High Levels Of PEEP Research Articles

High-frequency oscillatory ventilation in experimental lung injury: effects on gas exchange

To compare ventilation-perfusion (V(A)/Q) distributions during improvement of oxygenation caused by high-frequency oscillatory ventilation (HFOV) and pressure-controlled mechanical ventilation with high PEEP levels (CMV) in experimental acute lung injury (ALI). Prospective, controlled animal study. Animal research facility of a university hospital. Twelve pigs with oleic acid-induced ALI were randomised to HFOV ( n=6) or to CMV ( n=6) with a PEEP of 15 mbar for 1 h. The mean airway pressure was adjusted in both groups to achieve comparable improvements in arterial oxygen partial pressure (PaO(2)) and to avoid clinically relevant impairments of cardiac output, as assured by adequate mixed venous oxygen saturation and lactate levels. V(A)/Q distributions were determined by the multiple inert gas elimination technique (MIGET). Arterial oxygen partial pressure improved during CMV with a mean airway pressure of 20 mbar ( p<0.05) whereas HFOV revealed comparable improvements with a mean airway pressure of 40 mbar ( p<0.05). Shunt decreased and blood flow to normal V(A)/Q areas increased due to CMV and HFOV ( p<0.05). The perfusion of low V(A)/Q areas remained unchanged. Statistical analysis did not reveal differences of PaO(2), shunt or blood flow to low V(A)/Q areas between the groups. In this model of acute lung injury CMV and HFOV improved gas exchange due to similar changes in V(A)/Q distribution. However, mean airway pressure had to be adjusted twofold higher during HFOV then during CMV to achieve comparable improvements in gas exchange.

Ventilation liquide partielle dans le syndrome de détresse respiratoire aiguë

Partial liquid ventilation has known considerable development in both the experimental and the clinical fields of acute respiratory distress syndrome. This was due to the unique properties of perfluorocarbons that enable gas exchange enhancement and pulmonary mechanics restoration by acting as liquid-PEEP and surfactant. Current developments and highlights. – Recent experimental studies have paradoxically underlined both the dose-dependent improvement of gas exchanges during partial liquid ventilation and the increased risk of baro- and volutrauma with the use of high doses of perfluorocarbon and high levels of PEEP. Results from a recent international multicenter randomized controlled study have recently illustrated such risks. In this study, partial liquid ventilation had no beneficial effects in terms of mortality but was associated with a higher risk of barotrauma. New horizons and prospects. – These disappointing results have stimulated research on liquid ventilation in both new modalities of perfluorocarbon administration such as aerosol and new fields such as intrapulmonary drug delivery.

NEONATAL HIGH-FREQUENCY VENTILATION: Past, Present, and Future

High-frequency ventilation has become established as an effective treatment modality in a variety of clinical situations. The laboratory and clinical investigations of these techniques have contributed tremendously to our understanding of the pathophysiology of respiratory failure and the important concept of maintaining adequate lung volume. Clinicians have come to appreciate better the factors involved in lung injury and the potential for damage to distant organs. The place of HFV in the therapeutic armamentarium will undoubtedly continue to evolve in the years to come. Of particular interest is the advent of advanced modes of fully synchronized and volume-targeted conventional mechanical ventilatory modes, along with the trend to use smaller tidal volumes and higher levels of PEEP with conventional ventilation. With these developments there seems to be a certain convergence of HFV and tidal ventilation that is the logical result of our improved understanding of respiratory pathophysiology. The available controlled trials of HFV versus tidal ventilation do not clearly differentiate whether improved outcomes are the result of HFV per se, or a reflection of the effects of optimizing lung volume, a benefit that may not be unique to HFV.

High frequency oscillatory ventilation as therapy for acute lung injury and ARDS] .

Mechanical ventilation is the mainstay of therapy for acute lung injury, a disease with remainingly high morbidity and mortality. As a result of an improved understanding that mechanical ventilation itself can contribute to and aggravate the disease process, the term ventilator-associated lung injury (VALI) has been introduced. Main risk factor for VALI are (1) alveolar overdistention caused by excessivly high tidal volumes and/or inspiratory pressures (volu-/barotrauma), as well as (2) cyclic alveolar collapse promoted by insufficient endexpiratory pressure. So called "lung-protective ventilatory strategies" aim at minimizing these risk factors by the use of small tidal volumes and high PEEP levels. High frequency oscillatory ventilation (HFOV) can be regarded as an ultimate form of this approach, combining minimal pressure changes with a high continuous distending pressure (CDP). That CDP is generated using high fresh gas flows ("Super-CPAP"), while a piston pump incorporated into the system creates an oscillatory flow at frequencies ranging from 3-7 Hz. An initial lung volume recruitment manoeuvre is mandatory for the optimal use of HFOV. Whereas for many years HFOV is a well established therapy for the infant respiratory distress syndrome, experience in adults is still rare. First results, however, look promising, and HFOV might as well turn out as a valuable treatment modalitiy for ARDS.

A reply

The letter of Drs Latter and Mulvey touches upon several interesting issues. Yes, the four clamps do look cumbersome, but not so in use [1]. Clamp 1 initiates and maintains single lung ventilation, as is standard practice. Clamp 2 enables the double-lumen tube on the side of the nonventilated lung to be occluded for the short time it takes to connect the ambient pressure oxygen reservoir. Clamp 3 serves to contain the oxygen in the reservoir until such time as it is connected to the nonventilated lung. Clamp 4 can be omitted, if desired, as described in a separate correspondence [2]. The use of Clamp 2 is a simple, practical method of excluding nitrogen from the nonventilated lung, which is after all the object of the exercise [1]. If the airway of the nonventilated lung is momentarily left open to air as the reservoir is being connected, it is possible for more than 250 ml of ambient air to enter the nonventilated lung in the course of a single ventilation to the dependent, ventilated lung [3]. Drs Latter and Mulvey report that, once single-lung ventilation to the dependent lung is initiated, they separately ventilate the nondependent lung with 100% oxygen presumably by hand ‘shortly before the pleura is opened’. This practice will progressively wash out any nitrogen that may have entered the nondependent lung at the time their C-breathing system was connected. With nitrogen successfully excluded or eliminated, I suggest they try connecting an oxygen reservoir at ambient pressure rather than oxygen CPAP. The former practice offers several additional practical advantages [1], while the latter is contraindicated or counterproductive in many thoracoscopic procedures. I apply Clamp 1 at end-expiration to minimise the increase in inflation pressure in the ventilated lung over the short period before the nonventilated lung is opened to the oxygen reservoir. Whilst this is not of any importance in most cases, it may well be of relevance in patients with respiratory disease and high levels of intrinsic PEEP. Of greater importance, however, is the need to ensure that Clamp 2 is not left inadvertently applied for longer than is necessary [1]. As regards the apparent concern about ‘early atelectasis’, I hold the view that prompt passive and then absorptive collapse of the nonventilated lung does not prejudice patient wellbeing either intra-operatively or postoperatively. It does, on the other hand, serve to improve surgical access and operative conditions in many thoracic operations, especially those performed thoracoscopically.

HIGH-INFLATION PRESSURE AND POSITIVE END-EXPIRATORY: Injurious to the Lung? No

Survival rates in ARDS with conventional ventilation using high oxygen fractions and low PEEP levels have been reported to be less than 10%. In three prospective evaluations of ARDS in the 1980s, mortality rates remained greater than 60%. Early studies using high-level PEEP therapy in severe ARDS by Douglas, Downs, Kirby, and Civetta showed improved survival rates with ranges between 60% and 80%. In 1979 Gallagher reviewed 59 patients with ARDS who were treated with PEEP greater than 15 cm H2O titrated to improve FRC by achieving an intrapulmonary shunt fraction of 15%. The overall survival was 65%, with only 5% of the patients dying secondary to respiratory failure. In the more recent study by Miller in trauma patients and later by DiRusso in a variety of surgical patients, the overall mortality rate for those patients receiving PEEP greater than 15 cm H2O was 20% to 30%. Of the 14 patients who died, only seven (10% of the total) succumbed to respiratory failure. The remaining patients died from the primary underlying disease with normal oxygenation or after significant weaning from high PEEP levels. By using a goal-oriented approach to the management of patients with severe ARDS, we have found that high-level PEEP therapy was effective in lowering the intrapulmonary shunt and improving the SaO2 at acceptable levels of inspired oxygen. All of these patients were ventilated with traditional high tidal volumes (10 to 15 mL/kg) and therefore exhibited high peak inspiratory airway pressures. This support method did not seem to cause lung injury or an excessive amount of barotrauma in these patients, but in fact, was associated with a lower mortality rate (30%) than reported in other studies of patients with lesser degrees of lung oxygenation dysfunction and extrapulmonary organ system dysfunction. Currently available information indicates that increases in mean airway pressure (induced with PEEP or other modes of ventilatory support to restore losses in FRC that occur during ARDS) and limiting exposure to toxic concentrations of oxygen minimize ventilator-induced secondary lung injury and maximize chances for survival. Arbitrary limitations of peak inspiratory or end-expiratory airway pressure or mandatory tidal volume in patients with severe ARDS seem to be unfounded. Failure to achieve adequate physiologic end-points in these patients may increase morbidity and mortality rates.

HIGH-INFLATION PRESSURE AND POSITIVE END-EXPIRATORY: Injurious to the Lung? Yes

There is a growing body of evidence suggesting that high levels of inflation pressure and high levels of PEEP may be injurious to lung tissue and other organ systems. Limiting peak alveolar pressures below 35 cm H2O may help in avoiding these injuries. The findings have led to the development of a lung-protective strategy that is based on physiologic parameters. This strategy, often using permissive hypercapnia and pressure-limited modes of ventilation, may gain widespread use in the near future. If this strategy reduces barotrauma, a reduction in the length of time on mechanical ventilation and mortality rates can be anticipated. At our center we routinely initiate mechanical ventilation in patients with acute lung injury, using tidal volumes of approximately 6 mL/kg. This may be decreased further if peak alveolar pressures exceed 30 to 35 cm H2O. PEEP is added to maximize alveolar recruitment and oxygenation. Optimal PEEP is located at the inflection point of the respiratory compliance curve. Usually a PEEP of 8 to 12 cm H2O is sufficient. Although we usually initiate mechanical ventilation with a volume-cycled mode, we are not hesitant to switch rapidly to a pressure-limited mode if results are unsatisfactory. We believe that more attention to the potential harmful effects of pressure and volume on lung architecture may result in further improvement of survival in patients with acute respiratory failure.

Extracorporeal carbon dioxide removal technique improves oxygenation without causing overinflation.

Extracorporeal CO2 removal combined with low frequency positive pressure ventilation (ECCO2R-LFPPV) improves gas exchange and decreases peak pressures, respiratory rates, and tidal volumes in animals and in humans. Recent evidence suggests that pulmonary barotrauma results from lung overinflation rather than from high pressures. This study was to test the hypothesis whether ECCO2R-LFPPV could improve gas exchange without causing lung overinflation, despite the use of higher levels of PEEP, when compared with conventional mechanical ventilation. Eleven patients with severe adult respiratory distress syndrome (ARDS) who failed to respond to different modes of mechanical ventilation were treated with ECCO2R-LFPPV. Risk of pulmonary barotrauma was evaluated by static pressure-volume (P-V) curves and dynamic changes in volumes monitored by respiratory inductive plethysmography (Respitrace). ECCO2R-LFPPV PaO2/FIO2 increased from 79 +/- 21 to 207 +/- 108 (p = 0.003). Risk of barotrauma, as shown by the shape of the P-V curve, was present in all patients receiving mechanical ventilation even though most of them were treated with permissive hypoventilation. By contrast, no evidence of persistent lung overinflation could be detected by either static P-V curves or dynamic measurements in nine of 11 patients who were treated by ECCO2R-LFPPV. The two remaining patients had severe airway obstruction because of bleeding, and they remained ventilated with persistent risk of barotrauma. We conclude that ECCO2R-LFPPV improves gas exchange without causing lung overinflation in a majority of patients with ARDS.

Sequelae of the adult respiratory distress syndrome.

Most survivors of ARDS have persistent mild reductions of TLCO even as long as a year after their episode. The lung volumes and flows return to normal in most instances, although a subset of patients will have persistent impairment. Both obstructive and restrictive deficits may be seen. This group may be predicted by the degree of acute lung injury assessed by the level of FIO2, PEEP, and gas exchange abnormality that exists in the first few days. In the first year after ARDS most physiological abnormalities will improve, but if deficits persist at one year further improvement is unlikely. Although many patients report dyspnoea following ARDS, the symptom does not correlate with abnormalities of pulmonary function. The possibility that conventional management may augment the degree of acute injury and worsen outcome must be considered. The effects of chronic hyperoxia in humans with acute lung injury or those of high levels of PEEP compared with low levels are not known. Exploring new ventilator management strategies while we await more specific treatment directed at the primary problem of acute lung inflammation will hopefully reduce acute mortality as well as acute and chronic morbidity.

Pediatric characteristics of adult respiratory distress syndrome: a meta-analysis

A meta-analysis of the literature is carried out in order to present the current knowledge of ARDS in childhood (0-15 years). This might be helpful for planning controlled studies on new therapeutic measures (e.g. surfactant replacement, antioxidants, extracorporeal gas exchange). By means of Medline all available publications were taken into account referring to minimal criteria defined before. There were 4 studies (50 patients) and 48 individual case-reports (48 patients). Most often ARDS was caused by infection (21 resp. 30%), aspiration (12 resp. 23%) and trauma (10 resp. 23%). Therapeutically high respiration pressures and toxic oxygen tensions were applied generally (average maximum PEEP: 14.7 resp. 15.8 cm H2O; average maximum PIP: 59.6 resp. 61.9 cm H2O; FiO2 > 0.5 for an average of 10.1 resp. 10.4 days). The most frequent complications were barotrauma (43.8 resp. 78%), infection (60.4%) and multiorgan failure (66.7%). Mortality rate was 31.3 resp. 52%. So also in childhood almost every second case is fatal. Especially multiorgan failure as well as high levels of FiO2 and PEEP indicate bad prognosis at an early stage. Therefore it is necessary and justified to perform clinical trials on therapeutic agents successfully tested in animal studies before.

Effects of PEEP on Respiratory Mechanics after Open Heart Surgery

Respiratory dysfunction, particularly atelectasis, is common after open heart surgery. Routine use of PEEP (5 to 10 cm H2O) in these patients has been advocated. We studied the effects of different levels of PEEP on respiratory mechanics in ten mechanically ventilated open heart surgery patients in the immediate postoperative period. PEEP was studied in increasing increments and decreasing decrements. This procedure was repeated three times. Flow, tidal volume, and airway pressure were measured. We used the rapid airway occlusion technique to determine static compliance of the respiratory system (Cst,rs) and intrinsic PEEP (PEEPi). The changes in end-expiratory lung volume (delta EELV) were measured with respiratory inductive plethysmography. Recruitment of lung units (Vrec) was estimated as the difference in lung volume between PEEP and zero end-expiratory (ZEEP) for the same static inflation pressure (15 cm H2O). We found that (1) Cst,rs at ZEEP was significantly reduced (60 +/- 2 ml/cm H2O); (2) while PEEP of 5 cm H2O did not cause significant recruitment, higher levels of PEEP (10 to 15 cm H2O) were effective; (3) Cst,rs, Vrec, and delta EELV were higher during stepwise PEEP decrease; (4) after the first and second stepwise PEEP increase-decrease run, there was a small persistent increase in EELV and Cst,rs at ZEEP. No further changes were found after the third run. We conclude that after open heart surgery, PEEP less than 10 cm H2O is not effective to reopen atelectatic lung units.

Combined Thermodilution and Two-Dimensional Echocardiographic Evaluation of Right Ventricular Function during Respiratory Support with PEEP

In ten patients requiring respiratory support for an episode of acute respiratory failure (ARF), the best therapeutic level of PEEP was determined by measurement of changes in lung and chest wall compliance (CT) during a PEEP challenge from 0 to 20 cm H2O. During this challenge, hemodynamic monitoring combined with thermodilution measurement of right ventricular (RV) ejection fraction (EF) and two-dimensional echocardiographic measurement of RV size permitted assessment of the effects of increasing levels of PEEP on RV function. RV preload, as reflected by RV end-diastolic volume (EDV) and two-dimensional RV end-diastolic area (EDA), remained unchanged and RV diastolic compliance progressively decreased. On the other hand, RV systolic function, as assessed by RVEF and two-dimensional RV fractional area contraction (FAC), was progressively depressed. Substantial deleterious effects of PEEP were noted at high levels of PEEP including reduced CT and augmented pulmonary vascular resistance. Inadequate increase in RV preload to compensate for increased RV afterload resulted in depressed RV systolic function and contributed to the reduction in cardiac output. Finally, two-dimensional echocardiography proved to be more sensitive than fast-response thermodilution to evaluate change in RV function.

Lung management during cardiopulmonary bypass: Influence on extravascular lung water

Progressive respiratory insufficiency secondary to cardiopulmonary bypass (CPB) is still a hazard after cardiac surgery. Pathophysiologically, impaired capillary endothelial integrity seems to be the fundamental lesion, followed by increased interstitial fluid accumulation. The reasons for this pulmonary damage age controversial; however, management of the nonperfused lungs during CPB has been widely neglected and may be partly responsible. In this study, 90 patients undergoing coronary artery bypass grafting were randomly divided into six groups (15 patients each) with different management of the lungs during CPB: group 1, lungs collapsed (0/0); group 2, static inflation with +5 cm H 2O and F 1O 2 1.0 (+5/1.0); group 3, static inflation with +5 cm H 2O and F 1O 2 0.21 (+5/0.21); group 4, static inflation with +15 cm H 2O and F 1O 2 1.0 (+15/1.0); group 5, static inflation with +15 cm H 2O and F 1O 2 0.21 (+15/0.21); and group 6, controlled mechanical ventilation as before start of CPB (positive end-expiratory pressure [PEEP +5 cm H 2O; F 1O 2 1.0) (ventilation). In addition to hemodynamic monitoring, extravascular lung water (EVLW) was measured by means of a double-indicator dilution technique with heat and indocyanine green. Measurements were performed after induction of anesthesia, before onset of CPB, and immediately after weaning from bypass, as well as 60 minutes and 5 hours after termination of CPB. Pulmonary gas exchange (PaO 2) and intrapulmonary shunting (Q̇s/Q̇t) were also measured. Starting from comparable, normal baseline values, EVLW was increased in all groups after weaning from CPB, with the most pronounced increase in group 4 (maximum, +35%) and group 5 (+40%). Five hours after CPB, lung water content had normalized, except in groups 4 and 5, which still had values higher than those prior to CPB. (PaO 2) decreased most in these groups (group 4, −109 mm Hg; group 5, −130 mm Hg), and Q̇s/Q̇t increased significantly (group 4, +6.7%; group 5, +6.9%) after bypass. It is concluded that the handling of the lungs during CPB significantly influenced changes in EVLW after bypass. Static inflation with high levels of PEEP, particularly in combination with 100% oxygen, should be avoided due to the risk of an increase in lung water content. Static inflation with moderate PEEP and air seems to be the best way to optimize postbypass pulmonary function.

Percutaneous tracheostomy--a new method.

A rapid method of definitive low tracheostomy utilizing an original instrument kit and percutaneous approach is described. Through a horizontal, shallow (1.5-cm) skin incision in the neck, a 12-ga needle is introduced into the tracheal lumen. A flexible metal guidewire is gently introduced through the needle, which is then removed. A specially designed percutaneous tracheostomy tool slides over the guidewire into the trachea; by squeezing its handles, the intercartilaginous space is enlarged, securing the placement of a cuffed tracheal cannula. After extensive and successful investigation in both cadaver and animal trials, the procedure was carried out in 80 patients who had a variety of underlying diseases, without serious complications. Of these 80 patients, 33 required airway control after neurosurgical interventions or after severe head trauma, 23 had percutaneous tracheostomy performed before radical excision for head and neck cancer, and the remaining 24 suffered from severe cardiorespiratory problems. Twenty-nine procedures were performed in the operating theater, and 51 procedures were carried out at the patient's bedside in the ICU, ED, or in the ward. There was no infection at the stoma site, and late healing was remarkable. It should be strongly emphasized that in the majority of our patients the procedure was carried out safely at the bedside. This resolved the logistic problem of moving very sick ICU patients (who are sometimes on high PEEP levels) to the OR. Although our experience is totally restricted to elective situations, we do postulate that it could be as effective in a variety of urgent situations.

Pressure Control Inverse Ratio Ventilation as a Method to Reduce Peak Inspiratory Pressure and Provide Adequate Ventilation and Oxygenation

Nineteen patients with ARDS or pneumonia who were ventilated with PcIRV on the Siemens-Elema Servo 900 C were retrospectively reviewed. The PcIRV reduced peak airway pressure, PEEP, increased Paw, and improved ventilation and oxygenation in these patients. When these patients were compared with themselves on prior conventional IPPV, all had a decrease in PIP, an increase in Paw and most had a decrease in VE, with no change in PaCO2 and an increase in PaO2. The increase in Paw may have contributed to this improved arterial oxygenation. High levels of PIP and PEEP during IPPV have been identified as risk factors in the development of barotrauma and residual parenchymal pulmonary damage. We propose that PcIRV allows for adequate ventilation and oxygenation with decreases in PIP, extrinsically added PEEP and inspired O2 concentration. This mode of ventilation may decrease the morbidity associated with IPPV utilizing high PIP and PEEP.

Thoracic duct lymph and PEEP studies in anaesthetized dogs. II. Effect of a thoracic duct fistula on the development of a hyponcotic-hydrostatic pulmonary oedema.

PEEP impedes thoracic duct drainage (LF). This can be counteracted by a thoracic duct fistula. Consequently, lung oedema (LOE) should develop during PEEP more slowly with LF at atmospheric pressure (LFAP) than with LF against jugular venous pressure (LFJVP). In 12 anaesthetized dogs LOE was produced by Ringer's solution i.v. (2.5 ml/min per kg) for 6 h during PEEP (10 mmHg) with either LFAP or LFJVP. Ringer's + PEEP greatly increased aortic, pulmonary artery and wedge pressures, JVP, and cardiac output. Colloid osmotic pressures in plasma and lymph were drastically reduced, pulmonary effective filtration pressure (EFP) rose by about 20 mmHg. LFJVP increased 7-fold, LFAP about 19-fold, the respective loss of plasma proteins was 1.83 and 1.06 g/kg during 6 h. Thermal-dye extravascular lung water showed an increment of 68 with LFJVP versus 43 microliter/h/g per mmHg with LFAP. Final lung water content was at any delta EFP (12.8-31.9 mmHg) lower with LFAP than with LFJVP amounting 512 with LFJVP versus 377 microliters/g/per mmHg with LFAP. LFAP decreased the development of LOE during PEEP by bypassing the PEEP-induced high JVP and thus facilitating the removal of interstitial fluid. It is hypothesized that a thoracic duct fistula might aid the treatment of patients with LOE due to ARDS and therefore requiring high levels of PEEP.

OPTIMAL POSITIVE END-EXPIRATORY PRESSURE (PEEP) IN INFANTS AND CHILDREN WITH ACUTE RESPIRATORY FAILURE

PEEP has become the mainstay m the treatment of hypoxemic acute respiratory failure (ARF). While PEEP improves oxygenation by decreasing intrapulmonary shunting, it may also impair cardiac output and hence decrease oxygen delivery (O2D) despite increased arterial oxygen content (CaO2). Since optimizing O2D is the goal of therapy in ARF, we sought to determine whether the level of PEEP which results in maximal O2D can be estimated using noninvasive measurements of lung compliance (C). We studied 14 normovolemic children, aged 2 wks to II yrs, with ARF due to pneumonia or ARDS. Indicator dilution cardiac index (CI), arterial O2 partial pressure (PaO2), CaO2, C, and O2D were determined at 0, 3, 6, 9, 12 and 15 cm H2O PEEP. Tidal volume and FiO2 were held constant The level of PEEP (m±SD) at which O2D was maximal was 5.8±5.3 cm, and ranged from 0-15 cm in individual pts. PEEP of maximal C averaged 8.5±6 cm (range 0-15 cm) and was the same as PEEP of maximal O2D in 6 pts (43%) but higher (7 pts) or lower (1 pt) than PEEP of maximal O2D in the remainder; no consistent relationship between O2D and C was observed. PEEP of maximal CI was identical to PEEP of maximal O2D in all pts; higher levels of PEEP were associated with a significant decrease in CI, from 5.47±2 L/min/m2 at PEEP of maximal O2D to 4.39±2 L/min/m2 at PEEP 9 cm above that of maximal O2D. PEEP of maximal PaO2 averaged 12.2±4 cm (range 0-15 cm), and corresponded to PEEP of lowest O2D in 7 pts. At levels of PEEP above that of maximal O2D, PaO2 continued to increase significantly, from 115±40 torr at PEEP of maximal O2D to 145±75 torr at PEEP 9 cm above that of maximal O2D. We conclude that PEEP of maximal C does not reliably predict PEEP of best O2D in children with ARF. Because PaO2 continues to rise at levels of PEEP which cause significant decline in CI, maximizing PaO2 will not optimize O2D unless therapy to maintain CI is also employed.

Alternative modes of ventilation. Part I. Disadvantages of controlled mechanical ventilation: intermittent mandatory ventilation.

Controlled mechanical ventilation is an accepted therapy for acute respiratory failure but by virtue of the increase in intrathoracic pressure has a large number of disadvantages. It is to overcome these disadvantages that alternative modes of ventilation have been introduced. These aim to reduce the effects of abnormally high airway pressure on the lung whilst recruiting solid alveoli and at the same time maintaining effective blood volume. Intermittent mandatory ventilation is a mode of ventilation first introduced to aid weaning which may reduce the need for sedation, permit better tolerance of high levels of PEEP and maintain urine osmolar output. High frequency ventilation utilising low airway pressures can maintain pulmonary gas exchange whilst reducing the effects of stretch on the lung. Its major role would seem to be in cases of bronchopleural fistula and necrotising pneumonia where a low mean airway pressure is essential. Low frequency positive pressure ventilation with extra corporeal CO2 removal, whilst a very labour intensive technique, has produced a favourable outcome in patients with terminal respiratory failure. Use of PEEP is associated with further deleterious haemodynamic effects which are largely overcome with use of continuous positive airway pressure during spontaneous respiration. PEEP is widely used. Its effect on pulmonary compliance, dead space and oxygen delivery are unpredictable making haemodynamic monitoring mandatory. Inversed ratio ventilation requires further evaluation whereas differential lung ventilation is logical, complicated but very valuable where the time constants for each lung are significantly different.

Hemodynamic Management in the Adult Respiratory Distress Syndrome

SUMMARY The guiding principles of cardiovascular management of patients with ARDS should be to maintain adequate cardiac output and tissue oxygenation without worsening the degree of pulmonary edema. Because the pulmonary edema is caused by an increase in lung vascular permeability, therapy should be directed toward maintaining a low left ventricular filling pressure, or pulmonary capillary wedge pressure. A low pulmonary capillary wedge pressure will, based on experimental data, tend to decrease the rate of edema formation and to increase the forces for reabsorption of edema fluid once vascular permeability returns to normal. If cardiac output is low or falls as a function of therapy (PEEP), flow may be increased with inotropic or vasoactive agents, or both. These agents increase cardiac output and stroke volume by reducing ventricular end systolic volume so that flow increases despite constant or reduced left ventricular filling pressure. In the setting of low pressure pulmonary edema, with the exception of hydralazine, interventions that increase cardiac output will most likely increase right-to-left shunt and reduce arterial oxygen tension. If desaturation of arterial blood occurs with increased cardiac output, then this may be corrected with higher levels of PEEP. Because of potential deleterious effects of PEEP on left ventricular diastolic mechanics, right ventricular afterload, and barotrauma, hypoxemia should be treated with the lowest level of PEEP that provides 95 per cent saturation of the circulating hemoglobin during ventilation with nontoxic, inspired oxygen concentrations of 50 to 60 per cent. In the setting of ARDS and pulmonary hypertension, volume expansion may result in right ventricular dysfunction and/or a significant increase in the pulmonary capillary wedge pressure. Alternatively, inotropic agents may be preferable to maintain or increase cardiac output when right ventricular afterload is elevated. Also, while vasodilator therapy is designed to reduce an elevated systemic vascular resistance and may be appropriate when pulmonary vascular resistance is normal, such treatment may result in right ventricular dysfunction secondary to a fall in blood pressure and right ventricular perfusion pressure, when right ventricular afterload is increased. This approach attempts to incoporate principles deduced from clinical and experimental studies to provide a framework for approaching the cardiovascular management of patients with acute respiratory failure or ARDS.

Cardiopulmonary abnormalities in severe acute respiratory failure

Serial cardiopulmonary variables were recorded over 4 days in 23 children with severe acute respiratory failure. In all patients, pulmonary artery catheters were inserted within 24 hours of the diagnosis of respiratory failure, and all required greater than 10 cm H2O positive end-expiratory pressure to achieve adequate oxygenation. Eight patients died (35% mortality). Evaluation of systemic hemodynamic variables indicated that survivors had higher blood pressures than nonsurvivors, although neither group was in the hypotensive range. Systemic vascular resistance was lower in the nonsurvivors. Cardiac function as evaluated by cardiac index, right ventricular stroke work index, and left ventricular stroke work index was similar in both groups. Survivors demonstrated elevations in mean pulmonary artery pressure and pulmonary vascular resistance during the first 36 hours, with gradual improvement thereafter. In contrast, pulmonary artery pressure and resistance increased progressively in patients who died. Intrapulmonary shunt fractions remained high in the nonsurvivors despite the use of up to 25 cm H2O PEEP. Cardiac function and oxygen delivery were well maintained in both groups despite the high levels of PEEP.