

PURPOSE

To determine the role of platelet-derived thromboxane and endothelial-derived prostacyclin in the primary and secondary injury after unilateral blunt chest trauma, and to determine whether pretreatment with the cyclooxygenase inhibitor indomethacin alters the progression of secondary injury.

METHODS

Anesthetized, ventilated (FIO2 = 0.50) pigs received a unilateral, blunt injury to the right thorax (n = 20) or sham injury (n = 5) and were monitored for 24 hours. Either indomethacin (5 mg/kg i.v.; n = 10) or its saline vehicle (n = 10) were administered 15 minutes before injury. Serial bronchoalveolar lavages of each lung were analyzed for protein and neutrophil (polymorphonuclear neutrophil (PMN)) content.

RESULTS

Contusion caused profound hypoxemia; PaO2 partially recovered within 1 hour of injury to 50% of baseline. Thereafter, worsening hypoxemia required positive end-expiratory pressure. With indomethacin compared with vehicle, PaO2 was higher at any given level of positive end-expiratory pressure (p < 0.05). There was an early increase in serial bronchoalveolar lavage protein on the injured side (peak at 2 hours), with a delayed pulmonary capillary leak on the contralateral side (peak at 6 hours), which correlated with increasing PMN infiltration; this was reduced by 40 to 60% with indomethacin (p < 0.05). Thromboxane peaked within 1 hour after contusion at 800% baseline, then fell off rapidly. This peak preceded the maximal increase in permeability and was completely blocked by indomethacin. Prostacyclin slowly rose to 300% baseline by 3 hours and remained elevated; this change was blocked by indomethacin for 18 hours.

CONCLUSIONS

Contusion of the right thorax induced a delayed pulmonary capillary leak in the left lung, which reflects a progressive secondary inflammatory response. Elevations in thromboxane and prostacyclin preceded progressive bilateral PMN infiltration. Indomethacin blocked thromboxane and prostacyclin and attenuated, but did not prevent, the progression to pulmonary failure. Overall, these data suggest that prostanoids are released soon after unilateral contusion and initiate an inflammatory response in both lungs that is sustained by PMN infiltration.

Extreme hypoventilation reduces ventilator-induced lung injury during ventilation with low positive end-expiratory pressure in saline-lavaged rabbits

OBJECTIVE

To compare the degrees of ventilator-induced lung injury caused by two ventilation protocols.

DESIGN

Randomized trial.

SETTING

University animal laboratory.

SUBJECTS

Sixteen New Zealand white rabbits.

INTERVENTIONS

After five sequential saline lung lavages, eight pairs of anesthetized rabbits were allocated randomly to receive either of two ventilation protocols for 4 hrs during neuromuscular blockade. Both groups received 3 cm H2O of positive end-expiratory pressure and 100% oxygen. Control group animals received an estimated tidal volume of 12 mL/kg, an inspiratory time of 0.7 sec, and a ventilatory rate adjusted for a PaCO2 of 35 to 45 torr (4.7 to 6 kPa). Study group animals were ventilated through an intratracheal catheter, with inspiratory time of 1.5 secs, ventilatory rate of 20 breaths/min, and peak inspiratory pressure of 4 to 8 cm H2O, adjusted to maintain PaCO2 at 150 to 250 torr (20 to 33 kPa).

MEASUREMENTS AND MAIN RESULTS

Arterial blood gases were measured every 30 mins. After 4 hrs, a final lung lavage was performed. Physiologic parameters, cell counts and protein concentration in the final lavage, and lung histology were compared between groups. The alveolar-arterial oxygen tension gradient was higher in the study group over the first 1.5 hrs, but the time profile showed significantly (p = .001) greater improvement in the study group. After 4 hrs, the mean alveolar-arterial oxygen tension gradient was lower in the study group (94 torr [12.5 kPa] vs. 201 torr [26.8 kPa]). The increase in neutrophil count from the initial to the final lung lavage was lower in the study group (0.27 x 10(7) cells/L vs. 2.01 x 10(7) cells/L, p = .037), as was the absolute value of the neutrophil count in the final lavage (1.33 x 10(7) cells/L vs. 3.02 x 10(7) cells/L, p = .04). The median hyaline membrane score was lower in the study group (0.5 vs. 3.0) but the difference was not statistically significant.

CONCLUSION

These findings suggest that a very low tidal volume reduces ventilator-induced lung injury in saline-lavaged rabbits during ventilation at low lung volume.