Optimization of patient-ventilator interactions: closed-loop technology to turn the century.
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Algorithms↗
Biomedical subjects
V M Ranieri
Publications and source records attributed to V M Ranieri.
Effects of proportional assist ventilation on inspiratory muscle effort in patients with chronic obstructive pulmonary disease and acute respiratory failure.
BACKGROUND: Acute respiratory failure may develop in patients with chronic obstructive pulmonary disease because of intrinsic positive end-expiratory pressure (PEEPi) and increased resistive and elastic loads. Proportional assist ventilation is an experimental mode of partial ventilatory support in which the ventilator generates flow to unload the resistive burden (flow assistance: FA) and volume to unload the elastic burden (volume assistance: VA) proportionally to inspiratory muscle effort, and PEEPi can be counterbalanced by application of external PEEP. The authors assessed effects of proportional assist ventilation and optimal ventilatory settings in patients with chronic obstructive pulmonary disease and acute respiratory failure. METHODS: Inspiratory muscles and diaphragmatic efforts were evaluated by measurements of esophageal, gastric, and transdiaphragmatic pressures. Minute ventilation and breathing patterns were evaluated by measuring airway pressure and flow. Measurements were performed during spontaneous breathing, continuous positive airway pressure, FA, FA+PEEP, VA, VA+PEEP, FA+VA, and FA+VA+PEEP. RESULTS: FA+PEEP provided the greatest improvement in minute ventilation (89 +/- 3%) and dyspnea (62 +/- 2%). The largest reduction in pressure time product per breath of the respiratory muscles and diaphragm (44 +/- 3% and 33 +/- 2%, respectively) also was observed during FA+PEEP condition. When VA was added to this setting, a reduction in respiratory rate (50 +/- 3%), an increase in inspiratory time (102 +/- 6%), and a further reduction in pressure time product per minute (65 +/- 2% and 64% for the respiratory muscles and diaphragm, respectively) was observed. However, values of pressure time product per liter of minute ventilation during FA+VA+PEEP did not differ with those observed during FA+PEEP condition. Worsening of patient-ventilator interaction and breathing asynchrony occurred when VA was implemented. CONCLUSIONS: Application of PEEP to counterbalance PEEPi and FA to unload the resistive burden provided the optimal conditions in such patients. Ventilator over-assistance and patient-ventilator asynchrony was observed when VA was added to this setting. The clinical use of proportional assist ventilation should be based on continuous measurements of respiratory mechanics.
Acute Disease↗
Impairment of lung and chest wall mechanics in patients with acute respiratory distress syndrome: role of abdominal distension.
Recent data have suggested that the elastic properties of the chest wall (CW) may be compromised in patients with ARDS because of abdominal distension (4). We partitioned CW and lung (L) mechanics, assessed the role of abdominal distension, and verified whether the underlying disease responsible for ARDS affects the impairment of respiratory mechanics. Volume-pressure (V-P) curves (interrupter technique) were assessed in nine patients with surgical ARDS and nine patients with medical ARDS. Relative to nine patients undergoing heart surgery, V-P curves of the respiratory system (rs) and L of patients with surgical or medical ARDS showed a rightward displacement. V-P curves of the CW and the L showed an upward concavity in patients with medical ARDS and a downward concavity in patients with surgical ARDS. Although the CW and the abdomen (abd) V-P curves in patients with medical ARDS were similar to those obtained in patients undergoing heart surgery, they showed a rightward shift and a downward flattening in patients with surgical ARDS. In five of these patients, a reduction in static end-inspiratory pressure of the abd (69+/-4%), rs (30+/-3%), CW (41+/-2%), and L (27+/-3%) was observed after abdominal decompression for acute bleeding. Abdominal decompression therefore caused an upward and leftward shift of the V-P curves of the respiratory system, chest wall, lung, and abdomen. In conclusion we showed that impairment of the elastic properties of the respiratory system may vary with the underlying disease responsible for ARDS. The flattening of the V-P curve at high pressures observed in some patients with ARDS may be due to an increase in chest wall elastance related to abdominal distension. These observations have implications for the assessment and ventilatory management of patients with ARDS.
Abdomen↗
Patient-ventilator interaction.
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Humans↗
Right ventricular myocardial function in ARF patients. PEEP as a challenge for the right heart.
OBJECTIVE: To examine the hemodynamic effects of external positive end-expiratory pressure (PEEP) on right ventricular (RV) function in acute respiratory failure (ARF) patients. DESIGN: Prospective, with retrospective analysis on the basis of RV volume response to PEEP. SETTING: General intensive care unit in a university teaching hospital. PATIENTS: 20 mechanically ventilated ARF patients (mean lung injury score = 2.6 +/- 0.45 SD). INTERVENTION: Incremental levels of PEEP (0-5-10-15 cmH2O) were applied and RV hemodynamics were studied by means of a Swan-Ganz catheter with a fast-response thermistor for right ventricular ejection fraction (RVEF) measurement. According to their response to PEEP 15, two groups of patients were defined: group A (9 patients) with unchanged or increased RV end-diastolic volume index (RVEDVI) and group B (11 patients) with decreased RVEDVI. MEASUREMENTS AND RESULTS: At zero PEEP (ZEEP) the hemodynamic parameters of the two groups did not differ. In group A, cardiac index (CI) and stroke volume index (SI) decreased at all PEEP levels (5, 10, and 15 cmH2O), while RVEF started to decrease only at a PEEP of 10 cmH2O (-10.8%), and RVES(systolic)VI increased only at PEEP 15 cmH2O (+21.5%). RVEDVI was not affected by PEEP. In group B, CI and SI decreased at all PEEP levels (5, 10, and 15 cmH2O). Similarly, RVEDVI started to decrease at PEEP 5 cmH2O, while RVESVI decreased only at PEEP 15 cmH2O (-21.4%). RVEF was not affected by PEEP in this group. In each patient the slope of the relationship between RVEDVI and right ventricular stroke work index (RVSWI), expressing RV myocardial performance, was studied. This relationship was significant (no change in RV contractility) in 8 of 11 patients in group B and in only 2 patients in group A. In 4 patients in group A, PEEP shifted the RVSWI/RVEDVI ratio rightward in the plot, indicating a decrease in RV myocardial performance in these patients. CONCLUSIONS: PEEP affects RV function in ARF patients. The decrease in cardiac output is more often associated with a preload decrease and no change in RV contractility. On the other hand, the finding of increased RV volumes with PEEP may be associated with a reduction in RV myocardial performance. Thus, these results suggest that assessment of RV function by PEEP and preload recruitable stroke work may disclose otherwise unpredictable alterations in RV function.
Adult↗
Effects of positive end-expiratory pressure on right ventricular function in COPD patients during acute ventilatory failure.
OBJECTIVE: To examine the effects of external positive end-expiratory pressure (PEEP) on right ventricular function in chronic obstructive pulmonary disease (COPD) patients with intrinsic PEEP (PEEPi). DESIGN: Prospective study. SETTING: General intensive care unit in a university teaching hospital. PATIENTS: Seven mechanically ventilated flow-limited COPD patients (PEEPi = 9.7 +/- 1.3 cmH2O, mean +/- SD) with acute respiratory failure. INTERVENTION: Hemodynamic and respiratory mechanic data were collected at four different levels of PEEP (0-5-10-15 cmH2O). MEASUREMENTS AND RESULTS: Hemodynamic parameters were obtained by a Swan-Ganz catheter with a fast response thermistor. Cardiac index (CI) and end-expiratory lung volume (EELV) reductions started simultaneously when the applied PEEP was approximately 90% of PEEPi measured on 0 cmH2O (ZEEP). Changes in transmural intrathoracic pressure (PEEPi,cw) started only at a PEEP value much higher (120%) than PEEPi. The reduction in CI was related to a decrease in the right end-diastolic ventricular volume index (RVEDVI) (r = 0.61; p < 0.001). No correlation between CI and transmural right atrial pressure was observed. The RVEDVI was inversely correlated with PEEP-induced changes in EELV (r = -55; p < 0.001), but no with PEEPi,cw (r = -0.08; NS). The relationship between RVEDVI and right ventricular stroke work index, considered an index of contractility, was significant in three patients, i.e., PEEP did not change contractility. In the other patients, an increase in contractility seemed to occur. CONCLUSIONS: In COPD patients an external PEEP exceeding 90% of PEEPi causes lung hyperinflation and reduces the CI due to a preload effect. The reduction in RVEDVI seems related to changes in EELV, rather than to changes in transmural pressures, suggesting a lung/heart volume interaction in the cardiac fossa. Thus, in COPD patients, application of an external PEEP level lower than PEEPi may affect right ventricular function.
Acute Disease↗
Auto-positive end-expiratory pressure and dynamic hyperinflation.
PEEP is indicated in patients with COPD only to unload the respiratory muscles from the auto-PEEP resulting from expiratory flow limitation. If auto-PEEP is not caused by flow limitation, application of PEEP will cause further hyperinflation, worsening respiratory mechanics, muscle activity, and hemodynamics. To assess the presence of expiratory flow limitation correctly, to measure auto-PEEP correctly, and to identify the maximal PEEP level to be used, measurements of flow and opening pressure must be obtained during a brief period of suspended respiratory muscle activity (obtained by sedation) with the patient's own breathing pattern reproduced accurately.
Humans↗
Patient-ventilator interaction during acute hypercapnia: pressure-support vs. proportional-assist ventilation.
The objective of this study was to compare patient-ventilator interaction during pressure-support ventilation (PSV) and proportional-assist ventilation (PAV) in the course of increased ventilatory requirement obtained by adding a dead space in 12 patients on weaning from mechanical ventilation. With PSV, the level of unloading was provided by setting the inspiratory pressure at 20 and 10 cmH2O, whereas with PAV the level of unloading was at 80 and 40% of the elastic and resistive load. Hypercapnia increased (P < 0.001) tidal swing of esophageal pressure and pressure-time product per breath at both levels of PSV and PAV. During PSV, application of dead space increased ventilation (VE) during PSV (67 +/- 4 and 145 +/- 5% during 20 and 10 cmH2O PSV, respectively, P < 0.001). This was due to a relevant increase in respiratory rate (48 +/- 4 and 103 +/- 5% during 20 and 10 cmH2O PSV, respectively, P < 0.001), whereas the increase in tidal volume (VT) played a small role (13 +/- 1 and 21 +/- 2% during 20 and 10 cmH2O PSV, respectively, P < 0.001). With PAV, the increase in VE consequent to hypercapnia (27 +/- 3 and 64 +/- 4% during 80 and 40% PAV, respectively, P < 0.001) was related to the increase in VT (32 +/- 1 and 66 +/- 2% during 80 and 40% PAV, respectively, P < 0.001), respiratory rate remaining unchanged. The increase in pressure-time product per minute and per liter consequent to acute hypercapnia and the sense of breathlessness were significantly (P < 0.001) higher during PSV than during PAV. Our data show that, after hypercapnic stimulation of the respiratory drive, the capability to increase VE through changes in VT modulated by variations in inspiratory muscle effort is preserved only during PAV; the compensatory strategy used to increase VE during PSV requires greater muscle effort and causes more pronounced patient discomfort than during PAV.
Adult↗
Chest wall and lung contribution to the elastic properties of the respiratory system in patients with chronic obstructive pulmonary disease.
Conflicting data are available on the relative contribution of the chest wall (cw) to the intrinsic positive end-expiratory pressure of the total respiratory system (PEEPi,rs) in patients with chronic obstructive pulmonary disease (COPD). In order to assess the chest wall and lung contribution to the elastic properties of the respiratory system in COPD patients during acute ventilatory failure, using the "interrupter technique", static inflation volume-pressure (V-P) curves of the total respiratory system (rs), lung (L) and cw were obtained in seven mechanically-ventilated COPD patients during application of zero end-expiratory pressure (ZEEP) and different levels (0-15 cmH2O) of PEEP. On ZEEP, PEEPi,rs was present in all patients (range 10.5-13.1 cmH2O), to which PEEPi,cw and PEEPi,L contributed 17 +/- 2 and 83 +/- 1%, respectively. The static V-P curves of the rs, L, and cw on ZEEP were concave toward the horizontal axis, indicating that elastance increased with inflating volume. Application of PEEP did not affect lung and chest wall mechanics until PEEP levels exceeding 90% of PEEPi,cw on ZEEP (critical value of PEEP (Pcrit)). At PEEP levels higher than Pcrit, and relative to the V-P curves on ZEEP, we observed that: 1) the V-P curve of the rs showed an initial shift along the curve on ZEEP followed by a downward displacement with inflating volume; 2) the V-P curve of the L was shifted along the curve on ZEEP throughout inflating volume; and 3) the V-P curve of the cw was initially displaced along the curve on ZEEP, whilst a downward displacement appeared at higher lung volume. In conclusion, our data show that, in chronic obstructive pulmonary disease patients with flow limitation, the increase in pleural pressure does not make a significant contribution to the intrinsic positive end-expiratory pressure of the total respiratory system. However, during tidal ventilation, a substantial increase in elastance of the chest wall is present. The critical values of positive end-expiratory pressure below which there are no changes in chest wall and lung mechanics amount to 90% of the total PEEPi,rs on ZEEP. Positive end-expiratory pressure levels higher than such critical value cause important alterations of the elastic properties of the lung and chest wall.
Aged↗
Intrinsic PEEP and cardiopulmonary interaction in patients with COPD and acute ventilatory failure.
Deviation of end-expiratory lung volume from the elastic equilibrium volume of the respiratory system is recognized as a cardinal feature in mechanically-ventilated patients with severe chronic obstructive pulmonary disease (COPD) and acute ventilatory failure (AVF). The presence of dynamic hyperinflation implies that alveolar pressure remains positive throughout expiration. At the end of the expiration, this positive pressure is named intrinsic positive end-expiratory pressure (PEEPi). Recent studies have suggested that, in COPD patients with expiratory flow limitation, the application of external PEEP during assisted mechanical ventilation, or the use of continuous positive airway pressure (CPAP) in spontaneously breathing patients, can counterbalance and reduce the inspiratory threshold load imposed by PEEPi, without causing further increase in lung volume and alveolar and intrathoracic pressures until a critical value of PEEP (Pcrit) is reached. Above this critical limit further hyperinflation is observed. A specific and characteristic role of PEEPi in compromising the heart function in COPD patients during AVF may be identified based on: 1) an increase in right ventricular impedance due to lung hyperinflation; 2) an increase in the venous return to the right ventricle and, consequently, a leftward shift of the septum caused by the large negative deflections in intrathoracic pressure due to the inspiratory threshold load; 3) a further increase in venous return to the right ventricle, with the eventual collapse of the vena cava caused by the expiratory recruitment of abdominal muscles; and 4) hypoxia and hypercapnia consequent to acute ventilatory failure, which may further increase right ventricular impedance and venous return to the right ventricle. All these phenomenon are directly correlated to the large negative intrathoracic pressure developed by the respiratory muscles to overcome the inspiratory threshold caused by intrinsic positive end-expiratory pressure (preload effect), and to the increase in lung volume (afterload effect). Application of positive end-expiratory pressure/continuous positive airway pressure in chronic obstructive pulmonary disease patients during acute ventilatory failure may, hence, unload the respiratory muscles as well as the heart.
Humans↗
Survival despite almost complete fibrous obstruction of a Sorin tilting disc mitral prosthesis.
We report the case of a 40-year-old woman whose Sorin tilting disc mitral prosthesis was obstructed by fibrous overgrowth to the point of near occlusion. The unusual features of this case are that the patient survived reoperation and that her preoperative symptoms were mild despite an immobile disc and near occlusion of the valve. In most similar cases in the literature, preoperative symptoms have been acute and mortality has been high.
Adult↗
Inspiratory effort and measurement of dynamic intrinsic PEEP in COPD patients: effects of ventilator triggering systems.
OBJECTIVE: To investigate effects of ventilator triggering systems (pressure and flow triggering: PT and FT) on measurement of dynamic intrinsic PEEP (PEEPidyn) and patient-ventilator interaction in patients with chronic obstructive pulmonary disease during weaning from mechanical ventilation. DESIGN: Prospective study. SETTING: Medical/surgical intensive care unit of an academic hospital. PATIENTS AND PARTICIPANTS: 6 COPD patients with acute respiratory failure ready to wean. MEASUREMENTS: We measured flow, airway opening, esophageal and gastric pressures. Minute ventilation, breathing pattern and pressure time product (PTP) of the respiratory muscles and of the diaphragm were obtained during spontaneous ventilation through a mechanical ventilator (Puritan-Bennett 7200ae). Two triggering systems, namely PT and FT, were evaluated. RESULTS: The inspiratory muscles effort necessary to overcome the triggering system overestimated PEEPidyn measurement of an amount equal to 49 +/- 2 and 58 +/- 3% during respectively pressure and flow triggering. FT increased tidal volume and minute ventilation and decrease PTP/b and PTP/min of the respiratory muscles and diaphragm. CONCLUSIONS: To correctly measure PEEPidyn, the inspiratory effort produced to overcome PEEPi and to trigger the ventilator must be discriminated. Application of flow triggering requires less effort to initiate inspiration and provide a positive end-expiratory pressure level that is able to unload the respiratory muscles by reducing PEEPi. With flow triggering higher minute ventilation are obtained in COPD patients during the weaning phase.
Acute Disease↗
Cardiorespiratory effects of positive end-expiratory pressure during progressive tidal volume reduction (permissive hypercapnia) in patients with acute respiratory distress syndrome.
BACKGROUND: In patients with acute respiratory distress syndrome (ARDS), the ventilatory approach is based on tidal volume (VT) of 10-15 ml/kg and positive end-expiratory pressure (PEEP). To avoid further pulmonary injury, decreasing VT and allowing PaCO2 to increase (permissive hypercapnia) has been suggested. Effects of 10 cmH2O of PEEP on respiratory mechanics, hemodynamics, and gas exchange were compared during mechanical ventilation with conventional (10-15 ml/kg) and low (5-8 ml/kg) VT. METHODS: Nine sedated and paralyzed patients were studied. VT was decreased gradually (50 ml every 20-30 min). Static volume-pressure (V-P) curves, hemodynamics, and gas exchange were measured. RESULTS: During mechanical ventilation with conventional VT, V-P curves on PEEP 0 (ZEEP) exhibited an upward convexity in six patients reflecting a progressive reduction in compliance with inflating volume, whereas PEEP resulted in a volume displacement along the flat part of this curve. After VT reduction, V-P curves in the same patients showed an upward concavity, reflecting progressive alveolar recruitment with inflating volume, and application of PEEP resulted in alveolar recruitment. The other three patients showed a V-P curve with an upward concavity; VT reduction increased this concavity, and application of PEEP induced greater alveolar recruitment than during conventional VT. With PEEP, cardiac index decreased by, respectively, 31% during conventional VT and 11% during low VT (P < 0.01); PaO2 increased by 32% and 71% (P < 0.01), respectively, whereas right-to-left venous admixture (Qs/Qt) decreased by 11% and 40%, respectively (P < 0.01). The greatest values of PaO2, static compliance, and oxygen delivery and the lowest values of Qs/Qt (best PEEP) were obtained during application of PEEP with low VT (P < 0.01). CONCLUSIONS: Although PEEP induced alveolar hyperinflation in most patients during mechanical ventilation with conventional VT, at low VT, there appeared to be a significant alveolar collapse, and PEEP was able to expand these units, improving gas exchange and hemodynamics.
Female↗
Patient-ventilator interaction during synchronized intermittent mandatory ventilation. Effects of flow triggering.
Synchronized intermittent mandatory ventilation (SIMV) intermixes assisted and spontaneous breaths. Its ability as a weaning technique has been questioned on the basis that patients show little adaptation to ventilator assistance. We studied inspiratory effort and patient-ventilator interaction at different levels (SIMV, 100, 50, and 0%) of flow-triggered SIMV versus pressure-triggered SIMV in patients during the weaning period. The two triggering systems were evaluated during constant flow and constant pressure mandatory SIMV breaths. Inspiratory effort was estimated as the esophageal pressure time product (PTP) per breath (PTP/b) and per minute (PTP/min). The PTP/b and PTP/min of both mandatory and spontaneous breaths were significantly lower during flow triggering than during pressure triggering SIMV, irrespective of the ventilatory mode. During pressure-triggered SIMV PTP/b and PTP/min were identical for mandatory and spontaneous breaths, whereas during flow-triggered SIMV PTP/b and PTP/min were significantly lower for mandatory than for spontaneous breaths. This difference was greatest when flow triggering and constant pressure ventilation were associated. These data show that flow triggering reduces inspiratory effort during both mandatory and spontaneous SIMV breaths and obtains a better patient-ventilator interaction.
Acute Disease↗
PEEP, ARDS and alveolar recruitment--the physiologist point of view.
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Humans↗
Volume-pressure curve of the respiratory system predicts effects of PEEP in ARDS: "occlusion" versus "constant flow" technique.
The effects of positive end-expiratory pressure (PEEP) on static ("rapid airway occlusion" technique) and dynamic ("constant flow" technique) volume-pressure (V-P) curves were studied in 19 patients with adult respiratory distress syndrome (ARDS). To describe the shape of both curves, the nonlinear coefficient of a second-order polynomial equation fitted to the static (static nonlinear coefficient) and dynamic (dynamic nonlinear coefficient) V-P curves on zero end-expiratory pressure (ZEEP) was used. Two distinct patterns were observed: (1) in ten patients, the static and dynamic V-P curves on ZEEP exhibited a convex shape with a progressive decrease in slope with increasing inflation volume (nonlinear coefficients: negative). In these patients PEEP induced a volume displacement along the static and dynamic V-P curves on ZEEP (hyperinflation). (2) In nine patients, the static and dynamic V-P curves on ZEEP showed a concave shape with a progressive increase in slope with increasing volume (nonlinear coefficients: positive) and PEEP shifted both curves upward along the volume axis (alveolar recruitment). A correlation (p < 0.0001) between static and dynamic nonlinear coefficients was found at all levels of PEEP. Both static and dynamic nonlinear coefficients on ZEEP were correlated (p < 0.0001) with the amount of lung volume recruited with PEEP, and the variations of cardiac index (CI), O2 delivery (DO2), right-to-left venous admixture (Qs/Qt), and PaO2 with PEEP. Besides, the effects of PEEP on Cl, DO2, Qs/Qt, and PaO2 were less pronounced (p < 0.001) in patients with convex V-P curves than in patients with concave V-P curves.(ABSTRACT TRUNCATED AT 250 WORDS)
Aged↗
Comparison of static and dynamic measurements of intrinsic PEEP in mechanically ventilated patients.
Intrinsic positive end-expiratory pressure (PEEPi) is routinely determined under static conditions by occluding the airway at end-expiration (PEEPi,stat), the resulting plateau pressure representing the average PEEPi present within a nonhomogeneous lung. In contrast, PEEPi can also be evaluated dynamically (PEEPi,dyn) by recording the change in pressure required to initiate lung inflation. It has been suggested that PEEPi,dyn reflects the lowest regional PEEPi, and therefore underestimates PEEPi,stat in the presence of heterogenous mechanical properties. The purposes of this study were (1) to compare PEEPi obtained with these two methods in mechanically ventilated patients with significant airway obstruction (AWO) and those without (non-AWO), and (2) to relate any discrepancies observed with other indices of respiratory mechanics. PEEPi,stat, PEEPi,dyn, and respiratory mechanics were measured during controlled mechanical ventilation in 22 sedated, paralyzed patients. PEEPi,dyn was significantly less than PEEPi,stat in AWO, averaging 3.0 +/- 0.5 (SEM) and 9.3 +/- 1.1 (SEM) cm H2O, respectively (p < 0.0001). In contrast, these values were more comparable in non-AWO, averaging 4.6 +/- 0.8 and 5.4 +/- 1.0 cm H2O (p > 0.05). As a result, the ratio of PEEPi,dyn to PEEPi,stat amounted to 0.36 +/- 0.06 for AWO compared with 0.87 +/- 0.05 in non-AWO (p < 0.005). Maximal (Rmax) and minimal (Rmin) respiratory resistance were greater in AWO whereas respiratory compliance (Crs) was no different between groups. PEEPi,dyn/PEEPi,stat was inversely related to delta P, the pressure losses attributable to time constant inequalities and viscoelastic tissue properties (r = 0.64, p < 0.005). No correlation was found between this ratio and Rmax, Rmin, or Crs.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Oxygen delivery-consumption relationship in adult respiratory distress syndrome patients: the effects of sepsis.
The oxygen consumption-delivery relationship (VO2/DO2) was studied in 15 sedated paralyzed patients with the adult respiratory distress syndrome (ARDS) due to multiple trauma and in whom sepsis was absent. Different levels (0 to 15 cm H2O) of positive end-expiratory pressure (PEEP) were applied. Oxygen delivery was calculated from cardiac index (thermodilution technique) and arterial oxygen content measurements. Oxygen consumption was calculated using Fick's equation. Regression lines were obtained for each patient. Oxygen supply dependency was defined as a significant (P < .05) relationship between changes in VO2 and DO2 with PEEP. Results were compared with those obtained in 18 ARDS patients in whom ARDS was due to sepsis. In nonseptic ARDS patients no significant relationship between changes in VO2 and DO2 with PEEP was found within the experimental range of DO2 on zero end-expiratory pressure (ZEEP) (347 to 845 mL/min/m2). None of these patients had multiple organ system failure (MOSF), and 73% survived. In ARDS patients in whom sepsis was present, supply dependency was present only when DO2 on ZEEP ranged between 330 and 640 mL/min/m2. All these patients developed MOSF and died. When DO2 on ZEEP ranged between 686 and 951 mL/min/m2 in septic ARDS patients, the supply dependency phenomenon was absent and only three patients developed MOSF and died (70% survivors). In almost all patients PEEP reduced DO2 and therefore worsened O2 balance by either increasing O2 extraction ratio and approaching the critical threshold for supply dependency or dismissing DO2 from the range of non-supply dependency.
Adult↗