Using conventional infant ventilators at unconventional rates.

The effect of progressive increases in ventilator rate on delivered tidal and minute volumes, and the effect of changing peak inspiratory pressure (Pmax), positive end-expiratory pressure (PEEP), and inspiration to expiration (I:E) ratio at different ventilator rates were examined. Five different continuous-flow, time-cycled, pressure-preset infant ventilators were studied using a pneumotachograph, an airway pressure monitor, and a lung simulator. As rates increased from 10 to 150 breaths per minute, tidal volume stayed constant until 25 to 30 breaths per minute; then progressively decreased. In all, tidal volume began to decrease when proximal airway pressure waves lost inspiratory pressure plateaus. As rates increased, minute volume increased until 75 breaths per minute, then leveled off, then decreased. Substituting helium for O2 increased the ventilator rate at which this minute volume plateau effect occurred. Increasing peak inspiratory pressure consistently increased tidal volume. Increasing positive end-expiratory pressure decreased tidal volume. At rates less than 75 breaths per minute, inspiratory time (inspiration to expiration ratio) had little effect on delivered volume. At rates greater than 75 breaths per minute, inspiratory time became an important determinant of minute volume. For any given combination of lung compliance and airway resistance: there is a maximum ventilator rate beyond which tidal volume progressively decreases and another maximum ventilator rate beyond which minute volume progressively decreases; at slower rates, delivered volumes are determined primarily by changes in proximal airway pressures; at very rapid rates, inspiratory time becomes a key determinant of delivered volume.

High-frequency ventilation and tracheal injuries.

Recent reports linking serious tracheal injuries to various forms of high-frequency ventilation prompted this study. We compared the tracheal histopathology seen following standard-frequency, conventional mechanical ventilation with that seen following high-frequency, conventional mechanical ventilation, and two different forms of high-frequency jet ventilation. Twenty-six adult cats were examined. Each was mechanically ventilated for 16 hours. Seven received standard-frequency, conventional mechanical ventilation at 20 breaths per minute. Seven received high-frequency, conventional mechanical ventilation at 150 breaths per minute. Six received high-frequency jet ventilation at 250 breaths per minute via the Instrument Development Corporation VS600 jet ventilator (IDC). Six received high-frequency jet ventilation at 400 breaths per minute via the Bunnell Life Pulse jet ventilator (BLP). A semiquantitative histopathologic scoring system graded tracheal tissue changes. All forms of high-frequency ventilation produced significant inflammation (erosion, necrosis, and polymorphonuclear leukocyte infiltration) in the trachea in the region of the endotracheal tube tip. Conventional mechanical ventilation produced less histopathology than any form of high-frequency ventilation. Of all of the ventilators examined, the BLP, the ventilator operating at the fastest rate, produced the greatest loss of surface cilia and depletion of intracellular mucus. IDC high-frequency jet ventilation and high-frequency, conventional mechanical ventilation produced nearly identical histologic injuries. In this study, significant tracheal damage occurred with all forms of high-frequency ventilation.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal high-frequency jet ventilation: four years' experience.

During a 4-year period, 34 neonates were treated with high-frequency jet ventilation (HFJV) using two different HFJV systems. Twenty-three of the neonates had severe pulmonary air leaks, five had congenital left-sided diaphragmatic hernias, and six had end-stage respiratory failure without pulmonary air leaks. The two HFJV systems performed similarly in all pathologic conditions. Following HFJV, arterial blood gas values improved in 28 of the 34 patients (82%). Eleven patients (32%) ultimately survived. Of 23 patients with pulmonary air leaks, 17 (74%) improved, nine (39%) survived. One infant with diaphragmatic hernia and one with end-stage respiratory failure survived. Ten of 12 patients (85%) who died following eight or more hours of HFJV had significant tracheal histopathology in the region of the endotracheal tube tip. The lesions ranged from moderate erythema to severe necrotizing tracheobronchitis with total tracheal obstruction. HFJV can be useful in the treatment of severe pulmonary air leaks in neonates and may prove useful in the treatment of congenital diaphragmatic hernias. However, HFJV produces inflammatory injuries in the proximal trachea. More clinical and laboratory studies are needed to define the relative risks and benefits of this new therapy.

Monosomy 9p24-->pter and trisomy 5q31-->qter: case report and review of two cases.

Partial deletion of the short arm of chromosome 9 (p24-->pter) and partial duplication of the long arm of chromosome 5 (q32-->qter) were observed in an abnormal boy who died at age 8 weeks of a complex cyanotic cardiac defect. He also had minor anomalies, sagittal craniosynostosis, triphalangeal thumbs, hypospadias, and a bifid scrotum. Two other infants with similar cytogenetic abnormalities were described previously. These patients had severe congenital heart defect, genitourinary anomalies, broad nasal bridge, low hairline, apparently low-set ears, short neck, and triphalangeal thumbs, in common with our patient. We suggest that combined monosomy 9p23,24-->pter and trisomy 5q31,32-->qter may constitute a clinically recognizable syndrome.

Variations in end-expiratory pressure during partial liquid ventilation: impact on gas exchange, lung compliance, and end-expiratory lung volume.

STUDY OBJECTIVES: To determine the effects of different levels of positive end-expiratory pressure (PEEP) during partial liquid ventilation (PLV) on gas exchange, lung compliance, and end-expiratory lung volume (EELV). DESIGN: Prospective animal study. SETTING: Animal physiology research laboratory. SUBJECTS: Nine piglets. INTERVENTIONS: Animals underwent saline solution lavage to produce lung injury. Perflubron was instilled via the endotracheal tube in a volume estimated to represent functional residual capacity. The initial PEEP setting was 4 cm H(2)O, and stepwise changes in PEEP were made. At 30-min intervals, the PEEP was increased to 8, then 12, then decreased back down to 8, then 4 cm H(2)O. MEASUREMENTS AND RESULTS: After 30 min at each level of PEEP, arterial blood gases, aortic and central venous pressures, heart rates, dynamic lung compliance, and changes in EELV were recorded. Paired t tests with Bonferroni correction were used to evaluate the data. There were no differences in heart rate or mean BP at the different PEEP levels. CO(2) elimination and oxygenation improved directly with the PEEP level and mean airway pressure (Paw). Compliance did not change with increasing PEEP, but did increase when PEEP was lowered. EELV changes correlated directly with the level of PEEP. CONCLUSIONS: As previously reported during gas ventilation, oxygenation and CO(2) elimination vary directly with PEEP and proximal Paw during PLV. EELV also varies directly with PEEP. Dynamic lung compliance, however, improved only when PEEP was lowered, suggesting an alteration in the distribution of perflubron due to changes in pressure-volume relationships.

Lower respiratory rates without decreases in oxygen consumption during neonatal synchronized intermittent mandatory ventilation.

OBJECTIVE: We tested the hypothesis that synchronization to patient effort during intermittent mandatory ventilation (SIMV), when compared to conventional unsynchronized intermittent mandatory ventilation (IMV), will decrease energy expenditure, as reflected by decreased oxygen consumption (VO2). DESIGN: We used a four-period crossover design. Each patient was studied over four 30-min continuous time intervals. Patients were randomized to receive initially IMV or SIMV, then crossed over such that each patient was treated twice with each modality. Data were analyzed using an analysis of variance technique. SETTING: Patients were receiving treatment in the newborn intensive care unit of Children's Hospital, St. Paul. PATIENTS: We studied 17 patients, who ranged from 23 to 37 weeks gestation, were < or = 14 days old, and had study weights from 623 to 3015 g. All were mechanically ventilated for hyaline membrane disease. MEASUREMENTS AND RESULTS: We measured and compared VO2, carbon dioxide consumption (VCO2), minute ventilation (VE), total respiratory rate, heart rate, arterial blood pressure, and arterial oxygen saturation (SaO2) values during IMV and SIMV. Total respiratory rate fell significantly during SIMV (73 +/- 26 during IMV, 57 +/- 17 during SIMV, p < 0.01) in spite of no significant change in VO2 (0.6 +/- 0.16% fall in VO2 during SIMV) or VCO2 (4.2 +/- 0.19% increase in VCO2 during SIMV) values. Moreover, there were no significant differences in heart rate, blood pressure, VE, or SaO2 values with either form of therapy. CONCLUSIONS: Though total respiratory rate fell, these data do not support the hypothesis that SIMV significantly reduces respiratory rate by decreasing oxygen consumption and carbon dioxide production during infant mechanical ventilation. Rather, the marked fall in respiratory rate may be due to a more efficient respiratory pattern.

Neonatal apnea associated with maternal clonazepam therapy: a case report.

A 2750-g female infant was born at 36 weeks' gestation to a 40-year-old woman treated with clonazepam throughout her pregnancy. The infant developed apnea, cyanosis, and hypotonia within a few hours of birth. The mother's serum clonazepam level at delivery was 32 ng/mL; the cord blood level was 19 ng/mL. The infant had no congenital malformations, evidence of infection, or seizures. Clinical episodes ceased by ten days of age. The woman elected to breastfeed; breast milk clonazepam levels were between 11 and 13 ng/mL. She was discharged with a cardiorespiratory monitor. The authors suggest that infants of mothers receiving this agent during pregnancy or while nursing have serum levels measured. Additionally, these infants should be monitored for central nervous system depression or apnea.

Mechanical ventilation of the newborn. An overview.

Mechanical ventilation of the newborn infant is an ever-changing area. Its evolution has been hampered and stimulated by problems of small size, inadequate technology, unexpected complications, and changing expectations. With synchronized ventilation, a new technique in the neonatal ICU, clinicians again are reassessing their assumptions. HFV, a "new" technique for 15 years, has found a niche in the treatment of infants failing CV. Its use as an initial therapy for RDS, advocated by some, remains controversial. Monitoring gas flow patterns, tidal and minute volumes, and lung mechanics has become a part of the CV, but complications still occur. The only thing certain is that change will continue.

Suspected betamethasone-induced leukemoid reaction in a premature infant.

The use of antenatal corticosteroids in threatened pregnancies of less than 34 weeks' duration is a valuable tool for assisting fetal lung maturation. Although this practice has existed for over 20 years, little is known about a rare extreme elevation of the newborn's white blood cell count, called a leukemoid reaction. A case of leukemoid reaction is discussed to assist the clinician with the current thought processes and diagnostic differential behind this benign condition.

Hypercarbic ventilatory responses of infants at risk for SIDS.

We examined the hypercarbic ventilatory responses (HVR) of 143 infants at risk for sudden infant death syndrome (SIDS) and 34 normal control infants. Sixty-five of the at-risk infants had experienced apparent life-threatening events (ALTE), and 78 were siblings of SIDS victims. Twenty-three (35%) of the ALTE infants experienced subsequent apnea; one died of SIDS. Seven (9%) of the SIDS siblings experienced subsequent apnea; two ultimately died of SIDS. In the HVR studies, we measured tidal volume (VT), minute ventilation (VE), frequency of breathing (f), and end-tidal PCO2 (PETCO2) at rest and while breathing 2% and 4% CO2. Mean HVR vales for the ALTE, sibling, and control groups were all similar. The mean HVR values for those at-risk infants who experienced subsequent apnea were not different from those who did not experience subsequent apnea. However, those infants experiencing subsequent apnea had higher mean VT/kg values (P less than 0.01) and lower mean PETCO2 values (P less than 0.001) than those who did not. The SIDS siblings had significantly lower resting VT/kg values than either the near-miss infants or normal controls (P less than 0.01). We did not find depressed HVR values in infants at risk for SIDS. On the contrary, those infants who experienced subsequent apnea had evidence suggesting relative hyperventilation. SIDS siblings had evidence suggesting relative hypoventilation. These findings are interesting and thought-provoking. However, HVR studies do not appear to be sensitive, specific, or appropriate for the general screening of infants at risk for SIDS.

Effect of rapid thoracic compression on the cerebral blood flow-velocity patterns of small infants.

We measured the middle cerebral artery (MCA) flow-velocities of 12 small infants (mean weight, 2,882 +/- 602 g) before, during, and after the rapid thoracic compression (RTC) maneuvers of partial forced expiratory flow-volume studies. Cerebral flow-velocities were measured using transcranial Doppler ultrasonography. RTC increased MCA end diastolic flow-velocities and Pourcelot indices of all infants (P less than 0.001). These values returned to baseline immediately after the release of chest compression. We also measured the MCA flow-velocities of several preterm infants during their normal daily activities. The changes in flow-velocity patterns observed during normal daily life were similar to those observed during RTC. These findings demonstrate that RTC produces real, but likely not pathologic, changes in cerebral blood flow-velocities.

Identifying lung overdistention during mechanical ventilation by using volume-pressure loops.

We measured the pulmonary mechanics of 23 mechanically ventilated neonates. Airway pressures, inspiratory and expiratory flows were simultaneously measured. Values for respiratory system mechanics were then derived from these data by using a personal computer and a special software program. Volume-pressure (V-P) loops and respiratory system compliance values were determined for representative mechanical breaths. Twelve infants had normal-appearing V-P loops. Eleven had V-P loops characteristic of lung overdistention, showing decreasing changes in volume with progressive increases in pressure. To quantify this visual observation, we determined the change in compliance during the last 20% of inspiration (C20). We then compared this value to the total compliance value for the entire breath (C) using the ratio C20/C. Mean values for C, C20, and C20/C were compared for the two patient groups. Total respiratory system compliance values were similar. C20 values were decreased in those patients with V-P loops showing overdistention. C20/C values were significantly decreased in those patients with V-P loop evidence of overdistention. Patients with V-P loop evidence of overdistention all had C20/C values less than 0.8. Those with normal-appearing V-P loops all had C20/C values greater than 1.0. The C20/C ratio appears to effectively quantitate visual V-P loop evidence of lung overdistention during mechanical ventilation.

Determining optimum inspiratory time during intermittent positive pressure ventilation in surfactant-depleted cats.

This study compares two methods of selecting inspiratory time (Ti) during mechanical ventilation. One selects a standard Ti producing a brief inspiratory pressure plateau (P). The other uses simultaneous pressure, flow and tidal volume (VT) waveforms, generated by a computer-assisted lung mechanics analyzer, to reduce Ti to the point where Vt ceases to accumulate and flow returns to zero. This method does not produce a pressure plateau (NP). Following saline lung washout, ten intubated, paralyzed surfactant-depleted cats were ventilated with pressure-preset infant ventilators at constant measured VT and rates. Five animals were initially ventilated with P (Ti = 0.98 +/- 0.02 s) and five with NP (Ti = 0.77 +/- 0.10 s). Ti was then varied to produce P or NP by using a four-period crossover design. All other ventilator variables remained constant. Intravascular pressures, thermodilution cardiac outputs, arterial and mixed venous blood gases and oxygen saturations, airway pressures, Ti, VT, and gas flows were measured; respiratory system mechanics, alveolar-arterial oxygen gradients, and intrapulmonary shunts were determined for each study period. When P and NP states were compared, only mean airway pressures differed (10.1 vs. 8.9 cmH2O; P less than 0.001). Blood gas values, intravascular pressures, cardiac output, and respiratory system mechanics were all similar. Under the conditions of this study, there was no advantage to prolonging Ti beyond the point where VT ceased to accumulate.

Synchronized gas and partial liquid ventilation in lung-injured animals: improved gas exchange with decreased effort.

We hypothesized that partial liquid ventilation (PLV) with perflubron in spontaneously breathing lung-injured animals would increase respiratory workload compared to animals treated with gas ventilation (GV), and that a fully synchronized mode, assist-control ventilation (AC), would reduce the piglets' effort when compared to intermittent mandatory ventilation (IMV) or synchronized IMV (SIMV) during both GV and PLV. Newborn piglets with saline lavage-induced lung injury were randomized to sequential 30-min periods of IMV --> SIMV --> AC (n = 5), or AC --> SIMV --> IMV (n = 5) during GV followed by PLV. Pulmonary mechanics measurements and an esophageal patient effort index (PEI, defined as the product of the area below baseline of the esophageal pressure-time curve and respiratory rate [RR]) were determined to estimate the patient's nonmechanical work of breathing, using a computer-assisted lung mechanics analyzer. GV to PLV comparisons showed no change in PEI (IMV, 57.8 vs. 49.7; SIMV, 52.3 vs. 46.8; AC, 15.7 vs. 13.7 cm H2O x s/min); intermode comparisons showed significantly decreased PEI in AC vs. IMV and SIMV during GV, and in AC vs. SIMV (AC vs. IMV, P = 0.06) during PLV. AC consistently resulted in the highest minute ventilation, lowest total respiratory rate, most physiologic pH, and least tidal volume variability. These observations suggest that synchronization with AC during GV and PLV may have substantial physiologic benefits.

High-frequency oscillation versus conventional ventilation following surfactant administration and partial liquid ventilation.

Surfactant followed by partial liquid ventilation (PLV) with perfluorocarbon (PFC; LiquiVent) improves oxygenation, lung compliance, and lung pathology in lung-injured animals receiving conventional ventilation (CV). In this study, we hypothesize that high-frequency oscillation (HFO) and CV will provide equivalent oxygenation in lung-injured animals following surfactant repletion and PLV, once lung volume is optimized. After saline-lavage lung injury during CV, newborn piglets were randomized to either HFO (n = 10) or CV (n = 9). HFO animals were stabilized over 15 min without optimization of lung volume; CV animals continued treatment with time-cycled, pressure-limited, volume-targeted ventilation. All animals then received 100 mg/kg of surfactant (Survanta). Thirty minutes later, all received intratracheal PFC to approximate functional residual capacity. Thirty minutes after PLV began, mean airway pressure (MAP) in both groups was increased to improve oxygenation. MAP was directly adjusted during HFO; PEEP and PIP were adjusted during IMV, maintaining a pressure sufficient to deliver 15 mL/kg tidal volume. Animals were treated for 4 h. The CV group showed improved oxygenation following surfactant administration (OI: 26.79 +/- 1.98 vs. 8.59 +/- 6.29, P < 0.0004), with little further improvement following PFC administration or adjustments in MAP. Oxygenation in HFO-treated animals did not improve following surfactant, but did improve following PFC (0I: 27.78 +/- 6.84 vs. 15.86 +/- 5.53, P < 0.005) and adjustments in MAP (OI: 15.86 +/- 5.53 vs. 8.96 +/- 2.18, P < 0.03). After MAP adjustments, there were no significant intergroup differences in oxygenation. Animals in the CV group required lower MAP than animals in the HFO group to maintain similar oxygenation. We conclude that surfactant repletion followed by PLV improves oxygenation during both CV and HFO. The initial response to administration of surfactant and PFC was different for the conventional and high-frequency oscillation groups, likely reflecting the ventilation strategy used; animals in the CV group responded most to surfactant, whereas animals in the HFO group responded most after PFC instillation. The ultimately similar oxygenation of the two groups once lung volume had been optimized suggests that HFO may be used effectively during administration of, and treatment with, surfactant and perfluorocarbon.

Effect of spontaneous and mechanical breathing on dynamic lung mechanics in hyaline membrane disease.

We measured then compared the dynamic lung mechanics of spontaneous breaths and mechanical breaths in 9 mechanically ventilated neonates with hyaline membrane disease. All were receiving intermittent mandatory ventilation. All breathed spontaneously between ventilator breaths. Tidal volume, transpulmonary pressure, dynamic lung compliance, airways resistance, and peak inspiratory and peak expiratory gas flows were determined for both the mechanical and the spontaneous breaths. The mechanical breaths consistently had larger tidal volumes, higher transpulmonary pressures, higher airway resistance, and lower lung compliance values (P less than 0.05). Peak inspiratory and expiratory gas flows were also higher (P less than 0.01) during mechanical breathing. The spontaneous breaths generated by patients and the mechanical breaths generated by mechanical ventilators are different. The lung mechanics measurements of these two different types of breathing should be collected, analyzed, and reported separately.

The effects of bedside pulmonary mechanics testing during infant mechanical ventilation: a retrospective analysis.

We examined the effects of regular bedside testing of pulmonary mechanics (PM) on the outcome of 468 acutely ill, mechanically ventilated neonates. During the first of two 18-month study periods, 217 infants were mechanically ventilated without the assistance of PM measurements. During the second 18-month period, 251 infants were ventilated with the assistance of at least daily PM measurements. Using data obtained from the PM tests, we adjusted the infants' ventilators to maintain tidal volume, inspiratory time, and pressure-volume loops within predetermined limits. With the exception of the PM measurements, given the limitations of retrospective analyses, both groups of infants received identical medical and nursing care. The infants ventilated with the assistance of PM testing developed fewer pneumothoraces (4.0%; 10/251) vs. no PM testing, 10.1% (22/217); P < 0.05 by Chi-square analysis]. Infants weighing less than 1,500 g ventilated with the assistance of PM measurements had less intraventricular hemorrhage (IVH) overall, most notably, less grades I and II IVH (total IVH-PM testing, 39.1% vs. no PM testing, 65.7%; P < 0.01; Grades I-II IVH-PM testing, 30.4% vs. no PM testing, 54.9%; P < 0.01). IVH incidence was decreased independent of pneumothorax occurrence. Survival rates, incidences of bronchopulmonary dysplasia, and durations of mechanical ventilation and hospitalization were similar. This retrospective analysis suggests that PM testing during infant mechanical ventilation reduces common acute ventilator-associated complications.

Randomized controlled trial of volume-targeted synchronized ventilation and conventional intermittent mandatory ventilation following initial exogenous surfactant therapy.

We set out to evaluate the impact of volume-targeted synchronized ventilation and conventional intermittent mandatory ventilation (IMV) on the early physiologic response to surfactant replacement therapy in neonates with respiratory distress syndrome (RDS). We hypothesized that volume-targeted, patient-triggered synchronized ventilation would stabilize minute ventilation at a lower respiratory rate than that seen during volume-targeted IMV, and that synchronization would improve oxygenation and decrease variation in measured tidal volume (V(t)). This was a prospective, randomized study of 30 hospitalized neonates with RDS. Infants were randomly assigned to volume-targeted ventilation using IMV (n = 10), synchronized IMV (SIMV; n = 10), or assist/control ventilation (A/C; n = 10) after meeting eligibility requirements and before initial surfactant treatment. Following measurements of arterial blood gases and cardiovascular and respiratory parameters, infants received surfactant. Infants were studied for 6 hr following surfactant treatment. Infants assigned to each mode of ventilation had similar birth weight, gestational age, and Apgar scores at birth, and similar oxygenation indices at randomization. Three patients were eliminated from final data analysis because of exclusionary conditions unknown at randomization. Oxygenation improved significantly following surfactant therapy in all groups by 1 hr after surfactant treatment (P < 0.05). No further improvements occurred with time. Total respiratory rate was lowest (P < 0.05) and variation in tidal volume (V(t)) was least in the A/C group (P < 0. 05). Minute ventilation (V(')(E)), delivered airway pressures, respiratory system mechanics, and hemodynamic parameters were similar in all groups. We conclude that volume-targeted A/C ventilation resulted in more consistent tidal volumes at lower total respiratory rates than IMV or SIMV. Oxygenation and lung mechanics were not altered by synchronization, possibly due to the volume-targeting strategy. Of the modes studied, A/C, a fully-synchronized mode, may be the most efficient method of mechanical ventilator support in neonates receiving surfactant for treatment of RDS.

Dynamics of spontaneous breathing during patient-triggered partial liquid ventilation.

This study evaluates different ventilator strategies during gas (GV) and partial liquid ventilation (PLV) in spontaneously breathing animals. We hypothesized that during PLV, spontaneously breathing animals would self-regulate respiratory parameters by increasing respiratory rate (RR) and minute ventilation (V'E) when compared to animals mechanically ventilated with gas, and further that full synchronization of each animal's effort to the ventilator cycle would decrease RR at stable tidal volumes (V(T)). We studied 12 newborn piglets (1.54 +/- 0.24 kg) undergoing GV and PLV in 3 different modes: intermittent mandatory ventilation (IMV), synchronized IMV (SIMV), and assist control ventilation (AC). Modes occurred sequentially in random order during GV first, with the same order then repeated during PLV. Animals initially received continuous positive airway pressure (CPAP) and returned to CPAP during PLV at the end of the experiment. Pressure-limited, volume-targeted ventilation was used with a tidal volume goal of 13 cc/kg. Rate was set at 10/min during IMV and SIMV, with a back-up rate of 10/min during AC. RR, V'E, mechanical (V(T)) and spontaneous tidal volumes (sV(T)) were measured breath-to-breath using a computer-assisted lung mechanics analyzer; mean values were determined over 30-min periods. Data analysis used paired t-tests with Bonferroni correction as needed (P < 0.05). Blood gases were stable in all modes during GV and PLV. RR (min(-1)) and V'E (L x min(-1)/kg) increased in all modes from GV to PLV (RR: CPAP 71 vs. 128; IMV 69 vs. 112; SIMV 65 vs. 107; AC 33 vs. 47. V'E: CPAP 0.47 vs. 0.72; IMV 0.46 vs. 0.61; SIMV 0.45 vs. 0.61; AC 0.38 vs. 0.53; P < 0.05). Intermode comparisons during PLV showed a lower RR with AC (P < 0.02), and a higher V'E with CPAP (P < 0.05). V(T) and dynamic respiratory system compliance decreased from GV to PLV (V(T) P < 0.05; C(rs,dyn) P < 0.01); sV(T) remained unchanged. V(T) and sV(T) did not differ in intermode comparisons. We conclude that during PLV, spontaneously breathing piglets with normal lungs maintain physiologic blood gases by increasing V'E through increased RR. AC produced the most efficient respiratory pattern during PLV, with increased V'E achieved by a modest increase in RR.

Comparison of high-frequency oscillatory ventilation and high-frequency jet ventilation in cats with normal lungs.

Four adult cats received alternating high-frequency oscillatory ventilation (HFOV) and high-frequency jet ventilation (HFJV) at equivalent proximal airway pressures. Physiologic measurements were made before and after each ventilator change. Proximal airway pressures were then adjusted as necessary to reestablish normal pH and PaCO2 values. Aortic, pulmonary artery, and central venous pressures were monitored. Cardiac outputs were measured. Pulmonary and systemic vascular resistance, intrapulmonary shunt, and alveolar-arterial oxygen gradient were determined. Following the change from HFOV to HFJV at similar proximal airway pressures, HFJV always produced higher pH values (P less than 0.0001), higher PaO2 values (P less than 0.05), lower PaCO2 values (P less than 0.0001), as well as higher cardiac outputs (P less than 0.01), lower pulmonary artery pressures (P less than 0.001), and lower pulmonary vascular resistances (P less than 0.001). Following the reciprocal crossover, from HFJV to HFOV, HFJV pH values were again higher (P less than 0.001), and PaCO2 values were again lower (P less than 0.001). A comparison of HFOV and HFJV at similar pH and PaCO2 values showed that HFOV consistently required higher peak inspiratory pressures (P less than 0.001), higher mean airway pressure (P less than 0.001), and higher pressure wave amplitudes (P less than 0.001). Under the circumstances of this study, HFJV produced better gas exchange at lower proximal airway pressures.