High frequency mechanical ventilation in severe hyaline membrane disease an alternative treatment?

Has Quality Improvement Really Improved Outcomes for Babies in the Neonatal Intensive Care Unit?

During the past decade, the emergence of outcome measurement and quality improvement in the neonatal intensive care unit, far more than the introduction of new research approaches or novel therapies, has had a profound effect on improving outcomes for premature neonates. Collection of outcome data, review of those data, and strategies to identify and resolve problems using continuous quality improvement methods can dramatically improve patient outcomes. It is likely that further initiatives in quality improvement will continue to have additional beneficial effects for the neonate.

High-Frequency Ventilation in Newborn Infants

In the past mechanical ventilation always mimicked the tidal volumes and ventilatory frequencies of normal breathing. Recently, there has been great interest in techniques that use rapid rates (60 to 3,000 per minute) and tidal volumes approximating dead space. These techniques are known collectively as high-frequency ventilation, although they differ in circuit design, use, potential complications, and mechanism of gas transport. High-frequency ventilation can be divided into four categories: (1) high-frequency positive pressure ventilation, (2) high-frequency jet ventilation, (3) high-frequency oscillatory ventilation and high-frequency flow interruption, and (4) high-frequency chest wall oscillation. In this review we discuss the similarities and differences of these high-frequency techniques, their clinical applications, and some physiological mechanisms involved in gas transport.

Partial liquid ventilation in severely surfactant-depleted, spontaneously breathing rabbits supported by proportional assist ventilation

OBJECTIVE

We hypothesized that partial liquid ventilation (PLV) would improve oxygenation in nonparalyzed, surfactant-deficient rabbits breathing spontaneously while supported by proportional assist ventilation (PAV). This ventilation mode compensates for low pulmonary compliance and high resistance and thereby facilitates spontaneous breathing.

DESIGN

Randomized trial.

SETTING

University animal research facility.

SUBJECTS

Twenty-six anesthetized New Zealand white rabbits weighing 2592 +/- 237g (mean +/- sd).

INTERVENTIONS

After pulmonary lavage (target Pao2 <100 mm Hg on mechanical ventilation with 6 cm H2O of positive end-expiratory pressure [PEEP] and an Fio2 of 1.0), rabbits were randomized to PAV (PEEP of 8 cm H2O) with or without PLV. PLV rabbits received 25 mL/kg of perfluorocarbon by intratracheal infusion (1 mL/kg/min). Pao2, Paco2, tidal volume, respiratory rate, minute ventilation, mean airway pressure, arterial blood pressure, heart rate, pulmonary compliance, and airway resistance were measured. Evaporated perfluorocarbon was refilled every 30 mins in PLV animals. After 5 hrs, animals were killed and lungs were removed. Lung injury was evaluated using a histologic score.

MAIN RESULTS

Pao2 and compliance were significantly higher in PLV rabbits compared with controls (p <.05, analysis of variance for repeated measures). All other parameters were similar in both groups.

CONCLUSIONS

PLV improved oxygenation and pulmonary compliance in spontaneously breathing, severely surfactant-depleted rabbits supported by PAV. The severity of lung injury by histology was unaffected.

Delayed repair of congenital diaphragmatic hernia with early high-frequency oscillatory ventilation during preoperative stabilization

PURPOSE

The authors reviewed their experience in the management of CDH after the introduction of early high-frequency oscillatory ventilation (HFOV) during the preoperative stabilization period and delayed CDH repair.

METHODS

This is a retrospective analysis of 24 consecutive infants with CDH treated at University of California, Irvine Medical Center (UCIMC) during a 36-month period from January 1993 to December 1996.

RESULTS

Two patients were excluded from the study: one fetus with a prenatal diagnosis was referred for fetal surgery; one infant received CDH repair at another institution 2 weeks before transfer to UCIMC. Eight (36%) infants were inborn, and nine (41%) had a prenatal diagnosis of CDH. Median gestational age was 40 weeks (range, 29 to 42 weeks). Median birth weight was 3,019 g (range, 1,205 to 4,337 g). The defect was left sided in 18 infants (86%). Twenty-one infants were intubated within 5 hours of life, 15 had an AaDO2 greater than 610, 11 had an oxygenation index greater than 40, and 11 had a pH of less than 7.2. The median ratio of pulmonary artery pressure to systemic blood pressure was 0.93 (range, 0.51 to 1.15) in 12 infants. Eighteen infants were placed on HFOV within a median of 1 hour of life. Nitric oxide was given to six infants and surfactant to eight. Four infants were referred for extracorporeal membrane oxygenation (ECMO). Repair of CDH was performed on infants at a median age of 33.5 hours (range, 5.5 to 322). Six (30%) received a prosthetic patch. Overall 18 of 22 infants survived (81%); three survivors received ECMO. Two infants of the survivor group had congenital heart anomalies: one ventricular septal defect (VSD) and one double-outlet right ventricle with a VSD. Of the four nonsurvivors, one had lethal cardiac anomalies and bilateral CDH, two had severe bilateral pulmonary hypoplasia (one received ECMO), and one infant was a 29-week premature baby who did not qualify for ECMO.

CONCLUSION

We report a survival rate of 81% (18 of 22) with the management of CDH by delayed surgical repair, early postnatal HFOV, and selective referral for ECMO.

The new obstetrics: its integration into neonatal clinical practise, teaching and research

Most neonatologists have not yet incorporated into their teaching, clinical service and research the advances in high risk obstetrics particularly as it relates to fetal surveillance. This brief review emphasizes some of the "new obstetrics" from the viewpoint of perinatal medicine, particularly in terms of neonatal teaching and the design of future neonatal research. The information that can be obtained about an infant prenatally by the use of ultrasound. power doppler, computerized fetal heart rate monitoring, cordocentesis, etc is extensive and yet, has rarely been utilized in the design of neonatal research protocols. It is becoming imperative that the "new obstetrics" be recognized and utilized in modern neonatal thinking if a truly "perinatal medicine" is to be practised.

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.

Current treatment of severely burned patients

OBJECTIVE

The authors provide an update on a multidisciplinary approach to the treatment of severely burned patients. A review of studies and clinical trials from the past to the present include fluid resuscitation, sepsis, immune function, hypermetabolism, early excision, wound healing, scar formation, and inhalation injury.

SUMMARY BACKGROUND DATA

Advances in treating initial burn shock, infection control, early wound closure, and modulation of the hypermetabolic response have decreased morbidity and mortality in the last two decades. Specialized burn care centers, using a multidisciplinary approach, not only successfully treat large burns and their complications, but provide the necessary rehabilitation and psychological support required for readjustment back into society.

CONCLUSIONS

Thermal injury results in a number of physiologic alterations that can be minimized by adequate fluid resuscitation to maintain tissue perfusion, early excision of burn wounds, and rapid wound coverage. These measures, in combination with antibiotic coverage and nutritional support in the form of early enteral tube feedings, will decrease the hypermetabolic response and the incidence of sepsis that can lead to hemodynamic instability and organ failure. Ongoing clinical trials using anabolic agents (e.g., recombinant human growth hormone) and pharmacologic agents that modulate inflammatory and endocrine mediators (e.g., ibuprofen and propranolol) show promise in the treatment of severe burn injuries.

New concepts in the treatment of children with acute respiratory distress syndrome

Recent advances in mechanical ventilation, accompanied with a better understanding of the pathophysiology of ARDS, have resulted in a brighter outlook for the child who acquires this still dreaded disease. A greater understanding of the pathophysiology of ARDS has led to a heightened awareness that the care of these patients should be more than just supportive. The potential for exacerbation of lung injury by mechanical ventilation is real. Many new therapies are being evaluated for the treatment of ARDS; all are intended to reduce ventilator-induced injury. With the recognition of "volutrauma" as a serious complication of mechanical ventilation in ARDS, the mode of ventilation used should minimize the potential for this complication in a child with signs of progressive lung disease requiring mechanical ventilation. Optimal integration of the many new techniques into the treatment of pediatric ARDS will require more research and experience. Surfactant replacement in ARDS as an adjunct to the basic care of these patients may be beneficial. Liquid ventilation is another exciting new ventilation technique that has a significant protective effect in animal models of ARDS. Other therapies, such as tracheal gas insufflation, or other new modes of ventilation may also improve outcome. Techniques of high-frequency ventilation and ECMO in the treatment of children already show potential for improved outcome. The decision between using ECMO or "nonconventional" forms of mechanical ventilation should be considered carefully, after the morbidity of the procedures, the duration of therapy, and the cost have been weighed. Centers with experience using ECMO in the setting of pediatric ARDS have better results than those where ECMO is infrequently used for this purpose. It is imperative that future studies of both mechanical ventilation and ECMO describe ventilation strategy and prospectively identify protocols or algorithms for ventilator management. Coupled with severity scores, ventilator techniques and ECMO can then be systematically compared in children with ARDS.

High frequency ventilator therapy for newborns

High-frequency ventilation is a general term that refers to a family of ventilator techniques that utilize respirator rates greater than 60 breaths/minute and tidal volumes that are usually less than or equal to the anatomical dead space of the airways. These techniques include high frequency positive-pressure ventilation, high frequency jet ventilation, high frequency flow interruption, high frequency oscillatory ventilation, and high frequency chest wall oscillation. I review the proposed mechanisms of gas transport during high-frequency ventilation and the different ventilators capable of delivering this mode of ventilation. In addition, clinical studies involving infants treated with this new technology are reviewed, along with long-term patient follow-up and reported complications.

The clinical role of near infrared spectroscopy

Near infrared spectroscopy is a non-invasive bedside technique which allows cotside observation of cerebral haemodynamics and oxygenation in sick newborn infants. Methods have been described for measurement of cerebral blood flow and volume, as well as other tests of the cerebral circulation. The techniques is still under intensive development and further advances and refinements can be expected, but a present it is essentially a technique for research and investigations rather than a clinical tool.

Decreased incidence of extra-alveolar air leakage or death prior to air leakage in high versus low rate positive pressure ventilation: results of a randomised seven-centre trial in preterm infants

Two different ventilation techniques were compared in a seven-centre, randomised trial with 181 preterm infants up to and including 32 completed weeks gestational age, who needed mechanical ventilation because of lung disease of any type. Technique A used a constant rate (60 cycles/min), inspiratory time (IT) (0.33s) and inspiratory: expiratory ratio (I:E) (1:2). The tidal and minute volume was only changed by varying peak inspiratory pressure until weaning via continuous positive airway pressure. Technique B used a lower rate (30 cycles/min) with longer IT (1.0 s). The I:E ratio could be changed from 1:1 to 2:1 in case of hypoxaemia. Chest X-rays taken at fixed intervals were evaluated by a paediatric radiologist and a neonatologist unaware of the type of ventilation used in the patients. A reduction of at least 20% in extra-alveolar air leakage (EAL) or death prior to EAL was supposed in infants ventilated by method A. A sequential design was used to test this hypothesis. The null hypothesis was rejected (P = 0.05) when the 22nd untied pair was completed. The largest reduction in EAL (-55%) was observed in the subgroup 31-32 weeks of gestation and none in the most immature group (< 28 weeks). We conclude that in preterm infants requiring mechanical ventilation for any reason of lung insufficiency, ventilation at 60 cycles/min and short IT (0.33 s) significantly reduces EAL or prior death compared with 30 cycles/min and a longer IT of 1 s. We speculate that a further increase in rate and reduction of IT would also lower the risk of barotrauma in the most immature and susceptible infants.

Comparison of different rates of artificial ventilation for preterm infants ventilated beyond the first week of life

The effect on blood gases of different ventilator rates in preterm infants ventilated beyond the first week of life was assessed. Seventeen infants, median gestational age 25 weeks, were studied at median postnatal age of 11 days. The infants were ventilated through a set sequence of rates: 30, 60, 30, 100 and 30 breaths per min (bpm), each rate being maintained for 20 min. Peak and positive end expiratory pressure and I:E ratio (1:1) were unchanged at each rate and mean airway pressure was kept constant by altering flow as necessary. No significant change in oxygenation was demonstrated at either rates of 60 or 100 bpm compared to 30 bpm. PaCO2 levels were, however, significantly reduced at 60 bpm (P less than 0.001) compared to 30 bpm; but this improvement in PaCO2 was not seen at 100 bpm. These results suggest that increasing ventilator rate higher than 60 bpm in the majority of infants ventilated after the first week of life is not advantageous.

Multicentre randomised controlled trial of high against low frequency positive pressure ventilation. Oxford Region Controlled Trial of Artificial Ventilation OCTAVE Study Group

A total of 346 infants aged less than 72 hours were randomly allocated to be treated either by high frequency positive pressure ventilation (HFPPV; rate fixed at 60/minute throughout treatment and initial inspiratory:expiratory (I:E) ratio 1:2, increased to 1:1 if necessary) or by low frequency positive pressure ventilation (LFPPV; rate less than or equal to 40/minute and initial I:E ratio usually 1:1, both decreasing during weaning). The main hypotheses were that HFPPV reduces pneumothorax, chronic lung disease and death before discharge in all infants, as well as those with hyaline membrane disease, and that it reduces the incidence of later neurodevelopmental complications in infants of less than 33 weeks' gestation. Among all the infants the rate of pneumothorax was 19% in the HFPPV group and 26% in the LFPPV group (p = 0.13; odds ratio 0.7, 95% confidence intervals (CI) 0.4 to 1.1); there was no difference in mortality or the incidence of chronic lung disease. In infants of less than 33 weeks' gestation there were no differences in adverse neurodevelopmental outcomes. Among the subgroup of 237 infants with hyaline membrane disease, median fractional inspired oxygen at the time of entry to the trial was 0.6 in the HFPPV group and 0.7 in the LFPPV group, indicating that many had moderately severe disease. In patients with hyaline membrane disease HFPPV was associated with a lower rate of pneumothorax (18% in the HFPPV group compared with 33% in the LFPPV group, p = 0.013, odds ratio 0.5, 95% CI 0.3 to 0.8, with no differences in mortality, or in duration of intubation or supplementary oxygen in survivors. As used in this study, HFPPV was the preferred ventilator regimen for infants with hyaline membrane disease.

Prophylactic use of high-frequency percussive ventilation in patients with inhalation injury

Death and the incidence of pneumonia are significantly increased in burn patients with inhalation injury, despite application of conventional ventilatory support techniques. The effect of high-frequency percussive ventilation on mortality rate, incidence of pulmonary infection, and barotrauma were studied in 54 burn patients with documented inhalation injury admitted between March 1987 and September 1990 as compared to an historic cohort treated between 1980 and 1984. All patients satisfied clinical criteria for mechanical ventilation. High-frequency percussive ventilation was initiated within 24 hours of intubation. The patients' mean age and burn size were 32.2 years and 47.8%, respectively (ranges, 15 to 88 years; 0% to 90%). The mean number of ventilator days was 15.3 +/- 16.7 (range, 1 to 150 days), with 26% of patients ventilated for more than 2 weeks. Fourteen patients (25.9%) developed pneumonia compared to an historic frequency of 45.8% (p less than 0.005). Mortality rate was 18.5% (10 patients) with an expected historic number of deaths of 23 (95% confidence limits of 17 to 28 deaths). The documented improvement in survival rate and decrease in the incidence of pneumonia in patients treated with prophylactic high-frequency ventilation (HFV), as compared to a cohort of patients treated in the 7 years before the trial, indicates the importance of small airway patency in the pathogenesis of inhalation injury sequelae and supports further use and evaluation of HFV.

High-frequency oscillatory ventilation

The improved survival rate of premature infants with respiratory failure is attributable to advances in mechanical ventilation, although an adverse consequence has been an increased incidence of bronchopulmonary dysplasia (BPD) (1;32). Positive pressure ventilation with its attendant “barotrauma” is suspected in the causation of BPD. While many attempts to alter respirator variables, such as pressure and time components, have produced optimal patterns for gas exchange, evidence is lacking to support any one pattern that minimizes the incidence of chronic lung injury. The high incidence of BPD has promoted a search for alternative methods of ventilation that might reduce lung injury through a reduction in peak pressure applied to the lung. Additional motivation has come from the need for oxygenation when mechanical ventilation has failed or pulmonary interstitial emphysema has developed. Less compelling reasons have come from the desire to avoid high swings in thoracic pressure that might adversely affect cardiac output, venous return, and cerebral blood flow.

CPAP and mechanical ventilation

Respiratory insufficiency has previously been a frequent cause of neonatal death, especially in preterm infants. As late as in 1967, Silverman and associates (66) found that in infants with hyaline membrane disease (HMD), mechanical ventilation with a body-enclosing negative pressure respirator did not improve survival. Before 1970, the mortality among infants who required respiratory therapy was high (20;46;70). At the end of the 1960s and the beginning of the 1970s, several new methods were introduced to improve ventilation of newborn infants. Kirby and coworkers (41) introduced intermittent mandatory ventilation as a way of weaning from mechanical ventilation. In a series of studies, Reynolds and coworkers evaluated the effects of different peak airway pressures, respiratory frequencies, and inspiratory:expiratory ratios on arterial blood gases and right to left shunts (32;58;59;60).

Airway pressures during positive-pressure ventilation with superimposed oscillations

This study was undertaken to determine whether oscillations superimposed on a regular ventilatory pattern influence the arterial blood gases and pH and the airway pressures at adequate alveolar ventilation at the onset of inhibition of inspiratory activity. The peak, mean and end-expiratory airway pressures were therefore measured at inhibition of this activity with and without superimposition of oscillations on the ventilatory pattern. It was found that superimposed oscillations lowered the airway pressure only at a low ventilatory frequency, whereas inhibition occurred at almost equal arterial PCO2 and pH values with and without superimposed oscillations on the ventilatory pattern.

Randomised controlled trial of two methods of weaning from high frequency positive pressure ventilation

Forty preterm infants suffering from respiratory distress syndrome were entered into a randomised controlled trial to assess the importance of the length of inspiratory time during weaning from high frequency positive pressure ventilation (HFPPV). Two weaning regimes were compared: in one (group A) inspiratory time was limited to 0.5 seconds throughout weaning, in the other (group B) ventilator rate was reduced by increasing both inspiratory and expiratory time (inspiration:expiration ratio constant) until inspiratory time reached 1.0 seconds. At ventilator rates of 20 and 40 breaths/minute an acute comparison was made in all 40 infants of the two inspiratory times; despite the lower mean airway pressure associated with the shorter inspiratory time blood gases were maintained. There was no difference in the incidence of pneumothoraces or need for reventilation between the two regimens but infants in group A had a shorter duration of weaning. We conclude limitation of inspiratory time to 0.5 seconds during weaning from HFPPV is advantageous to preterm infants with respiratory distress syndrome.

Cardiopulmonary effects of high frequency positive-pressure ventilation versus jet ventilation in respiratory failure

Conventional ventilators are frequently used at high rates in the intensive care nursery to achieve adequate oxygenation and ventilation with reduced peak inspiratory pressure. The efficacy and limitations of high frequency positive-pressure ventilation (HFPPV) using a conventional ventilator were studied by comparing the cardiopulmonary effects of HFPPV with those of high frequency jet ventilation (HFJV) in an animal model of respiratory failure. Sixteen saline-lavaged rabbits were ventilated with either HFPPV or HFJV for 2 h using rates of 200 breaths/min, inspiratory to expiratory ratio of 1:2, and FIO2 of 1.0. As controls an additional eight lavaged rabbits were ventilated at conventional rates (40 to 60 breaths/min). Proximal peak inspiratory pressure as indicated on the ventilator manometer or drive pressure was adjusted to maintain acceptable blood gases. Cardiac output (CO) was measured by thermodilution. Although there was a significant decrease in cardiac function over time, there were no significant differences between the groups in CO or stroke volume. Satisfactory oxygenation and ventilation were maintained in all groups. Static respiratory system compliance and mean airway pressure were similar among the groups. Histologic examination of the lungs revealed no differences between the three ventilator groups. The results of this study indicate that both HFPPV and HFJV are effective in short-term maintenance of normal blood gases in respiratory failure without any discernable differences in their effects on cardiovascular function. At very high rates, however, increases in VT are not possible with HFPPV, which limits its usefulness and flexibility in respiratory failure.

Neonatal pneumopericardium with high-frequency ventilation

Pneumopericardium is an uncommon condition in the neonate and has not, to our knowledge, previously been reported in patients treated with high-frequency ventilation. The results of such treatment in 8 neonates seen in the Neonatal Intensive Care Unit, Wilford Hall USAF Medical Center, San Antonio, Texas, are presented. The mean gestational age was 35 weeks, and birth weight averaged 2.7 kg. The pneumopericardium developed while the patients were on high-frequency ventilation, and the diagnosis was confirmed with a chest radiogram. Treatment included pericardiocentesis with a needle catheter followed by placement of a 10F to 14F chest tube into the pericardial space. The pneumopericardium resolved in all 8 patients. Three of the newborns died of underlying disease; 5 survived and were discharged from the hospital. Pneumopericardium in the neonate is a life-threatening complication, and appropriate therapy includes drainage with a pericardial tube placed under direct vision.

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.

Reducing inadvertent PEEP by controlling end-tidal pressures in the trachea

Mechanical ventilation using exhalation times too brief for completion of exhalation results in inadvertent positive end-expiratory pressure (IPEEP) and increased functional residual capacity (FRC). The endotracheal (ET) tube with side lumen allows us to monitor tracheal airway pressures and to determine the contributions of the ET tube to IPEEP. We hypothesized that, during rapid rate ventilation, controlling the positive end-expiratory pressure (PEEP) level in the trachea rather than at the ET tube adaptor will counter effects of the ET tube on IPEEP and result in less increase of FRC. Thirteen anesthetized rabbits were ventilated at rates of 30, 60, 90, and 120 breaths per minute (BPM). Peak inspiratory pressure was held constant, and PEEP was adjusted to 2 cmH2O, measured conventionally at the proximal end of the ET tube. Pulmonary function tests were made and then repeated while PEEP was held constant in the trachea, measured at the distal end of the ET tube. Controlling PEEP conventionally resulted in mean (+/- SE) FRC values of 13.7 +/- 3.8, 14.8 +/- 3.9, 17.1 +/- 4.5, and 21.1 +/- 5.3 ml/kg at 30, 60, 90, and 120 BPM, respectively. Controlling PEEP at the trachea yielded FRC values of 13.7 +/- 3.8, 13.6 +/- 3.4, 15.3 +/- 4.4, and 16.3 +/- 5.4 ml/kg, respectively. Increasing the ventilator rate above 60 BPM did not affect minute ventilation or blood gases. These results suggest that controlling PEEP in the trachea reduces effects of IPEEP on FRC by countering the contribution of the ET tube to the resistance of gas flow during exhalation.

Intrathoracic pressure fluctuations and periventricular haemorrhage in the newborn

The incidence of periventricular intraventricular haemorrhage (PVH-IVH) in premature infants with respiratory distress syndrome (RDS) remains at about 40%. Although there are certain inherent anatomical and physiological features of a neonate which predisposes them to PVH-IVH, there is conflicting evidence about the precipitating cause. The most significant antecedents of PVH-IVH are mechanical ventilation and barotrauma. This report examines the hypothesis that the final common pathway in PVH-IVH is fluctuation in intrathoracic pressure, and then attempts to explain previous anomalies and conflicting results which have implicated other causes. Suggestions are offered as to how changes in current management could contribute to a decreased incidence of this serious and widespread complication in premature infants with RDS.

Performance of respirators at fast rates commonly used in neonatal intensive care units

The effect on tidal volume and airway pressure of increasing ventilator rate (30, 60, and 120/min) was tested in six commonly used neonatal ventilators. In all six ventilators increased flow was necessary to maintain mean airway pressure at the higher rates. Tidal volume decreased at rates of both 60 and 120/min in all six ventilators, associated with a change in pressure waveform. The most marked reduction in tidal volume, however, was associated with increased positive end-expiratory pressure (PEEP). This was only demonstrated in four ventilators, all incorporating nonassisted expiratory valves. These results stress the necessity for appropriately designed ventilators if fast rates are to be used routinely in clinical practice.

High frequency ventilation in the neonatal period

There are three forms of high frequency ventilation, high frequency jet ventilation (HFJV, up to 400/min), high frequency oscillation (HFO, up to 40 Hz), and high frequency positive pressure ventilation (HFPPV, rates between 60 and 150/min). The first two forms of ventilation are still experimental and have been used only in critically ill children where respiratory failure has been unresponsive to more conventional therapy. Unfortunately, however, HFJV has already been associated with a high incidence of tracheal lesions. High-frequency positive pressure ventilation, on the other hand, using conventional ventilators, has been used and studied widely. Certain neonatal ventilators function suboptimally at increased rates, resulting in a reduction in tidal exchange with a consequent clinical deterioration. Using appropriate ventilators, arterial oxygen tensions improve and carbon dioxide tensions are reduced at fast rates in non-paralysed infants. Air-trapping, however, may be a problem in infants paralysed and ventilated at fast rates. HFPPV have been associated with a reduced incidence of pneumothoraces, but there is no knowledge of the effect of this form of ventilation on subsequent lung growth.

Synchronous respiration: which ventilator rate is best?

Twenty-four infants were ventilated through a series of rates (30, 60 and 120/min), to determine which rate was most successful in provoking synchronous respiration. Their spontaneous respiratory rate was also documented during a temporary disconnection from the ventilator: respiratory rate and gestational age were significantly correlated (r = -0.85). Seventeen infants showed synchronous respiration at a ventilator rate of 120/min and 4 at 60/min. Of the remaining 3 infants, 2 only showed synchrony if ventilated at their own spontaneous respiratory frequency (between 60-75/min) and 1 infant was asynchronous at all rates including her own spontaneous respiratory frequency. The 17 infants synchronous at a ventilator rate of 120/min were significantly less mature (p less than 0.01) and had a faster spontaneous respiratory rate (p less than 0.01) than the 6 infants synchronous at ventilator rates of 60-75/min.

Peak intratracheal pressure during controlled ventilation in infants and children. A computer study of the Servo 900C ventilator

The mathematical relationship between peak ventilator breathing system pressure displayed on the digital meter of the Siemens SV900C ventilator, and peak intratracheal pressure measured at the distal end of the tracheal tube, was defined and incorporated into a computer program. The mean difference between peak airway pressure calculated by the computer and directly measured peak intratracheal pressure was 0.02 kPa (SD 0.10) in 18 infants and children. The mean difference between ventilator breathing system pressure and intratracheal pressure in the same group was 0.82 kPa (SD 0.91). Bench tests established that the decrease in peak pressure displayed by the ventilator (from 1.36 to 0.38 kPa) while inspiratory time was increased from 20 to 80% of the respiratory period, concealed an increase (from 0.2 to 0.38 kPa) in intratracheal pressure which occurs during this process; and that the large increase in pressure displayed by the ventilator (from 0.3 to 6 kPa) while respiratory frequency was increased from 20 to 120 breaths/minute, concealed a small increase in peak intratracheal pressure (0.2-0.3 kPa) which occurs during this process. These changes were accurately predicted by the computer program. The increase in intratracheal pressure associated with prolonged inspiratory times explains the high incidence of barotrauma which has recently been associated with this procedure in infants.

Randomized trial of high-frequency jet ventilation versus conventional ventilation in respiratory distress syndrome

To compare high-frequency jet ventilation (HFJV) with pressure-limited time-cycled conventional ventilation (CV), we randomized 41 infants with clinical and radiographic evidence of respiratory distress syndrome during the first day of life to receive either HFJV or CV. Standardized ventilatory protocols were used for 48 hours, after which CV was administered to both groups. Despite comparable oxygenation (arterial/alveolar oxygen tension ratio), mean airway pressure was lower in the HFJV group (9 +/- 2 vs 13 +/- 2 cm H2O, P less than 0.001), and thus the arterial/alveolar oxygen tension ratio corrected for mean airway pressure was improved in the HFJV group (P less than 0.05). PaCO2 was lower during HFJV (37 +/- 3 vs 42 +/- 3 mm Hg, P less than 0.05) despite a comparable peak inspiratory pressure. The incidence of air leaks, progression of intraventricular hemorrhage, and mortality during the 48-hour period did not differ between the two groups. Bronchoscopies in eight infants given HFJV and five given CV revealed no microscopic evidence of necrotizing tracheobronchitis, but one infant given HFJV had evidence of necrotizing tracheitis at autopsy. We conclude that for 48 hours during the acute stage of respiratory distress syndrome, HFJV can maintain adequate gas exchange at lower mean airway pressure than during CV, without an increase in the incidence of side effects.

High frequency ventilation

High frequency ventilation (HFV) presents a new respiratory therapy modality that has taught us much about the theories of gas transport in the lung. Both experimental and clinical applications are summarized. Although the future clinical role of HFV remains uncertain, pediatric applications and investigation continue at the forefront of this new technology.

Positive-pressure ventilation at moderately high frequency in newborn infants with respiratory distress syndrome (IRDS)

In 24 seriously ill newborn infants with respiratory distress syndrome (IRDS) and ensuing respiratory failure, high-frequency positive-pressure ventilation was administered. The mean gestational age of the infants was 32 +/- 3 weeks. In the infant ventilator employed, the compressible volume had been reduced in order to give higher flow rates but lower intratracheal pressures. The ventilation frequency was kept constant at 60-66 per min and the insufflation period at 32-35% of the ventilatory cycle. A positive end-expiratory pressure (PEEP) of 0.2-0.6 kPa was used. Arterial PCO2 was maintained at 4.0-5.3 kPa and PO2 at 8.5-10.5 kPa by adjusting the gas flow through the patient circuit, the peak tubing pressure, the PEEP and the oxygen concentration in inspired gas. High-frequency positive-pressure ventilation improved oxygenation and gave adequate alveolar ventilation in all infants, in most cases at a low peak pressure. Only one infant developed pneumothorax during intermittent positive pressure ventilation, and in no infant did bronchopulmonary dysplasia or retrolental fibroplasia occur. One infant died from intracranial hemorrhage during the ventilation period, giving a survival rate of 96%.

High-frequency ventilation

Over the last six years high-frequency ventilation has been extensively evaluated both in the clinical and laboratory settings. It is now no longer the great mystery it once was, and it is now no longer believed (as many had hoped), that it will solve all the problems associated with mechanical pulmonary ventilation. Although the technique is safe and appears to cause no harm even in the long term, it has not yet been shown to offer any major advantages over conventional mechanical ventilation.

Inadvertent positive end-expiratory pressure in mechanically ventilated newborn infants: detection and effect on lung mechanics and gas exchange

During mechanical ventilation, inadvertent positive end-expiratory pressure (PEEP) can have deleterious effects, including decreasing lung compliance and alveolar ventilation. To detect and quantitate inadvertent PEEP in 10 preterm neonates receiving mechanical ventilation, we clamped the connection between the endotracheal tube and the respirator at end-expiration and, after about 5 seconds, measured the airway pressure resulting from the trapped gas that emptied into the airways and the measuring system. To study the effect of decreasing inadvertent PEEP on lung mechanics and gas exchange, we measured the compliance of the respiratory system and blood gases. Inadvertent PEEP greater than 1 cm H2O was detected in 19 of 29 measurements. Decreasing inadvertent PEEP by lengthening the expiratory time increased the compliance of the respiratory system (r = -0.74, n = 10, P less than 0.02). Decreasing inadvertent PEEP by greater than 1 cm H2O (mean 2.1 +/- 0.8 cm H2O) in six newborn infants increased respiratory compliance from 0.57 +/- 0.09 to 0.73 +/- 0.13 ml/cm H2O, or approximately 30%, and lowered Pco2 from 40.6 +/- 14.4 to 38.2 +/- 14.1 mm Hg despite a reduction in the level of ventilation set on the respirator. Knowing the amount of inadvertent PEEP and its effects can help improve mechanical ventilation in newborn infants.

Principles of neonatal assisted ventilation

Based on the current knowledge of pulmonary mechanics and the results of clinical studies, we have reviewed principles that govern gas exchange during assisted ventilation in infants with RDS. Guidelines for changes in ventilator settings have been presented with respect to their specific effects on CO2 elimination and O2 uptake. In addition, their possible mechanisms of action and potential side effects have been addressed. General strategies have been presented, but they must be employed with caution. All infants will not exhibit the expected response to changes in ventilator setting, and thus their ventilatory management, as well as their general medical care, will need to be individualized.

The Penlon Bromsgrove high frequency jet ventilator for adult and paediatric use. A solution to the problem of humidification

The Penlon Bromsgrove is a new high frequency jet ventilator, suitable for use in both adults and children. The jet stream is humidified by means of an integral in-line Bernoulli nebuliser. The self-recharging nebuliser can also convey drugs directly to the airways in the form of an aerosol. Alarms and fail-safe systems are incorporated. A pressure gauge continuously displays the jet drive pressure. There are two digital LED displays; one shows jet frequency in breaths per minute; the other, the jet drive pressure, minimum, maximum and mean patient airway pressures. Two fail-safe systems ensure that these pressures do not become excessive. Both audible and visual alarms are provided. The prototype has proved to be quiet and completely reliable over more than 4000 hours use, with no bronchoscopic or histological evidence of ineffective humidification.

Combined high-frequency oscillatory ventilation and intermittent mandatory ventilation in critically ill neonates

Combined high-frequency oscillatory ventilation (HFOV) and intermittent mandatory ventilation (IMV) was used in 12 neonates with inadequate gas exchange with conventional IMV. Diagnoses included diaphragmatic hernia with hypoplastic lungs, pneumonia, persistent fetal circulation, and severe respiratory distress syndrome. In most patients there was severe air leak. Within 10 hours of beginning HFOV-IMV the mean arterial PCO2 fell from 60 +/- 5 (means +/- SEM) to 38 +/- 2 mm Hg (P less than 0.01) and the mean IMV rate was reduced from 96 +/- 8 to 17 +/- 4 breaths per minute (P less than 0.001). The mean arterial-alveolar oxygen tension ratio rose from 0.05 +/- 0.01 to 0.09 +/- 0.01 (P less than 0.005). Mean airway pressure in the trachea was reduced from 16 +/- 2 to 10 +/- 3 cm H2O (P less than 0.05). Four patients died, three of whom had diaphragmatic hernias with hypoplastic lungs. Five of the eight survivors had mild bronchopulmonary dysplasia requiring supplemental oxygen. These studies demonstrate that in some neonates with respiratory failure who fail to respond to conventional IMV, combined HFOV-IMV can be successful.

Decrease in airway pressure during high-frequency jet ventilation in infants with respiratory distress syndrome

Using a crossover study design, we compared a system of high-frequency jet ventilation with appropriate humidification to pressure-limited conventional ventilation in 12 preterm infants with a birth weight of 1.9 +/- 0.6 kg and gestational age of 32 +/- 2 weeks who had severe respiratory distress syndrome. After a control period of conventional ventilation, high-frequency jet ventilation was administered for 1 to 3 hours at a constant rate (250/min) and inspiratory to expiratory time (1:3 or 1:4) in the same fraction of inspired oxygen as during conventional ventilation. Average peak inspiratory pressure decreased from 29 +/- 3 cm H2O during conventional ventilation to 20 +/- 4 cm H2O during high-frequency jet ventilation (P less than 0.001), whereas positive end expiratory pressure was unchanged, resulting in a reduction in mean airway pressure from 14 +/- 3 to 10 +/- 2 cm H2O (P less than 0.001). There was a simultaneous decrease in PaCO2 (39 +/- 4 to 34 +/- 4 mm Hg, P less than 0.01), but PaO2 did not change. These data indicate that short-term high-frequency jet ventilation maintains gas exchange in infants with respiratory distress syndrome despite a lower PIP and Paw, and results in smaller airway pressure swings than during conventional ventilation. Thus, high-frequency jet ventilation may offer hope for reducing barotrauma in this population.

A new endotracheal tube for neonatal use

A new endotracheal tube for neonatal use is described in which a 5FG pressure measuring catheter is attached to the convex surface of a conventional polyvinylchloride endotracheal tube. Using this modified endotracheal tube intratracheal pressure can be measured. Preliminary studies were undertaken in three infants. It was found that at high ventilatory rates (above 80/min) pressures measured within the trachea were different from those measured in the ventilator circuit.

[High frequency ventilation during the surgical treatment of esophageal atresia]

The preliminary results of the use of high frequency positive pressure ventilation (HFPPV) in six newborn infants presenting a type III oesophageal atresia during the thoracic stage of surgical repair are reported. HFPPV allowed correct gas exchange during the surgical procedure. In the six cases, a significant decrease in PaCO2 (p less than 0.05) was observed, whereas the effects on PaO2 were variable. The technical problems, mechanism of gas transport during HFPPV and clinical interest of HFPPV in thoracic surgery are discussed.

Ventilation at high respiratory frequencies. High frequency positive pressure ventilation, high frequency jet ventilation and high frequency oscillation

This paper reviews the development of different methods of ventilation at respiratory rates higher than 60 per minute (1 Hz) along with data on experimental and clinical uses of the techniques. The definitions and terms that have been used for these high rates at the present time are confusing. An attempt to clarify the terms has been made. Whereas high frequency positive pressure ventilation (HFPPV) refers to respiratory rates between 60-110 per minute (1-1.8 Hz), high frequency jet ventilation (HFJV) usually refers to rates between 110-400 per minute (1.8-6.7 Hz) and high frequency oscillation (HFO) refers to rates above 400 and up to 2400 per minute (40 Hz). It should be recognised that this differentiation in terminology is rather arbitrary and does not necessarily represent a sudden switch to different physiological methods of ventilation. In view of the various techniques which are involved in ventilation methods utilising rates greater than 60 per minute (1 Hz), it is the purpose of the present work to review the literature. In so doing, the contrasting rates, mechanical equipment, and experimental and clinical uses of these different methods will be discussed in order to clarify their potential contribution to clinical medicine.