Partial liquid ventilation combined with inhaled nitric oxide in acute respiratory failure with pulmonary hypertension in piglets

Effects of perfluorochemical evaporative properties on oxygenation during partial liquid ventilation

BACKGROUND

The physical-chemical properties of perfluorochemical (PFC) liquids have been shown to influence physiological and cellular responses during partial liquid ventilation (PLV). The aim of this study is to compare the relationship between patho-physiological endpoints and the physical properties of three PFC liquids used in treating acute lung injury.

METHODS

A total of 18 juvenile rabbits were randomized into conventional mechanical ventilation or PLV groups after lung saline lavages. Three PFC liquids, including Flutec perfluoro-1,3,5-trimethylcyclohexane (PP4; vapor pressure, 28.8 mmHg at 37 degrees C), Perfluorodecalin (PFD; vapor pressure, 13.6 mmHg at 37 degrees C), and Perflubron (PFB; vapor pressure, 10.4 mmHg at 37 degrees C) were used for PLV with no replacement for 4 h. A thermal detector was used to measure PFC loss rate. Physiological measurements and evaporative loss rate of PFC were done every 30 min, and lung histology was examined.

RESULTS

The mean evaporative loss rate was significantly higher in the PP4 group (4.75 +/- 0.24 mL/kg per h) than in either the PFD (1.43 +/- 0.11 mL/kg per h) or the PFB (1.18 +/- 0.05 mL/kg per h) group (P < 0.05). The oxygenation of PFD and PFB was maintained good for 4 h, however, the PP4 group showed a fast deterioration since 2 h post-treatment due to fast dropping of the residual PP4 amount in lungs. Histology showed good alveolar integrity in the PFD and PFB groups.

CONCLUSIONS

The effects of PLV are directly influenced by the evaporative property of the PFC liquid. With no replacement over 4 h, PLV effects could be maintained with utilizing a PFC liquid with low, rather than high, vapor pressure. PFC with high vapor pressure has a high loss rate and low residual volume that causes poor maintenance on oxygenation during PLV. Therefore, measuring PFC loss rate is important in future studies and clinical application of PLV.

Arterial oxygen fluctuation and retinopathy of prematurity in very-low-birth-weight infants

OBJECTIVE

To determine the influence of arterial oxygen fluctuation on development of threshold ROP.

STUDY DESIGN

Retrospective study of 231 infants, < or =1500 g birth weight, who were admitted to Arkansas Children's Hospital NICU from January 1993 to June 1995. Fluctuation in partial pressure of dissolved arterial oxygen (PaO(2)) was expressed as coefficient of variation (CoV) for each infant. We investigated the relationship between CoV at three intervals and the risk of developing threshold ROP.

RESULTS

The odds ratio (OR) of developing threshold ROP versus prethreshold ROP or less associated with a 10% increase in the CoV during the first 5 days of oxygen therapy was 1.44, and during the first 10 days was 1.51. When analysis was restricted to infants receiving 30 days of therapy, the OR during the first 5 days of therapy was 1.67, during the first 10 days was 1.82, and during days 11-30 was 1.68.

CONCLUSIONS

Very-low-birth-weight infants experiencing fluctuating PaO(2) are at higher risk of threshold ROP.

Perfluorochemical (PFC) combinations for acute lung injury: an in vitro and in vivo study in juvenile rabbits

Perfluorochemical (PFC) fluids of different physical properties were titrated and tested in vitro for physical properties that are appropriate for respiratory application. Two PFC liquids were studied: perfluoromethylcyclohexane (PP2), a liquid with high vapor pressure and low viscosity, and perfluoromethyldecalin (PP9), a fluid with low vapor pressure and high viscosity. Eighteen rabbits (2.05 +/- 0.07 kg; mean +/- SEM) were lung-lavaged and randomized: group I, control group; group II, partial liquid ventilation with 75% PP2 and 25% PP9; group III, partial liquid ventilation with 50% PP2 and 50% PP9; and group IV, partial liquid ventilation with 25% PP2 and 75% PP9. Ventilator volumes were kept constant during the 4-h experiment. Cardiopulmonary measurements were performed every 30 min. The lung histology was examined. The in vitro study showed PFC [viscosity/vapor pressure (in cS and mm Hg, respectively)] as follows: 100% PP2 (0.88/141); 100% PP9 (3.32/2.9); 75% PP2 and 25% PP9 (1.26/107); 50% PP2 and 50% PP9 (1.63/13.7); and 25% PP2 and 75% PP9 (2.21/4.4). The in vivo experiments found that combinations of moderate vapor pressure (groups 3 and 4) demonstrated good gas exchange, compliance, and histologic findings. Thus, combinations of PFC liquids can be formulated to modulate the physiologic outcome in acutely injured lungs, and may prove useful for alternative PFC liquid applications.

Partial liquid ventilation versus conventional mechanical ventilation with high PEEP and moderate tidal volume in acute respiratory failure in piglets

This prospective randomized pilot study aimed to test the hypotheses that partial liquid ventilation combined with a high positive end-expiratory pressure (PEEP) and a moderate tidal volume results in improved gas exchange and lung mechanics without negative hemodynamic influences compared with conventional mechanical ventilation in acute lung injury in piglets. Acute lung injury was induced in 12 piglets weighing 9.0 +/- 2.4 kg by repeated i.v. injections of oleic acid and repeated lung lavages. Thereafter, the animals were randomly assigned either to partial liquid ventilation (n = 6) or conventional mechanical ventilation (n = 6) at a fractional concentration of inspired O(2) of 1.0, a PEEP of 1.2 kPa, a tidal volume < 10 mL/kg body weight (bw), a respiratory rate of 24 breaths/min, and an inspiratory/expiratory ratio of 1:2. Perfluorocarbon liquid 30 mL/kg bw was instilled into the endotracheal tube over 10 min followed by 5 mL/kg bw/h. Continuous monitoring included ECG, mean right atrial, pulmonary artery, pulmonary capillary, and arterial pressures, arterial blood gas, and partial pressure of end-tidal CO(2) measurements. When compared with control animals, partial liquid ventilation resulted in significantly better oxygenation with improved cardiac output and oxygen delivery. Dead space ventilation appeared to be lower during partial liquid ventilation compared with conventional mechanical ventilation. No significant differences were observed in airway pressures, pulmonary compliance, and airway resistance between both groups. The results of this pilot study suggest that partial liquid ventilation combined with high PEEP and moderate tidal volume improves oxygenation, dead space ventilation, cardiac output, and oxygen delivery compared with conventional mechanical ventilation in acute lung injury in piglets but has no significant influence on lung mechanics.

Effects of partial liquid ventilation with FC-77 on acute lung injury in newborn piglets

Partial liquid ventilation (PLV) with various types of perfluorochemicals (PFC) has been shown to be beneficial in treating acute lung injury. FC-77 is a type of PFC with relatively high vapor pressure and evaporative losses during PLV. This study tested the hypothesis that using FC-77 for PLV with hourly replacement is effective in treating acute lung injury. Fifteen neonatal piglets were randomly and evenly divided into 3 study groups: 1) lavage-induced lung injury followed by conventional mechanical ventilation (Lavage-CMV); 2) lavage-induced lung injury followed by PLV using FC-77 with hourly replacement (11.2 +/- 1.5 mL/kg/hr) (Lavage-PLV); and 3) sham lavage injury followed by conventional mechanical ventilation (Control). Immediately after induction, repeated saline lavages induced acute lung injury characterized by decreases in dynamic lung compliance, arterial oxygen tension, and arterial pH, and increases in arterial CO(2) tension and oxygenation index, whereas the sham lavage procedure failed to do so. During the 3-hr period of CMV, these pulmonary and cardiovascular parameters remained stable in the Control group, but deteriorated in the Lavage-CMV group. In contrast, after acute lung injury, low lung compliance, abnormal gas exchange, acidosis, and inadequate oxygenation significantly improved in the Lavage-PLV group. Histological analysis of these 3 study groups revealed that the Lavage-CMV group had the highest lung injury score and the Control group had the lowest. These results suggest that, in comparison to CMV, PLV with FC-77 and hourly replacement of FC-77 promotes more favorable pulmonary mechanics, gas exchange, oxygenation, and lung histology in a piglet model of acute lung injury.

Partial liquid ventilation with and without inhaled nitric oxide in a newborn piglet model of meconium aspiration

The object of this study was to determine the effects of partial liquid ventilation (PLV) with and without inhaled nitric oxide (NO) over a 4-h period on lung mechanics, gas exchange, and hemodynamics in an animal model of meconium aspiration syndrome (MAS). Twenty-four fentanyl-anesthetized piglets were instrumented and administered a slurry of human meconium to create a model with hypoxia, hypercarbia, acidosis, and pulmonary hypertension. They were then randomly assigned to conventional ventilation, conventional ventilation plus inhaled NO at 40 ppm, PLV using perfluorodecalin, or PLV plus inhaled NO. The perfluorocarbon was added until a meniscus was visible in the endotracheal tube during expiration. Hemodynamics, lung mechanics, and gas exchange were monitored for 4 h, and then the animals were killed. The conventionally ventilated animals continued to deteriorate, and three of the six died prior to 4 h. All the animals in the remaining groups survived. Oxygenation improved significantly immediately with the start of inhaled NO (from 43.8 SD 10.3 to 62.6 SD 11.7 mm Hg after 30 min) and stayed elevated compared with the control group for the remainder of the study (62.4 SD 21.8 mm Hg at 4 h compared with 44.9 SD 1.6 mm Hg for the control group, p < 0.05). Oxygenation improved more slowly in the PLV alone group, being slightly less than control at 30 min (p = NS) but increasing to 104 SD 34.9 after 4 h (p < 0.01 compared with the control group), at which time it was also greater than inhaled NO alone (p < 0.05). The combined group had an acute increase in oxygenation indistinguishable from the NO alone group and maintained this until the end of the study. Lung compliance was unaffected in the inhaled NO group. In both the liquid ventilation groups the lung compliance improved with the instillation of perfluorodecalin (from 0.46 SD 0.18 to 0.62 SD 0.09 ml/cm H(2)O/kg in the PLV alone group at 1 h, p < 0.05 compared with the control group) and remained stable for the remainder of the study. Cardiac output and pulmonary vascular resistance were not significantly affected by any of the treatments. It was concluded that in this animal model of MAS, inhaled NO led to an acute improvement in gas exchange and prolonged survival compared with conventional therapy. PLV improved lung mechanics, which was maintained over the course of the study. The combination of PLV and inhaled NO produced both effects, acutely improving both gas exchange and lung mechanics. Combined therapy with PLV and inhaled NO may have benefits in the MAS.

Liquid ventilation: a future modality?

Liquid ventilation, an idea currently being trialled in the United States, is increasingly being discussed as a possible future trend in ventilation. A review of the available literature indicates that this treatment provides effective gas exchange and has a number of potential advantages. These include lower airway pressures, decreased alveolar surface tension, alveolar recruitment and removal of pulmonary exudate. While yet to be seen in this country, liquid ventilation may be introduced in the future. If it is, those caring for patients treated in that way will require knowledge of the mechanics and physiological changes involved, as well as the potential hazards of this modality.

Perfluorocarbon priming and surfactant: physiologic and pathologic effects

OBJECTIVE

To test the hypothesis that perfluorocarbon (PFC) priming before surfactant administration improves gas exchange and lung compliance, and also decreases lung injury, more than surfactant alone.

DESIGN

Prospective, randomized animal study.

SETTING

Animal research laboratory of Children's Hospital of St. Paul.

SUBJECTS

Thirty-two newborn piglets, weighing 1.55 +/- 0.18 kg.

INTERVENTIONS

We studied four groups of eight animals randomized after anesthesia, paralysis, tracheostomy, and establishment of lung injury using saline washout to receive one of the following treatments: a) surfactant alone (n = 8); b) priming with the PFC perflubron alone (n = 8); c) priming with perflubron followed by surfactant (n = 8); and d) no treatment (control; n = 8). Perflubron priming was achieved by instilling perflubron via the endotracheal tube in an amount estimated to represent the functional residual capacity, ventilating the animal for 30 mins, and then removing perflubron by suctioning. After all treatments were given, animals were mechanically ventilated for 4 hrs.

MEASUREMENTS AND MAIN RESULTS

We evaluated oxygenation, airway pressures, respiratory system compliance, and hemodynamics at baseline, after induction of lung injury, and at 30-min intervals for 4 hrs. Histopathologic evaluation was carried out using a semiquantitative scoring system and by computer-assisted morphometric analysis. After all treatments, animals had decreased oxygenation indices (p < .001) and increased respiratory system compliance (p < .05). Animals in PFC groups had similar physiologic responses to treatments as animals treated with surfactant only; both the PFC-treated groups and the surfactant-treated animals required lower mean airway pressures throughout the experiment (p < .001) and had higher pH levels at 90 and 120 mins (p < .05) compared with the control group. Pathologic analysis demonstrated decreased lung injury in surfactant-treated animals compared with animals treated with PFC or the controls (p < .02).

CONCLUSIONS

Priming the lung with PFC neither improved the physiologic effects of exogenous surfactant nor improved lung pathology in this animal model.

The effect of positive end-expiratory pressure during partial liquid ventilation in acute lung injury in piglets

OBJECTIVES

To investigate the effects of positive end-expiratory pressure (PEEP) application during partial liquid ventilation (PLV) on gas exchange, lung mechanics, and hemodynamics in acute lung injury.

DESIGN

Prospective, randomized, experimental study.

SETTING

University research laboratory.

SUBJECTS

Six piglets weighing 7 to 12 kg.

INTERVENTIONS

After induction of anesthesia, tracheostomy, and controlled mechanical ventilation, animals were instrumented with two central venous catheters, a pulmonary artery catheter and two arterial catheters, and an ultrasonic flow probe around the pulmonary artery. Acute lung injury was induced by the infusion of oleic acid (0.08 mL/kg) and repeated lung lavage procedures with 0.9% sodium chloride (20 mL/kg). The protocol consisted of four different PEEP levels (0, 5, 10, and 15 cm H2O) randomly applied during PLV. The oxygenated and warmed perfluorocarbon liquid (30 mL/kg) was instilled into the trachea over 5 mins without changing the ventilator settings.

MEASUREMENTS AND MAIN RESULTS

Airway pressures, tidal volumes, dynamic and static pulmonary compliance, mean and expiratory airway resistances, and arterial blood gases were measured. In addition, dynamic pressure/volume loops were recorded. Hemodynamic monitoring included right atrial, mean pulmonary artery, pulmonary capillary wedge, and mean systemic arterial pressures and continuous flow recording at the pulmonary artery. The infusion of oleic acid combined with two to five lung lavage procedures induced a significant reduction in PaO2/FI(O2) from 485 +/- 28 torr (64 +/- 3.6 kPa) to 68 +/- 3.2 torr (9.0 +/- 0.4 kPa) (p < .01) and in static pulmonary compliance from 1.3 +/- 0.06 to 0.67 +/- 0.04 mL/cm H2O/kg (p < .01). During PLV, PaO2/FI(O2) increased significantly from 68 +/- 3.2 torr (8.9 +/- 0.4 kPa) to >200 torr (>26 kPa) (p < .01). The highest PaO2 values were observed during PLV with PEEP of 15 cm H2O. Deadspace ventilation was lower during PLV when PEEP levels of 10 to 15 cm H2O were applied. There were no differences in hemodynamic data during PLV with PEEP levels up to 10 cm H2O. However, PEEP levels of 15 cm H2O resulted in a significant decrease in cardiac output. Dynamic pressure/volume loops showed early inspiratory pressure spikes during PLV with PEEP levels of 0 and 5 cm H2O.

CONCLUSIONS

Partial liquid ventilation is a useful technique to improve oxygenation in severe acute lung injury. The application of PEEP during PLV further improves oxygenation and lung mechanics. PEEP levels of 10 cm H2O seem to be optimal to improve oxygenation and lung mechanics.

Recognition and management of pulmonary hypertension

Pulmonary hypertension (mean pulmonary arterial pressure > 20mm Hg at rest or > 30mm Hg during exercise) occurs (i) as primary pulmonary hypertension (no known underlying cause), (ii) as persistent pulmonary hypertension of the newborn or (iii) secondary to a variety of lung and cardiovascular diseases. In the last 10 to 15 years there have been significant advances in the medical management of this debilitating and life-threatening disorder. The main drugs in current use are anticoagulants (warfarin, heparin) and vasodilators, especially oral calcium antagonists, intravenous prostacyclin (prostaglandin I2; epoprostenol) and inhaled nitric oxide. Calcium antagonists, (e.g. nifedipine, diltiazem) are used chiefly in primary pulmonary hypertension. They are effective in patients who give a pulmonary vasodilator response to an acute challenge with a short acting vasodilator (e.g. prostacyclin, nitric oxide or adenosine), and are used in doses greater than are usual in the treatment of other cardiovascular disorders. Prostacyclin, given by continuous intravenous infusion, is effective in patients even if they do not respond to an acute vasodilator challenge. The long term benefit in these patients is thought to reflect the antiproliferative effects of the drug and/or its ability to inhibit platelet aggregation. It is used either as long term therapy or as a bridge to transplantation. Inhaled nitric oxide, which is used mainly in persistent pulmonary hypertension of the newborn, has the particular benefit of being pulmonary selective, due to its route of administration and rapid inactivation. Anticoagulants have a specific role in the treatment of pulmonary thromboembolic pulmonary hypertension and are also used routinely in patients with primary pulmonary hypertension. Nondrug treatments for pulmonary hypertension include (i) supplemental oxygen (> or = 15 h/day), which is the primary therapy in patients with pulmonary hypertension secondary to chronic obstructive pulmonary disease and (ii) heart-lung or lung transplantation, which nowadays is regarded as a last resort. Different types of pulmonary hypertension require different treatment strategies. Future advances in the treatment of pulmonary hypertension may come from the use of drug combinations, the development of new drugs, such as endothelin antagonists, nitric oxide donors and potassium channel openers, or the application of gene therapy.

Non-conventional respiratory support modalities applicable in the older child. High frequency ventilation and liquid ventilation

HFV, LV, and several other novel therapies offer promise to adults and children that the mortality associated with respiratory failure may be affected. Although there are several forms of HFV, HFOV is presently gaining favor in the treatment of severe respiratory failure and has generally supplanted HFJV in pediatric critical care. HFOV has the advantage of having an active expiratory phase, which helps to minimize air trapping and better modulate mean lung volume. Ventilators with sufficient power to perform HFOV in adults are currently under investigation, although there is a growing experience in using current ventilators in larger patients. To date, however, demonstration of lowered mortality with HFOV is lacking although intermediate outcome indicators are improved. PLV also offers promise in the treatment of ARF through its drastic ability to improve oxygenation, ventilation, and compliance in many lung injury models. Human trials are presently underway, but the optimal delivery of this novel therapy still necessitates extensive investigation. TLV is likely even more removed from general clinical application given the necessity of developing a new generation of ventilators for the delivery of liquid tidal volumes. How these and other modalities may piece together to improve the condition of our patients who have respiratory failure remains to be seen, but certainly, present and future investigation will be intriguing for years to come.