Effects Of High-frequency Ventilation Research Articles

Effect of High-frequency Ventilation on the Development of Alveolar Edema in Premature Monkeys at Risk for Hyaline Membrane Disease

High-frequency oscillatory ventilation (HFOV) permits adequate gas exchange but avoids the large phasic pressure-volume excursions of conventional mechanical ventilation (CMV); such avoidance may reduce the lung injury associated with hyaline membrane disease (HMD). We hypothesized that premature monkeys ventilated from birth with HFOV would have reduced lung injury compared to those assigned to CMV. Macaca nemestrina were delivered at 134 days (80% of term gestation) and ventilated from the first breath with either HFOV (n = 10) or CMV (n = 10). The mean airway pressure (Paw) was kept at 15 cm H2O pressure in HFOV animals; in CMV animals Paw was increased from 8 cm H2O at 2 h to 13 cm H2O at 6 h to prevent hypoxemia. At the conclusion of the 6-h experiment the HFOV animals had better oxygenation (p less than 0.05) and less evidence of HMD by chest radiograph (p less than 0.05). At 6 h of age a piece of the right middle lung lobe was removed, divided, and placed in fixatives for light and transmission electron microscopy. The lungs were subsequently inflated to 30 cm H2O pressure, and the right lower lobe was rapidly frozen in situ for morphometric studies. The proportion of peripheral lung tissue occupied by clear alveoli was greater in HFOV animals (66.3 +/- 14.8%) than in those assigned to CMV (44.2 +/- 16.9%, p less than 0.01); less alveolar debris and fluid was present in the HFOV animals (12.7 +/- 9.9%) compared with CMV animals (27.1 +/- 12.5%, p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Breathing and ventilation model, and simulation of high-frequency ventilation.

An extended model is proposed for lung respiration. This model is made to cope with the gas diffusion by convection in the airway network, gas exchange at the alveolus and gas transport by blood flow, as well as the gas flow phenomenon in the airway network induced by diaphragm motion. The submodel for each phenomenon is combined into a total respiratory system, and it is formulated in the context of discretization based on the adjoint variational principle. The model is examined first in terms of normal respiration and then high-frequency ventilation (HFV) is studied. The effects of HFV are evaluated through several case studies and it is quantitatively shown that the influence of the convective diffusion is dominant in the gas transport in the case of HFV. The proposed model is expected to be used to determine proper parameters of HFV.

INFLUENCE OF HIGH FREQUENCY VENTILATION AT DIFFERENT END-EXPIRATORY LUNG VOLUMES ON THE DEVELOPMENT OF LUNG DAMAGE DURING LUNG LAVAGE IN RABBITS

The effects of high frequency ventilation in combination with sustained inflations was studied in the surfactant-deficient lungs of 18 New Zealand White rabbits (weight 1.9-2.1 kg) during anaesthesia with urethane and neuromuscular block with pancuronium. Lung damage was induced by repeated lung lavage. In nine rabbits (group I) baseline ventilator settings were maintained constant throughout the study and airway pressure was readjusted to achieve a constant tidal volume. In the other nine rabbits (group II), ventilation was reinstituted after lung lavage with one period of four sustained inflations followed immediately by high frequency ventilation. In group I there was a significant decrease in gas exchange for oxygen and deterioration in pulmonary mechanics, whereas in group II there was little change in baseline blood-gas values or pulmonary mechanics. These data suggest that, with adequate ventilatory management during the period of lung lavage, the lung damage produced by this manoeuvre may be obviated.

Gas exchange during high-frequency ventilation in the pigeon (Columba livia).

We studied the effect of high-frequency ventilation (HFV) on the gas exchange of tracheotomized pigeons. The pigeons were artificially ventilated using a piston pump, which alternately connected the pigeons' airways to a constant-flow source. Two-minute periods of HFV were interposed between long periods of normal ventilation. The effect of HFV was assessed by the recorded changes in the PO2, PCO2 and pH of arterial blood and from the changes in the composition of the gas in the interclavicular air sacs. The results showed that HFV can augment gas exchange when the tidal volume (VT) is less than the volume of the anatomical dead space (VD). However, normal arterial gas composition can only be maintained if respiratory frequency is high (greater than 20 Hz). At the normal panting frequency of pigeons (7.8 Hz), gas exchange can thus only be maintained if tidal volume is approximately 125% of the dead space. When panting the VT must be greater than the VD. This finding agrees with the results of recent work showing flush-out- or compound-panting in birds: i.e. if, during panting, VT approaches close to the VD, intermittent interruptions, by taking deeper breaths in order to ensure a supply of fresh air to the lungs, are necessary.

Hemodynamic effects of high frequency ventilation superimposed on intermittent positive pressure ventilation in dogs.

The hemodynamic effects of high frequency ventilation (HFV) superimposed on intermittent positive pressure ventilation (IPPV) in seven dogs before and after thrombin infusion were investigated. HFV was superimposed on a Servo 900 B ventilator by a Siemens Elema HFV prototype unit. Mean arterial blood pressure, heart rate, central venous pressure, pulmonary artery pressure, cardiac output, right and left ventricular pressures, pleural pressure, arterial blood gases, and right and left ventricular ejection fractions were recorded. Measurements were done during IPPV alone and during HFV superimposed on IPPV. The HFV frequencies were 5, 15, and 20 Hz at a constant minute volume of 5 1. When HFV was started, the IPPV minute volume was reduced to one third of the initial volume. No significant changes in the measured parameters were observed during the different ventilatory modes either before or after thrombin infusion which doubled the pulmonary vascular resistance. It is concluded that high frequency ventilation superimposed on IPPV might be a ventilatory mode that offers cardiovascular stability and reduces the risk of barotrauma.

Viscoelasticity of Tracheobronchial Secretions in High-Frequency Ventilation

The effects of high-frequency ventilation (HFV) on the rheological properties of tracheobronchial secretions were investigated in vitro. The mucus was obtained by suction from 16 patients after major surgery of the larynx or hypopharynx. After short centrifugation, the mucus was treated with high-frequency vibrations of 3 or 30 Hz for 10 min in a chamber saturated with water vapor. The mucus was separated by a nonpermeable membrane from the ventilator's gas stream. Maximal, minimal, and mean viscosity were determined by a modified capillary viscosimeter. Under high-frequency vibrations, maximal, minimal, and mean viscosity slightly increased (p less than or equal to .001). Conversely, the mean viscosity of control samples did not change significantly. These findings indicate that high-frequency vibrations do not improve mucous rheology. Observed beneficial effects of HFV on mucociliary clearance may be caused by changes in the mucous subphase during HFV.

Cardiovascular effects of high frequency ventilation - the possible involvement of thromboxane

Recent studies with high frequency ventilation (HFV) have noted that HFV-induced increases in mean airway pressure leads to a marked cardiovascular depression, especially in cardiac output (CO).Aside from mechanical events a negative inotropic agent possibly prostaglandin in nature may also be involved. This study examined the possible involvement of thromboxane A 2 (TXA 2) in the HFV-induced cardiovascular deterioration. Chloralose-anesthetized mechanically-ventilated dogs were subjected to HFV 4, 10, and 20 mm Hg for 30 min. Some animals were also treated with imidazole (25 mg/Kg/hr) prior to HFV. Arterial levels of TXB 2 (stable metabolite of TXA 2) where monitored by radioimmunoassay. During HFV, tracheal pressure-related decreases in both CO and stroke volume (SV) were noted. Imidazole treatment significantly reduced the decrement in SV. Application of HFV resulted in variable changes in circulating TXB 2 levels. Overall, application of HFV did not result in a significant change from baseline levels.Furthermore there was no correlation between changes in CO and SV with changes in arterial TXB 2 concentration. These results do not support the hypothesis that hyperexpansion of the lungs during HFV causes the release of a cardiodepressant prostanoid.

HIGH FREQUENCY VENTILATION DECREASES THE BAROREFLEX

Effects of high frequency ventilation (HFV), such as apnea, are mediated by the autonomic nervous'system. To further define these effects, we studied an important autonomic reflex, the baroreflex, during HFV and conventional ventilation (CV) of 5 young adult male rabbits anesthetized with chloralose and paralyzed with metocurine. To keep FRC constant during change from CV to HFV, we continuously measured FRC by jacket plethysmograph, adjusting jet ventilator driving pressure as necessary. Ventilation (PaCO2) was also kept constant by adding CO2 to inspired gas as needed. We tested the baroreflex by increasing blood pressure (BP) with phenylephrine and plotting change in BP against change in R-R interval obtained from EKG. Measurements were made during baseline CV (pressure 25/4, rate 45), HFV (rate 600), and after return to CV. BP and heart rate were not affected by changes in ventilator frequency. The slope of the line of BP vs R-R interval was 31% to 62% less during HFV compared to CV (P<0.04). The slope of the baroreflex returned to baseline when CV was re-instituted. Since FRC and ventilation were constant, we speculate the decrease in baroreflex results from rapid pressure changes (dp/dt) during HFV stimulating pulmonary mechanoreceptors causing centrally mediated baroreceptor inhibition. We conclude that close cardiovascular monitoring of patients during HFV is warranted, especially when there is cardiovascular compromise (shock, hypovolemia, sedation or anesthesia). Neonates are especially at risk when the baroreflex is diminished because their relatively fixed stroke volume makes cardiac output heart rate dependent.

The effect of high frequency ventilation on phrenic nerve activity and the role of pulmonary stretch receptors

The effect of high frequency ventilation on phrenic nerve activity and the role of pulmonary stretch receptors

Hemodynamic effects of high frequency ventilation in surfactant-treated preterm lambs.

We compared the hemodynamic status and left ventricular (LV) performance in 7 twin pairs of preterm lambs delivered at 124 days gestational age (83% of term gestation) and ventilated by either conventional ventilation (CV) or high frequency ventilation (HFV) at 15 Hz. The lambs were treated with suspensions of natural sheep surfactant to permit ventilation and survival, and ventilatory settings were adjusted to maintain physiologic blood gas values. The ductus arteriosus was occluded with a balloon catheter at 40-45 min of age to eliminate the variable of a left to right ductal shunt. Cineangiocardiographic, pressure, and blood flow measurements were made 1 and 2 h after ductal occlusion. At the same mean airway pressures, the heart rates, LV end-diastolic volumes, and mean arterial pressures were similar in both groups. LV stroke volumes, ejection fractions, LV outputs, and organ blood flows also did not differ between the two groups. When compared with CV, HFV provides comparable ventilation with no apparent deleterious hemodynamic effects in preterm surfactant-treated lambs with occluded ductus arteriosus.

Effects of high frequency ventilation on lung lymph dynamics and prostaglandin generation

We examined the consequences of high frequency ventilation (HFV) on pulmonary lymph flow (Q̇lym) in normal lungs and after the lymph flow was increased with an i.v. histamine infusion (2 μg/kg/min). Studies were made in the anesthetized sheep prepared with lung lymph fistulas. The sheep were subjected to intermittent positive pressure ventilation (IPPV) (frequency (f) = 12 breaths/min, tidal volume (TV) = 10 ml/kg) and HFV (f = 7 Hz, TV = 1−2 /kg). TV was adjusted prior to the experiment to achieve normal arterial blood gases. Histamine infusion doubled Q̇lym and lymph protein clearance (Q̇lym × lymphto-plasma protein concentration ratio) from baseline. HFV did not affect Q̇lym and lymph protein clearance in normal lungs or after histamine infusion. HFV also did not result in generation of thromboxane (T×A 2) and prostacyclin because plasma concentrations of their stable degradation product, T×B 2 and 6-ketoprostaglandin F 1α, were not significantly elevated. The results indicate that HFV does not alter lymphatic pumping in the normal lung and during increased transcapillary protein transport.

High-frequency ventilation attenuation of hypoxic pulmonary vasoconstriction. The role of prostacyclin.

In order to determine the effects of high-frequency ventilation on the pulmonary vascular response to hypoxia, we assessed pulmonary vascular resistance at 2 levels of inspired oxygen tension (PlO2), 200 and 30 mmHg, during conventional and high-frequency ventilation in the isolated, blood-perfused lungs of 10 sheep, 5 treated with indomethacin (40 micrograms/ml of perfusate) and 5 untreated. Resistance was assessed by measuring pulmonary artery pressure-flow curves generated over a wide range of flows (20 to 140 ml X min-1 X kg body wt-1). Conventional ventilation was provided by an animal ventilator at a rate of 10 min-1 and a tidal volume of 10 ml X kg body wt-1. High-frequency ventilation was provided by a flow interrupter at a rate of 1,200 min-1 and a tidal volume less than 1.5 ml X kg body wt-1. In the 5 untreated lungs, the normoxic pressure-flow curve was unaltered by high-frequency ventilation, but the hypoxic pulmonary vasoconstrictor response was significantly attenuated. Furthermore, the net rate of change of 6-keto-prostaglandin F1 alpha concentration in the perfusate during hypoxia was significantly greater with high-frequency ventilation (65.4 +/- 8.9 pg X ml-1 X min-1) than with conventional ventilation (2.8 +/- 18.7 pg X ml-1 X min-1). In the 5 indomethacin-treated lungs, production of 6-keto-prostaglandin F1 alpha was markedly depressed, and the attenuation of the hypoxic vasoconstrictor response by high-frequency ventilation was abolished.(ABSTRACT TRUNCATED AT 250 WORDS)

PaO2 and intratracheal pressure in oscillatory ventilation in experimental respiratory failure.

In order to study the effect of high frequency ventilation on the pulmonary gas exchange in respiratory failure, we measured the PaO2 and other pulmonary gas exchange parameters for ventilatory rates of 0.5, 1.2, 4.8, and 16 Hz in dogs, to which acute respiratory failure was created by intravenous infusion of oleic acid. Either the mean intratracheal pressure or the end-tidal intratracheal pressure was controlled. A loud-speaker ventilator of our own design was modified so as to enable variation of the intratracheal pressure. Ventilation was measured using an ultrasonic instrument which counts the number of turbulent eddies. The tidal volume was set slightly higher than that was determined for healthy animals, but the resulting PaCO2 values were consistently higher than normal when PaO2 values were low. With the mean intratracheal pressure kept constant at 5, 10, and 15 cm H2O, PaO2 values with the FIO2 of 1 were between 41 and 46 at mPit of 5, 65 and 82 at 10 cm H2O, 278 and 423 at 15 cm H2O. No increase in PaO2 was observed with the increase in respiratory frequencies. If anything, a slight reduction in PaO2 at 8 and 16 Hz was observed, though not statistically significant. With the end-tidal intratracheal pressure constant, PaO2 varied but again correlated well with the mPit.