High frequency oscillatory ventilation near resonant frequency of the respiratory system in rabbits with normal and surfactant depleted lungs

It has been suggested that high frequency oscillatory ventilation (HFV) might improve gas exchange and reduce the risk of pressure-related side-effects compared to conventional mechanical ventilation (CMV). Whereas most studies have used arbitrarily set frequencies for HFV, we evaluated the effects of HFV near resonant frequency (f_r). Anaesthetised and tracheotomised adult rabbits (n=10; 3.8–5.1 kg body weight) were ventilated by alternating periods of CMV and HFV near f_r. Negative ventilator resistance was used for complete resistive unloading of the respiratory system before each HFV period. This enabled a continuous swinging at resonance thus allowing measurement of f_r and selection of exactly that frequency for the HFV run. Intra-animal CMV-HFV comparisons (n=4) were performed on each animal: with healthy lungs at a mean airway pressure (MAP) of 0.5 kPa and after saline lung lavage at MAPs of <1.5 kPa; 1.5–1.8 kPa; >1.8 kPa. Surfactant removal caused total respiratory system compliance (C_tot) to decrease from 44±5 to 22±3 ml/kPa. Corresponding f_r was 244±48 and 360±30 min^–1, respectively. HFV produced effective pulmonary gas exchange but did not improve arterial oxygenation in comparison with CMV at matched MAPs both before and after surfactant depletion. Volume amplitudes of oscillation necessary to achieve normocapnia were slightly above the natural plus equipment (2 ml) dead space. Maximum intra-alveolar pressure (P_max) was calculated for the HFV runs from MAP, C_tot, and the volume amplitude of oscillation. P_max during CMV was nearly twice that during HFV at equivalent PaCo₂ and equivalent MAPs throught the experiments.     

High frequency oscillatory ventilation in acute respiratory failure

High frequency oscillatory ventilation (HFOV) has emerged over the past 20 years as a safe and effective means of mechanical ventilatory support in patients with acute respiratory failure. During HFOV, lung recruitment is maintained by application of a relatively high mean airway pressure with superimposed pressure oscillations at a frequency of 3 to 15Hz, creating adequate ventilation using tidal volumes less than or equal to the patient's dead space volume. The physiologic rationale for the application of HFOV in the clinical arena comes from its ability to preserve end-expiratory lung volume while avoiding parenchymal overdistension at end-inspiration and theoretically limiting the potential for ventilator-associated lung injury. Data in the neonatal population suggests significant benefits in pulmonary outcomes when HFOV is applied with a recruitment strategy in preterm infants with respiratory distress syndrome (RDS). Use of HFOV in the paediatric and adult populations has not as yet been associated with significant improvements in clinically important outcome measures.     

Hemodynamic effects of conventional and high frequency oscillatory ventilation in normal and septic piglets

The cardiovascular effects of high frequency oscillation (HFO) and conventional ventilation (CMV) were evaluated in 10 piglets prior to and during an infusion of group B streptococci (GBS). Animals were randomized to begin ventilation with either HFO or CMV. Arterial blood gases, cardiac output (CO), and pulmonary artery (Ppa), pulmonary wedge (Ppw) and arterial blood pressures were measured. These values were recorded at a mean airway pressure (MAP) of 2 cm H2O for both modes of ventilation after which a continuous infusion of GBS (4 X 10(7) CFU/kg/min) was begun. MAP was increased in both ventilators in the following sequence: 4, 8 and 12 cm H2O. Prior to GBS infusion, HFO was associated with small but significant changes in hemodynamic parameters when compared to CMV for the following: Ppa (15 +/- 4 vs. 13 +/- 4.0 mm Hg; p less than 0.03), Ppw (3 +/- 1 vs. 2 +/- 1 mm Hg; p less than 0.02), and CO (0.24 +/- 0.08 vs. 0.25 +/- 0.09 l/min/kg; p less than 0.05). Similar statistically significant increases in Ppa (p less than 0.005) and Ppw (p less than 0.0001), and decrease in CO (p less than 0.007) were present during GBS infusion when animals were ventilated with HFO, irrespective of the MAP used. Our results suggest that the use of HFO in both normal piglets and those receiving an infusion of GBS results in mild but consistent impairment in cardiovascular function compared to CMV. In summary, these data demonstrate that HFO has no beneficial effect compared to CMV at similar MAP in the management of the septic piglet model and may in fact further compromise the animal's hemodynamic status.     

新生儿呼吸衰竭28例高频振荡通气治疗的疗效观察

目的　探讨高频振荡呼吸机在治疗新生儿呼吸衰竭中的应用价值及其安全性。方法　 2 8例新生儿呼吸衰竭患儿经常频呼吸机治疗无效后改用高频振荡呼吸机治疗 ,观察其治疗前后血气指标的变化及临床疗效。结果　 2 8例患儿中治愈 2 0例 ,治愈率 71 4% ,死亡 5例 ,放弃治疗 3例 ;出现颅内出血等并发症 6例 ;治愈患儿中高频振荡通气 (HFOV)治疗后 8～ 12h的PaCO2 迅速下降 [(6 8± 0 4)kPavs(5 5± 0 6 )kPa,P <0 0 5 ],2 4～ 48h的PaO2 明显上升 [(5 8± 1 1)kPavs(9 2± 1 7)kPa ,P <0 0 5 ];同时吸入氧浓度、平均气道压、振荡压力幅度、血氧合指数等均有明显下降 (P均 <0 0 5 )。结论　HFOV用于治疗新生儿呼吸衰竭 ,是一种疗效肯定、安全性好的新型机械通气方法 ,具有常频机械通气 (CMV)难以比拟的优越性。     

High frequency oscillatory ventilation: Initial experience in 22 patients

Objectives: To report the outcome of a consecutive cohort of neonates treated with high frequency oscillatory ventilation (HFOV).
Methodology Prospective cohort study of 22 neonates failing conventional mechanical ventilation (CMV) between October 1992 and August 1993. Outcomes evaluated were in-hospital survival rate, comorbidities including patent ductus arteriosus (PDA), cerebroventricular haemorrhages (CVH), necrotizing enterocolitis (NEC), bronchopulmonary dysplasia (BPD) and retinopathy of prematurity (ROP), and acute changes in respiratory status.
Results Eighteen of 22 (81.8%) survived. Of the four children who died, one did not respond to HFOV and died within 24 h of treatment. Two died of respiratory failure complicated by pulmonary haemorrhage. The remaining infant responded to HFOV but later developed severe NEC while on minimal CMV and died at 2 weeks of age.
Three subjects were ≥34 weeks' gestation; each responded well to HFOV with no substantial comorbidity. Of the remaining 19 infants <34 weeks' gestation, six (31.6%) had a PDA, and seven (36.8%) had a CVH. One infant developed cystic periventricular leucomalacia. Three infants (15.8%) had NEC. Respiratory failure in the 15 survivors with gestational ages <34 weeks improved dramatically with HFOV. Ten (66.7%) survivors <34 weeks developed BPD and 10 (66.7%) ROP.
Conclusion High frequency oscillatory ventilation was associated with a survival rate of 81.8%, but with significant comorbidity.     

高频振荡通气治疗新生儿肺透明膜病疗效观察

目的探讨高频振荡通气在治疗新生儿肺透明膜病(HMD)中的应用价值及其安全性.方法分别以常频机械通气(CMV)和高频振荡通气(HFOV)治疗HMD患儿,并比较其治疗前后血气指标的变化及临床疗效.结果 1.HFOV组患儿治愈率明显高于CMV组(68.0%vs 38.8%,P<0.05);2.HFOV治疗后肺通、换气功能迅速改善,表现为PaCO2迅速下降(6.8±0.4 vs 5.5±0.6 kPa,P<0.05),PaO2明显上升(5.8±1.1vs 9.2±1.7 kPa,P<0.05),同时所需吸入氧浓度(FiO2)、血氧合指数(OI)等均迅速明显下降(p均<0.05);3.HFOv组并发症及后遗症等均少于CMV组.结论 HFOV用于治疗HMD,是一种疗效肯定、安全性好的新型机械通气方法.     

High frequency oscillatory ventilation in infants with increased intra-abdominal pressure

AIMS—To describe the short term effect of high frequency oscillatory ventilation on infants with severe abdominal distension who could not be conventionally ventilated.METHODS—Eight infants (25 to 38 gestational weeks, birthweight 600-3200 g, postnatal age 1 to 190 days) with a variety of intra-abdominal pathologies, resulting in severe abdominal distension and failure of conventional ventilation, were studied.RESULTS—The oxygenation status of all infants significantly improved within an hour of changing from conventional to high frequency oscillatory ventilation. Infants who were hypercapneic on conventional ventilation also showed a reduction in PaCO₂. As a group, the mean (SD) PaO₂/FIO₂ improved from 4.99 (0.98) kpa to 11.55 (3.8) kpa (P = 0.002), and the PaCO₂ from 6.48 (2.12) kpa to 4.89 (1.22) kpa (P= 0.028). These improvements were sustained throughout the next 48 hours.CONCLUSION—High frequency oscillatory ventilation seems to be an effective rescue measure for infants with respiratory failure secondary to increased intra-abdominal pressure.     

高频振荡通气在新生儿重症肺疾病中的应用

目的　 为观察高频振荡通气 (HFOV)治疗新生儿重症肺疾病的应用价值及安全性。 方法 　 2 7例新生儿重症肺疾病 ,15例应用HFOV治疗 ,12例应用常频机械通气 (CMV)治疗 ,观察治疗前后 2 4h内氧合参数及呼吸机参数的变化。 结果 　HFOV组 2 4h内氧浓度 (FiO2 )迅速从 ( 0 85± 0 11)降至 ( 0 5 3± 0 0 8) ,P <0 0 1;平均气道压 (MAP)从 ( 15 3± 1 8)cmH2 O降至( 11 2± 2 1)cmH2 O(P <0 0 1) ,氧合指数 (OI)从 ( 3 4± 10 )降至 ( 2 0± 4) (P <0 0 1) ,二氧化碳分压 (PCO2 )从 ( 6 9± 0 8)kPa迅速降至 ( 5 4± 0 9)kPa(P <0 0 1)。HFOV期间血压、心率无明显变化。 结论 　HFOV治疗新生儿重症肺疾病 ,通气效果优于CMV ,氧合改善快 ,短时间内氧浓度、MAP下降更快 ,是一种疗效肯定、安全性好的新型机械通气方法。     

高频振荡通气对新生儿呼吸衰竭氧合改善的临床研究

目的探讨高频振荡通气(HFOV)治疗新生儿呼吸衰竭的疗效。方法采用HFOV结合肺复张策略,实现经皮氧饱和度≥0.9。结果 20例患儿在施行HFOV治疗48h后,吸入氧体积分数由(0.72±0.17)降至(0.37±0.17),差异有统计学意义(H=17.820,P<0.05);氧合指数(OI)由(11.94±4.50)降至(2.73±2.24),差异有统计学意义(F=9.662,P<0.05);振荡压力幅度由(31.67±5.92)cmH2O逐渐降至(22.00±5.66)cmH2O,差异有统计学意义(F=3.954,P<0.05);平均气道压由(10.44±1.88)cmH2O降至(8.75±1.39)cmH2O,但差异无统计学意义(F=1.030,P>0.05)。治疗过程中,患儿的心率、血压无明显变化。结论 HFOV治疗新生儿呼吸衰竭安全有效。     

High frequency oscillation ventilation compared to conventional mechanical ventilation plus exogenous surfactant replacement in rabbits

OBJECTIVES: (a) to evaluate the effect on oxygenation and ventilation of rabbits with induced surfactant depletion when they are submitted to a conventional mechanical ventilation, plus a small dose of exogenous surfactant; (b) to compare this group with another group submitted to a High Frequency Oscillation (HFO) without exogenous surfactant administration.METHODS: Twenty New Zealand White rabbits weighing (-/+ 3 kg) were anaesthetized and artificially induced to a endogenous surfactant depletion by successively lung lavage with normal saline (aliquots of 25 ml/kg) until to reach a persistent PaO(2) less than 100 mmHg when submitted to a mechanical ventilation in a pressure control mode with a target tidal volume of 10ml/kg, PEEP of 5cm H(2)O, FiO(2) 1.0, respiratory rate 30/min, and inspiratory time of 0.65 s. Then the rabbits were divided in (a) CMV+S group, submitted to a conventional mechanical ventilation plus exogenous surfactant replacement; (b) HFO group, submitted to a High Frequency Oscillation Ventilation. Arterial blood gases were measured at control period, post lung lavage, 15, 16 and 120 minutes after treatment started. The groups were compared using Student t test.RESULTS: The post lung lavage PaO(2) in both groups was lower than 50mmHg (p=0.154), increasing after 15 min of treatment to 254 mmHg (CMV+S) and 288 mmHg (HFO, p=0.626). The PaO(2) at 60 and 120 minutes were higher (p=0.001) in the HFO group (431 e 431 mmHg) when compared with the CMV+S group, which showed a progressive fall (148 e 126 mmHg). At 60 minutes of treatment, the PaCO(2) was lower (p=0.008) in the CMV+S group (29 versus 41 mmHg).CONCLUSIONS: In ARDS animal model a protect mechanical ventilation strategy as HFO by itself promotes a fast and persistent increase in the oxygenation, with superior levels than those observed in animals treated with conventional mechanical ventilation plus exogenous surfactant replacement.     

机械通气方式对新生猪支气管肺泡灌洗液炎性因子及肺表面活性物质相关蛋白A的影响

目的 观察常频机械通气(CMV)、高频振荡通气(HFOV)、部分液体通气(PLV)3种机械通气方式对急性肺损伤(ALI)新生猪BALF炎性因子及肺表面活性物质相关蛋白A(SP-A)水平的影响.方法 出生1～3 d健康新生猪24只,用9 g/L盐水(38 ℃,35 mL/kg)灌洗制备ALI模型.随机分为4组:对照组(6只,模型制备成功后不予通气,直接处死)、CMV组(6只)、HFOV组(6只)、PLV组(6只),行机械通气24 h后处死动物,用ELISA法检测BALF中TNF-α、IL-8、IL-1及SP-A水平.结果 3种方式机械通气24 h后,BALF中3种炎性因子及SP-A总体均数比较差异均有统计学意义(P均=0.000),PLV组、HFOV组SP-A水平较CMV组高(P均＜0.05),PLV组IL-8、IL-1、TNF-α水平较CMV组低(P均＜0.05),PLV组IL-8、TNF-α水平较HFOV组低(P均＜0.05),HFOV组IL-8、TNF-α水平较CMV组低(P均＜0.05).结论 不同机械通气方式致肺部炎性反应不同,PLV致肺部炎性反应最轻.PLV较CMV、HFOV更能增加SP-A表达,降低SP-A的降解.     

High frequency oscillatory ventilation and extracorporeal membrane oxygenation in severe persistent pulmonary hypertension of the newborn

We report on 50 term and near-term neonates (birth weight > 1800 g, gestational age > 33 weeks) with severe persistent pulmonary hypertension of the newborn (PPHN), referred to us from January 1987 to July 1991 after failure of maximum conventional treatment. All infants had paO₂<45 mm Hg when ventilated with peak inspiratory pressure >38 cm H₂O and FiO₂=1.0, hence meeting entry criteria for extracorporeal membrane oxygenation (ECMO). High frequency oscillatory ventilation (HFOV) was tried in all patients. If sufficient oxygenation could not be achieved (paO₂<40 mm Hg for at least 2 h), ECMO therapy was begun, which was the case in 25 children. Neonates responding to HFOV (n=25) were of a slightly younger gestational age (37.0 weeks vs 38.8 weeks,P<0.05), had higher Apgar scores and were less hypoxaemic before HFOV (paO₂ 36.6 mm Hg vs 28.8 mm Hg,P<0.01); during HFOV there was a significant rise in paO₂ (> 150 mm Hg;P<0.001) and a fall in pCO₂ to 21.6 mm Hg (P<0.001). Due to air leaks, which was the main complication of HFOV (52%), ECMO therapy had to be begun in two additional infants after an initial positive effect. HFOV tended to be successful in cases of primary PPHN, meconium aspiration and sepsis, but not in infants with lung hypoplasia as a result of diaphragmatic hernia or other reasons. Success or failure of HFOV could not be reliably predicted by any parameter. Mean duration of HFOV was 37.8 h vs 84.9 h of ECMO. PPHN could be overcome in 88% of the HFOV-treated and in 76% of the ECMO-treated infants; overall survival rate was 74% (predicted probability of survival using maximum conventional treatment <10%). There were no significant differences between HFOV/ECMO groups with regard to duration of ventilation following HFOV/ECMO, total time in hospital, rate of bronchopulmonary dysplasia and neurological complications (intracranial haemorrhage, brain infarction). Among the survivors, the rate of mentally handicapped children was equal in both groups (overall 18.9%). Our analysis shows that about 50% of neonates with PPHN who fail to respond to conventional ventilatory support and maximum treatment can be treated successfully with HFOV, thus avoiding ECMO. By applying both forms of therapy, the survival rate of infants with severe PPHN can be increased from an estimated rate of <10% up to 80%.     

Distribution of surfactant, lung compliance, and aeration of preterm rabbit lungs after surfactant therapy and conventional and high-frequency oscillatory ventilation.

Previous studies in preterm lambs have shown that exogenous surfactant is more uniformly distributed if given at birth before ventilation or if followed by high-frequency ventilation (HFV) after establishing conventional ventilation (CV). We hypothesized that the pre-term rabbit pup would respond similarly and that improved respiratory system compliance (Crs) would accompany improved surfactant distribution. We randomized pups (27 d gestation) into three groups: control, surfactant at birth, and surfactant after 15 min of CV (rescue). We administered dipalmitoylphosphatidyl-[3H]choline-labeled natural surfactant by tracheostomy to each of the treated groups. The two treatment groups were treated for 15 min with either HFV or CV and subsequently with CV. We measured Crs at 15, 25, 35, and 45 min after surfactant. Lungs from pups treated with CV or HFV (n = 89) for 15 min, with and without 30 min of subsequent CV, were cut into 32 pieces that were counted for distribution of label or were sectioned for quantitative morphometry (n = 36). Pups receiving surfactant after 15 min of CV had higher Crs 15 min after surfactant than either pups treated with surfactant at birth or controls (p less than 0.001). The Crs of pups 15 min after rescue surfactant followed by HFV was lower than that of pups treated with CV (p less than 0.05) but was higher than that of either control or pups treated at birth groups (p less than 0.05). Crs at 35 and 45 min after surfactant were the same in all treatment groups. Application of HFV appeared to delay the delivery of surfactant to the distal airspaces.(ABSTRACT TRUNCATED AT 250 WORDS)     

Combination therapy of high frequency oscillatory ventilation, NO inhalation and surfactant replacement in a child with acute respiratory distress syndrome]

We report about a child with severe ARDS after burning trauma who did not respond to conventional treatment with controlled pressure ventilation under conditions of permissive hypercapnia and changing of the infants's body position. A combined treatment with high frequency oscillatory ventilation, inhalation of nitric oxide and surfactant replacement improved the pulmonary status. Twelve days after the accident the boy could be extubated and 5 weeks later he could be discharged without any pulmonary and neurologic handicap. The use of these therapeutic tools may help to avoid the necessity of the invasive extracorporeal life support.     

新生儿高频振荡通气时呼出气潮气量与动脉二氧化碳分压的研究

目的 探讨新生儿高频振荡通气(HFOV)时呼出气潮气量与动脉二氧化碳分压(PaCO2)变化的关系.方法 对应用HFOV的重症呼吸衰竭新生儿,监测记录PaCO2、呼出气潮气量(VTe)、呼吸机频率(F)、平均气道压(MAP)、振幅(ΔP)及吸入氧浓度(FiO2)等指标,并进行相关性分析.结果 共46例患儿接受HFOV治疗,平均胎龄(36.37 ± 3.53)周,出生体质量(2.76 ± 7.77)kg.不同PaCO2组( ＜ 35 mmHg、35 ～ 45 mmHg、46 ～ 55 mmHg、＞ 55 mmHg)对应VTe/kg平均值分别为(2.20 ± 0.40)ml/kg、(1.96 ± 0.46)ml/kg、(1.71 ± 0.44)ml/kg、(1.21 ± 0.33)ml/kg,组间比较差异有统计学意义(χ2 ＝ 85.74,P ＜ 0.05),Vte/kg与PaCO2呈负相关(r ＝ -0.59,P ＜ 0.05).当PaCO2于正常范围(35 ～ 45 mmHg),MAP为8 ～ 10 cmH2O、11 ～ 13 cmH2O、14 ～ 16 cmH2O、＞ 16 cmH2O时,对应VTe/kg平均值分别为(1.73 ± 0.33)ml/kg、(1.90 ± 0.39)ml/kg、(2.19 ± 0.54)ml/kg、(2.53 ± 0.53)ml/kg,组间比较差异有统计学意义(χ2 ＝ 18.96,P ＜ 0.05),VTe/kg和MAP呈正相关(r ＝ 0.43,P ＜ 0.05).经多元线性回归分析逐步法建立回归方程:PaCO2 ＝ -13 VTe/kg + 4.32 F + 0.13 FiO2 + 19.68(r ＝ 0.68,r2 ＝ 0.47,P ＜ 0.05).结论 新生儿高频振荡通气时VTe/kg与PaCO2呈负相关,PaCO2正常范围时VTe/kg和MAP呈正相关;高频振荡通气时监测VTe可作为调控PaCO2的参考.     

The role of high frequency oscillatory ventilation in the management of children with severe traumatic brain injury and concomitant lung pathology

OBJECTIVE: To report the use of high frequency oscillatory ventilation (HFOV) in two children with severe traumatic brain injury and concurrent lung pathology where conventional mechanical ventilation was ineffective. DESIGN:: Case report. SETTING: Regional intensive care unit in a pediatric teaching hospital. PATIENTS: Two severely head-injured children (both with postresuscitation Glasgow Coma Scores of 3), one of whom was age 11 yrs and developed an invasive fungal (rhizomucor) pneumonia, while the other age 5 yrs had bilateral lung contusions. Both were treated according to local head injury guidelines, which included conventional ventilation. Despite increasing conventional ventilatory support, CO2 removal became problematic in both cases, making the intracranial pressure control and consequent maintenance of adequate cerebral perfusion pressure difficult. In both patients, a dramatic reduction in intracranial pressure and improvement in cerebral perfusion pressure was observed soon after the use of HFOV. Additionally, inotropic support was weaned by 50% in both children after commencing HFOV. A significant increase in the mean arterial blood pressure occurred in one child with HFOV. INTERVENTION: Use of HFOV as an alternative to conventional mechanical ventilation. CONCLUSION: HFOV may have utility in the management of selected cases of severe brain trauma with concurrent lung pathology where conventional ventilation is ineffective.     

Proximal, tracheal, and alveolar pressures during high-frequency oscillatory ventilation in a normal rabbit model

To study the effect of different high-frequency oscillatory ventilation parameters on airway pressure, we measured oscillatory pressure amplitude ([Paw[) and mean airway pressure (Paw) at three sites in open-chested normal rabbits: proximal, trachea, and alveolus. Five animals were studied to test a new pleural capsule design, which was then used in two groups of animals to measure right upper (n = 4) or middle (n = 5) lobe alveolar pressures. Animals were randomly sequenced through combinations of frequency (10, 15, and 20 Hz) and fractional inspiratory time (Ti) (0.3 and 0.5) while normoxic and eucapnic. During capsule testing, we noted that alveolar pressures increased (p less than 0.05) with increasing capsule mass, suggesting that compressive forces from the capsule may alter the capsule measurement. We thus used a low-mass (430 mg) transducer system in the rabbit high-frequency oscillatory ventilation experiments. Using multifactorial analysis of variance, we found significant main effects of Ti on Paw, and of measurement site on both [Paw[ and Paw (all p less than 0.009). Frequency did not influence variations in either [Paw[ or Paw. For both Ti settings, alveolar upper lobe Paw was lower compared with that of the middle lobe (p less than 0.0005). Lengthening Ti (0.3 to 0.5) increased tracheal Paw in each capsule group (p less than 0.0005). At Ti = 0.5, tracheal Paw exceeded Paw measured proximally (p less than 0.05). Our data support in vivo alveolar Paw inhomogeneity and demonstrate significant changes in pressures within the lung related to Ti during high-frequency oscillatory ventilation.     

Frequency, tidal volume, and mean airway pressure combinations that provide adequate gas exchange and low alveolar pressure during high frequency oscillatory ventilation in rabbits

We studied healthy and saline lavaged rabbits during high frequency oscillatory ventilation to determine what combination of frequency (f), tidal volume (Vt), and mean airway pressure (Paw) produced the lowest peak-to-peak alveolar pressure amplitude (Palv) and physiologic blood gas tensions. Sinusoidal volume changes were delivered through a tracheostomy by a piston pump driven by a linear motor. Tracheal pressure amplitude (Ptr) was measured through a tracheal catheter and alveolar pressure amplitude was measured in a capsule glued to the right lower lobe. PaO2, PaCO2, Ptr, and Palv were measured at the following settings: FiO2 = 0.5, frequency 2-28 Hz, Vt 1-3 mL/kg (50 150% dead space) and Paw 5-15 cm H2O. Many combinations of frequency and Vt resulted in the same PaO2 and PaCO2. Paw had a large effect on Palv and minimal effect on blood gas tensions. In lavaged rabbits, the composite variable f x Vt2 described the trends in Palv and blood gas tensions. As the product of f x Vt2 increased, PaO2 initially increased and then decreased, whereas PaCO2 decreased and Palv increased. No single combination of frequency, Vt and Paw simultaneously provided the lowest Palv and physiologic blood gas tensions. Adequate blood gas tensions and low Palv were obtained at frequencies less than 12 Hz, a Vt of 2 mL/kg and a Paw of 10 cm H2O. In healthy and lavaged rabbits PaO2 increased and PaCO2 decreased as frequency increased at lower Vt.PaO2 decreased as frequency increased at higher Vt in lavaged rabbits only. Palv tended to be greater in lavaged rabbits.