Left ventricular function after acute myocardial infarction

10 patients with their first AMI were studied within the first 48 hours and again after 3 weeks. Central and peripheral haemodynamics (CI, SV, SW, TPR) were examined, including indices of contractility (dp/dlmax) and wall stiffness (P/V, relation P/V to P) of the left ventricle.In the early phase CI and SW, as well as LV dp/dt_max were depressed in accordance with symptoms of LV failure. P/V was increased. Elevation of LVEDP correlated well with ventricular gallop rhythm, but less consistently with LV functional disturbance.During convalescence CI increased uniformly, both in digitalized and non-digitalized individuals. In contrast heart rate, aortic pressure, LVEDP and dp/dt_max remained unchanged. The increase of CI, SV and SW was accompanied by a fall of TPR and P/V. LV wall stiffness was still elevatedabove normal after 3 weeks. The improvement of cardiac pumping during infarct convalescence may have been effected through a fall of TPR and LV wall stiffness. Recovery of depressed contractile performance was generally not observed, and does therefore not seem to contribute to recuperation.Herrn Prof. Dr. med. P. Schölmerich zum 60. Geburtstag.     

Effects of the heat-moisture exchangers on dynamic hyperinflation of mechanically ventilated COPD patients

In recent years the use of devices called Heat and Moisture Exchangers (HME) has become widespread as gas conditioners for ICU patients requiring mechanical ventilation. As an important variation of the resistive properties of the HME, related to flow and duration of use, has recently been pointed out during in vitro studies, the use of these devices in COPD patients could increase the levels of auto PEEP and dynamic hyperinflation. In this study we have compared the levels of auto PEEP and difference in functional residual capacity ( FRC) in a group of COPD patients, requiring controlled mechanical ventilation (CMV), at basal conditions and after the insertion into the circuit of three HMEs (Dar Hygrobac, Pall Ultipor, Engstrom Edith) at random: the results obtained excluded a significant increase of auto PEEP and (FRC) both with new HMEs and after 12 h of continuous use.     

Increased oxygen consumption after cardiac surgery is associated with the inflammatory response to endotoxemia

Objective The aim of this study was to determine whether the increase in post-operative oxygen consumption (VO₂) in cardiac surgery patients in related to endotoxemia and subsequent cytokine release and whether VO₂ can be used as a parameter of post-perfusion syndrome.Design Prospective study.Setting Operating room and intensive care unit of a university hospital.Patients Twenty-one consecutive male patients undergoing elective coronary artery bypass surgery without major organ dysfunction and not receiving corticosteroids.Measurements and results Plasma levels of endotoxin, tumor necrosis factor (TNF) and interleukin-6 (IL-6) were measured before, during and for 18 h after cardiac surgery. Oxygen consumption, haemodynamics, the use of IV fluids and dopamine, body temperature and the time of extubation were also measured. Measurements from patients with high VO₂ (median value of the entire group) were compared with measurements from patients with low VO₂ (2 had higher levels of circulating endotoxin (P=0.004), TNF (P=0.04) and IL-6 (P=0.009) received more IV fluids and dopamine while in the ICU, and were extubated later than patients with low VO₂. Several hours after VO₂ the patient's body temperature rose, Forward stepwise regression analysis showed that circulating endotoxin and TNF explained 50% of the variability of VO₂.Conclusions This study demonstrates that patients with high post operative oxygen comsumption after elective cardiac surgery have higher circulating levels of endotoxin, TNF and IL-6 and also have more symptoms of post-perfusion syndrome. Early detection of high VO₂ might be used as a clinical signal to improve circulation in order to meet the high oxygen demand of inflammation. In addition, continuous measurement of VO₂ provides us with a clinical parameter of inflammation in interventional studies aiming at a reduction of endotoxemia or circulating cytokines.Part of this study was supported financially by Jaussen Pharmaceutica B.V. (Tilburg, The Netherlands)     

Alveolar pressure during high-frequency jet ventilation

We studied the influence of ventilatory frequency (1–5 Hz), tidal volume, lung volume and body position on the end-expiratory alveolar-to-tracheal pressure difference during high-frequency jet ventilation (HFJV) in Yorkshire piglets. The animals were anesthetized and paralysed. Alveolar pressure was estimated with the clamp off method, which was performed by a computer controlled ventilator and which had been extensively tested on its feasibility. The alveolar-to-tracheal pressure difference increased with increasing frequency and with increasing tidal volume, the common determinant appearing to be the mean expiratory flow. The effects in prone and in supine position were similar. Increasing thoracic volume decreased the alveolar-to-tracheal pressure difference indicating a dependence of this pressure difference on airway resistance. We concluded that the main factors determining the alveolar-to-tracheal pressure difference (P) during HFJV are expiratory flow (V_E) and airway resistance (R), PV_E×R.     

Plasma catecholamines and oxygen consumption during weaning from mechanical ventilation

Previous studies on oxygen consumption ( ) during weaning from mechanical ventilation assumed that an increase in ( ) reflected oxygen consumption by respiratory muscles ( ), and proposed as a weaning predictor. We measured CO₂ production ( ) and plasma catecholamines in 20 short-term ventilated patients during weaning by SIMV and CPAP. as a percentage of during spontaneous ventilation ( %) ranged from 4.8% to 41.5%. also increased and correlated with . Plasma adrenaline and noradrenaline increased significantly to levels known to produce considerable increases in metabolic rate. Mean arterial pressure and heart rate concomitantly increased, but spontaneous minute ventilation decreased. Thus, since the increased plasma catecholamines are calorigenic, the assumption that represents is incorrect. Although mean % of successfully weaned patients was significantly less than that of failure-to-wean patients, the wide scatter of individual values in the latter group excludes % as an accurate weaning predictor.     

Stroke output variations calculated by esophageal Doppler is a reliable predictor of fluid response

Objective Esophageal Doppler allows continuous monitoring of stroke volume index (SVI) and corrected flow time (FTc). We hypothesized that variations in stroke output index SOI (SVI/FTc) during volume expansion can predict the hemodynamic response to subsequent fluid loading better than the static values.Design and setting Prospective study in the intensive care unit of a university hospital.Patients Fifty-one patients with circulatory failure were monitored by esophageal Doppler.Interventions Patients who responded to a first fluid challenge received a second one. Patients who responded to both were classified as responders-responders, and those who did not respond to the second as responders-nonresponders. In these two groups we compared SVI, FTc, and SOI during each fluid challenge and also static values at the end of each fluid challenge.Measurements and results After the first fluid challenge SOI and SVI were significantly higher in patients who responded to subsequent volume expansion than in patients who no longer responded. ROC curves showed that SOI was a better predictor of fluid responsiveness than SVI. During volume expansion a SOI value of 11% discriminated between responders and nonresponders to subsequent volume expansion with a sensitivity of 91% and a specificity of 97%. There was no significant difference between the two groups for FTc value at the end of first fluid challenge.Conclusions Analysis of SOI during fluid challenge predicts response to subsequent fluid challenge and FTc is not a reliable indicator of cardiac preload.     

Volume/pressure curve of total respiratory system in paralysed patients: artefacts and correction factors

The volume/pressure (V/P) curve of the total respiratory system in paralysed patients is drawn assuming that volume changes of the respiratory system (V resp) equals volume displacement of the measuring apparatus (V syr), usually a supersyringe. However, in 93 VP curves we found that O₂ removed from the lung-syringe system during the procedure (proportional to the time) largely exceedes the CO₂ added to the lung-syringe system (V gas). This results in a net loss of volume from the system (V resp相似文献   

Transthoracic electrical impedance during extracorporeal hemodialysis in acute respiratory failure (“Shocked lung syndrome”)

The alteration (Z ₀ ) of transthoracic electrical impedance (TEI) during extracorporeal hemodialysis (EHD) was investigated in two Groups of patients with acute renal and acute respiratory failure, that differed with respect to the severity of respiratory insufficiency. Group I had moderate respiratory failure (Fi _{O 2} 0.31±0.10, Pa _{0 2} 84±14 mmHg), and Group II had severe respiratory failure (Fi _{0 2} 0.75±0.17, Pa _{O O} 77±14 mmHg). There was a significant correlation between increase in TEI (Z₀) and decrease in body weight (BW) in each individual patient, but the slope of regression lines was remarkably flattened in Group II. In Group I, TEI was 1.9±0.9 , the calculated TEI for 500 gr decrease in BW (Z_{0–500 gr}) was 0.59±0.21 , and a significant correlation existed between pooled data of Z₀ and BW. In Group II TEI increased less significantly, TEI was 0.6±0.3 (P<0.001), Z_{0–500 gr} was 0.26±0.27 (P<0.01), and there was no correlation between pooled data of Z₀ and BW. Increase of TEI in Group II could be completely attributed to increase in hematocrit. It is concluded that patients of Group I with acute renal failure and moderate respiratory failure lost intrathoracic fluid during EHD, whereas patients of Group II with severe respiratory failure did not. TEI during EHD may serve as a test for detection of fixed fluid within the pulmonary interstitium indicating a poor prognosis of the acute respiratory failure.     

Intrathoracic blood volume accurately reflects circulatory volume status in critically ill patients with mechanical ventilation

Positive pressure ventilation in patients with acute respiratory failure (ARF) may render the interpretation of central venous pressure (CVP) or pulmonary wedge pressure (PCWP) difficult as indicators of circulating volume. The preload component of cardiac (CI) and stroke index (SI) is also influenced by the increased intrathoracic pressures of positive pressure ventilation. Moreover CI and SI do not indicate volume status exclusively but also contractility and afterload. We investigated whether intrathoracic blood volume (ITBV) more accurately reflects blood volume status and the resulting oxygen transport (DO₂). CVP, PCWP, cardiac (CI) and stroke index (SI) were measured, oxygen transport index (DO₂I) and oxygen consumption index (VO₂I) were calculated in 21 ARF-patients. Ventilatory patterns were adjusted as necessary. CI, SI and intrathoracic blood volume index (ITBVI) were derived from thermal dye dilution curves which were detected with a 5 F fiberoptic thermistor femoral artery catheter and fed into a thermaldye-computer. All data were collected in intervals of 6h. There were 224 data sets obtained. Linear regression analysis was performed between absolute values as well as between the 6 h changes (prefix ). The following correlation coefficients were determined: CVP/CI and PCWP/CI 0.01 and –0.142 (p<0.05); CVP/SI and PCWP/SI –0.108 and –0.228 (p<0.01); ITBVI/CI and ITBV/SI 0.488 (p<0.01) and 0.480 (p<0.01); ITBVI and DO₂I 0.460 (p<0.01); CVP/CI and PCWP/CI –0.069 and–0.018; CVP/SI and PCWP/SI –0.083 and –0.009; ITBVI/CI and ITBVI/SI 0.715 (p<0.01) and 0.646 (p<0.01); ITBVI and DO₂I 0.707 (p<0.01). We conclude that in mechanically ventilated patients ITBV is a suitable indicator of circulating blood volume.     

A comparison between continuous central venous pressure measurement from right atrium and abdominal vena cava or common iliac vein

Objective To determine the accuracy of meancontinuous central venous pressure (CVP) measurements in the abdominal vena cava.Design We simultaneously measured the CVP at the superior vena cava or right atrium and at the abdominal vena cava or common iliac vein. The study was conducted at the pediatric intensive care unit of a major university-affiliated medical center.Patients Nine patients, aged 6 months to 14 years, were included in our study.Measurements and results Elevencontinuous recordings of 12 to 68 min were taken, eight of them while the children were mechanically ventilated. Mean overall CVP ranged from 3 to 30 mmHg. A total of 519 simultaneous recordings were made, of which 515 (99.2%) were within the accepted limits of agreement of ±2 mmHg: 301 (58%) with CVP of ±mmHg, 189 (36,4%) with CVP of ±1 mmHg, and 25 (4.8%) with CVP of ±2 mmHg. The mean pressure difference was –0.22±1.52 mmHg. Accuracy was maintained within all ranges of CVP (3–10, 11–20, and 21–30 mmHg) and was not influenced by mechanical ventilation or abdominal fluid colection.Conclusion In children with no obstruction of blood flow from the abdominal vena cava to the right atrium, the pressure in the abdominal vena cava or common iliac vein accurately reflects the pressure in the right atrium.     

Transbronchial regional electroplethysmography of the lungs

Objective. The objective of this study was to describe a method of transbronchial regional electroplethysmography of the lungs.Methods. The electrical resistance of a division of a lung, such as a segment or subsegment, as well as its pulsatile oscillation, were measured using a two-part process: A catheter-transducer was wedged into a small bronchus and the electrical resistance of a blood sample obtained from the same patient was measured. The electroplethysmograph (EPG) was developed for this purpose. The theory behind our method is based on a model of the lung as a three-component structure (blood-tissue-air). We performed experiments on isolated lung lobes of animals, using simultaneous electrometric and direct determination of physiologic indices for regional lung function.Results. Equations have been proposed to calculate blood volume, V_b (±10%); air volume, V_a (±11%); pulsatile increment of the blood volume, V (±10%); and regional stroke volume, RSV (±20%) per 100 cm³ of the lung. The proposed formulas yield an accuracy that is adequate for the clinical range of variations in V_b and V_a, as well as V and RSV. Experiments on lung lobes indicate that the conductivity of lung tissue (_t) is not large. This allows one to calculate the above indices without our having obtained accurate values for conductivity.Conclusions. The method of Transbronchial regional electroplethysmography of the lungs is described and cases in which this method was used for clinical investigation are presented.Glossary Resistivity of suspension - ₁ Resistivity of conducting medium - ₂ Resistivity of conducting spheres - Electroconductivity of the lung at diastole - ₁ Electroconductivity of the lung at systole - _b Electroconductivity of blood - _t Electroconductivity of tissue - _bt Electroconductivity of conducting medium (blood+tissue) - Pulsatile electroconductivity increment in the lung - _e Electrical equivalent of regional stroke volume - V Volume of lung - V_B Blood volume of lung - V_T Tissue volume of lung - V_A Air volume of lung - V_b Regional blood volume per lung volume unit - V_a Regional air volume per lung volume unit - V Regional pulsatile blood increment in the lungs - Ratio of the conducting medium (blood+tissue) volume to the lung volume unit - Ratio of the blood volume to the volume unit of the conducting medium - R Electrical resistance - R Electrical resistance increment - RSV Regional stroke volume per the organ volume unit - F Form-factor - l Cell length - S Cell cross-section area - K Empirical correction coefficient - K₁ Coefficient of electrode installation     

Positive end-expiratory pressure: Implications for tidal volume changes in anesthesia machine ventilation

In clinical practice, the addition of positive end-expiratory pressure (PEEP) into a standard anesthesia circle circuit decreases the delivered tidal volume (DTV) to a patient. We studied the magnitude of the DTV/PEEP relationship in two commonly used anesthesia systems. In addition, the magnitude of the DTV/PEEP relationship varies with both pulmonary compliance and volume of gas contained in the patient's breathing system between the ventilator and PEEP valve site, and this was also evaluated. Routine monitoring of expired tidal volume should be used whenever PEEP is added to an anesthesia circuit.     

Endotracheal suctioning: from principles to practice

Objective This paper aims to highlight the physical principles of pressure and flow dynamics underlying endotracheal suctioning, to demonstrate these on a simple illustrative model and to discuss the clinical implications of these principles for paediatric practice.Design Prospective in vitro study.Setting Paediatric intensive care unit of a tertiary, academic hospital.Measurements and main results The peak pressure change (P) obtained in a bag-in-box model was recorded during suctioning, using different combinations of endotracheal tube (ETT) and catheter sizes, different suction pressures and techniques. Suction was also performed on three different consistencies of gelatine, using a range of suction catheter sizes and vacuum pressures. The mass of gelatine suctioned per second was calculated. A large P was measured when using neonatal-sized ETTs. There was a linear relationship (r=0,8, p<0.05) between P and the ratio of external catheter area to area difference (internal ETT area–external catheter area). Significantly greater P was measured when using a short versus long suction catheter (p<0.001) and when applying suction for longer duration (p<0,001) and with higher vacuum pressures (p<0.05). The amount of mucus suctioned in a given time was related to catheter size, suction pressure and mucus density.Conclusion Applied clinically, these results indicate that intrapulmonary pressure changes generated by the endotracheal suctioning of intubated neonates are likely to be considerable, possibly translating into loss of lung volume.This work was supported by research grants from Institute of Child Health (University of Cape Town) and South African Physiotherapy Research Foundation. Consumables were funded by EMS – VEN Medical (RSA).     

Noninvasive monitoring of renal blood flow characteristics during acute renal failure in man

Duplex Doppler ultrasound (DDU) was used to study the blood flow characteristics of the renal interlobar artery in 20 subjects with acute renal failure (ARF), 14 subjects with transient impairment of renal function and 23 control subjects with normal function. Renovascular resistance was assessed by pulsatility index (PI) and change in flow velocity by change in mean frequency shift (f). The 99% confidence intervals for PI in the three groups were 3.32–5.46, 1.58–2.34 and 0.99–1.33 respectively. Values for f were 0.2–0.38, 0.5–0.62 and 0.7–1.02 kHz respectively. Ten ARF patients recovered function, 99% confidence intervals for PI just prior to recovery were 0.9–1.48 and for f 0.52–1.02 kHz. There was increased renovascular resistance and reduced intrarenal blood flow velocity at the onset of ARF. These changes persisted during ARF; recovery of function occurred after they returned to normal. Similar, but less marked, changes were found in patients with a transient impairment of function.This study was conducted at Charing Cross Hospital, London     

Tracheal gas insufflation improves ventilatory efficiency during methacholine-induced bronchospasm

Barotrauma and cardiovascular insufficiency are frequently encountered problems in patients with acute bronchospastic disease who require mechanical ventilation. Permissive hypercapnia is a recognized strategy for minimizing these adverse effects; however, it has potential risks. Tracheal gas insufflation (TGI) has been shown to increase carbon dioxide elimination efficiency and thus could permit mechanical ventilation at lower peak airway pressures without inducing hypercapnia. However, caution exists as to the impact of TO on lung volumes, given that expiratory flow limitation is a hallmark of broncho-spastic disease. To examine these issues, we studied ventilatory and hemodynamic effects of continuous TGI as an adjunct to mechanical ventilation before and after methacholine-induced bronchospasm. Ten anesthetized, paralyzed dogs were ventilated on volume-controlled mechanical ventilation during administration of continuous TGI (0, 2, 6, and 10 L/min) while total inspired minute ventilation (ventilator-derived minute ventilation plus TGI) was kept constant. In an additional step, with TGI flow of 10 L/min, total inspired minute ventilation was decreased by 30%. Paco₂ decreased (44 ± 7 mm Hg at zero flow to 34 ± 7 mm Hg at 6 L/min and 31 ± 6 mm Hg at 10 L/min, respectively, P < .05), as did the dead space to tidal volume ratio at TGI of 6 and 10 L/min compared with zero flow. There were no significant changes in end-expiratory transpulmonary pressure, mean arterial pressure, or cardiac output. During the highest TGI flow (10 L/min), with a 30% reduction of total inspired minute ventilation, both Paco₂ and peak airway pressure remained less than during zero flow conditions. We conclude that TO increases carbon dioxide elimination efficiency during constant and decreased minute ventilation conditions without any evidence of hyperinflation or hemodynamic instability during methacholine-induced bronchospasm.     

Respiratory compliance and resistance in mechanically ventilated patients with acute respiratory failure

The purpose of this study was noninvasive assessment of respiratory compliance and resistance in mechanically ventilated patients with acute respiratory failure (ARF). To this end, flow, change in lung volume, and airway pressure were measured at the proximal tip of the endotracheal tubes in twenty nine critically ill unselected patients. Eleven had acute exacerbation of chronic obstructive pulmonary disease (COPD), 8 had adult respiratory distress syndrome (ARDS) and 10 had ARF of various etiologies. Static compliance (Cst,rs), intrinsic PEEP (PEEPi), as well as minimum and maximum resistance (Rrs,min and Rrs,max, respectively) were obtained with end-inspiratory and end-expiratory airway occlusions. We found that: (1) PEEPi was present in all patients with COPD (up to 11.4 cmH₂O) and it was not uncommon in patients with ARF without history of chronic airway disease (up to 4.1 cmH₂O). (2) Without correction for PEEPi average Cst,rs was not significantly different between ARDS and COPD patients, whereas the average corrected compliance was significantly lower in ARDS patients. (3) Substantial frequency-dependence of resistance was exhibited not only by COPD patients, but also by ARDS patients.A preliminary report was presented at the joint meeting SEP-SEPCR, Paris, September 3rd, 1986.Currently on sabbatical leave from the University of Cincinnaty     

Continuous tracheal gas insufflation during protective mechanical ventilation in juvenile piglets with acute lung injury induced by endotoxin

BACKGROUND:

Low tidal volume mechanical ventilation is difficult to correct hypoxemia, and prolonged inhalation of pure oxygen can lead to oxygen poisoning. We suggest that continuous tracheal gas insufflation (TGI) during protective mechanical ventilation could improve cardiopulmonary function in acute lung injury.

METHODS:

Totally 12 healthy juvenile piglets were anesthetized and mechanically ventilated at PEEP of 2 cmH₂O with a peak inspiratory pressure of 10 cmH₂O. The piglets were challenged with lipopolysaccharide and randomly assigned into two groups (n=6 each group): mechanical ventilation (MV) alone and TGI with continuous airway flow 2 l/min. FIO₂ was set at 0.4 to avoid oxygen toxicity and continuously monitored with an oxygen analyzer.

RESULTS:

Tidal volume, ventilation efficacy index and mean airway resistant pressure were significantly improved in the TGI group (P<0.01 or P<0.05). At 4 hours post ALI, pH decreased to below 7.20 in the MV group, and improved in the TGI group (P<0.01). Similarly, PaCO₂ was stable and was significantly lower in the TGI group than in the MV group (P<0.01). PaO₂ and PaO₂/FIO₂ increased also in the TGI group (P<0.05). There was no significant difference in heart rate, respiratory rate, mean artery pressure, central venous pressure, dynamic lung compliance and mean resistance of airway between the two groups. Lung histological examination showed reduced inflammation, reduced intra-alveolar and interstitial patchy hemorrhage, and homogenously expanded lungs in the TGI group.

CONCLUSION:

Continuous TGI during MV can significantly improve gas exchange and ventilation efficacy and may provide a better treatment for acute lung injury.KEY WORDS: Acute lung injury, Tracheal gas insufflation, Lung protective strategy, Mechanical ventilation     

Dead-space reduction and tracheal pressure measurements using a coaxial inner tube in an endotracheal tube

OBJECTIVE: To evaluate the effects on CO(2) washout of the coaxial double lumen tube (DLT) as compared to a standard endotracheal tube (ETT) and tracheal gas insufflation (TGI). Precision of tracheal pressure monitoring through the DLT and safety issues, including intrinsic PEEP (PEEPi) formation during DLT ventilation, were also evaluated. DESIGN: Lung model study. SETTING: University research laboratory. MEASUREMENTS AND RESULTS: CO(2) washout was analysed in a lung model by measuring single alveolar CO(2) concentration during DLT ventilation as compared to standard ETT ventilation, at different minute ventilation (6-14 l/min) and different CO(2)-output levels (180 ml/min, 240 ml/min, and 300 ml/min). At a CO(2) output level of 240 ml/min the CO(2) washout was also compared to tidal volume-adjusted continuous TGI and expiratory synchronised TGI. Precision of tracheal pressure monitoring and PEEPi formation during DLT ventilation was evaluated by comparing pressure in each limb above the tube to reference tracheal pressure, varying I:E ratios (1:2, 1:1, and 2:1), tidal volumes (300-700 ml), breathing frequencies (15-25), and compliance (20-50 ml/cmH(2)O). DLT ventilation had the same efficacy in removing CO(2) as continuous and expiratory synchronised TGI, reducing single alveolar CO(2) concentration by 9-21% compared to normal ventilation. Tracheal pressure could be measured through the DLT with high precision. There was only marginal formation of PEEPi at tidal volumes 相似文献   

Is proximal airway pressure a good reflection of peripheral airspace pressure in infants and children models under HFJV?

This expriimental study was carried out to determine if an alveolar positive end-expiratory pressure (PEEP) could occur during high frequency jet ventilation (HFJV) in infants, and if tracheal pressure is a good estimation of alveolar pressure.We used physical models simulating a 1.5 kg premature (P), a 3 kg newborn (N) and a 6 kg child (C) with normal compliance and normal resistance. Moreover, in the N model, we used two different resistances and lung compliance heterogeneity was studied in the P model. Pressure was measured simultaneously in the tube simulating trachea (Paw) and in the bottle simulating the lung (Palv). HFJV was performed either via an endotracheal tube (ETT) or via a long catheter as in laryngoscopy. The ratio of injection time upon cycle duration (Ti/Ttot) was 20% or 30%, jet frequency was altered from 150 to 300 min^–1 and the driving pressure was set as in clinical practice (0.5 and 0.6 bar).PEEP occurred mainly in N (1.1 to 3.2 cm H2O) and C models (0 to 3.5 cm H2O). It was inversely related to expiratory time (Te). The end-expiratory pressure drop between Palv and Paw (EEP) was higher in N and increased from 0.5 to 2 cm H2O with the shortening of Te and with airway resistances, i.e. the presence of ETT. In the heterogeneous model, PEEP and EEP were greater in the higher compliance alveolus.This study shows that the end-expiratory Palv is underestimated by end-expiratory Paw. This is particularly important in the presence of an heterogeneity of distribution in lung compliance. In this case the airway PEEP overestimates the PEEP in the lower compliance alveolus and underestimates the PEEP in the higher compliance alveolus.     

New aspects of pulmonary mechanics: ‘slowly” distensible compartments of the respiratory system,identified by a PEEP step maneuver

Objective The aims of the present study were 1) to evaluate a method for identification of slowly distensible compartments of the respiratory system (rs), which are characterized by long mechanical time constants (RC) and 2) to identify slowly distensible rs-compartments in mechanically ventilated patients.Design Prospective studyon a physical lung model.Setting Intensive Care Unit, University Hospital, Tübingen.Patients and participants 19 patients with severe lung injury (acute respiratory distress syndrome, ARDS) and on 10 patients with mild lung injury.Measurements and results Positive end-expiratory pressure (PEEP)-increasing and-decreasing steps of about 5 cmH₂O were applied and the breath-by-breath differences of inspiratory and expiratory volumes (V) were measured. The sequence of Vs were analyzed in terms of volume change in the fast compartment (V_fast), the slow compartment (V_slow), total change in lung volume (VL) and mechanical time constant of the slow compartment (RC_slow). Thirty-eight measurements in a lung model revealed a good correlation between the preset V_slow/VL and V_slow/VL measured: r²=0.91 The V_slow/VL measured amounted to 0.94±0.15 of V_slow/VL in the lung model. RC_slow measured was 0.92±0.43 of the RC_slow reference. Starting from a PEEP level of 11 cmH₂O PEEP-increasing and PEEP-decreasing steps were applied to the mechanically ventilated patients. Three out of ten patients with mild lung injury (30%) and 7/19 patients with ARDS (36.8%) revealed slowly distensible rscompartments in a PEEP-increasing step, whereas 15/19 ARDS patients and 1/10 patients with mild lung injury showed slowly distensible rs-compartments in a PEEP-decreasing step (78.9% vs 10%,P<0.002, chi-square test).Conclusions The gas distribution properties of the respiratory system can be easily studied by a PEEP-step maneuver. The relative contribution of the slow units to the total increase of lung volume following a PEEP step could be adequately assessed. Slowly distensible rs-compartments could be detected in patients with severe and mild lung injury, however significantly more ARDS patients revealed slow rs-compartments in PEEP-decreasing steps. The influence of slowly distensible rs-compartments on pulmonary gas exchange is unknown and has yet to be studied.The work was performed at the Klinik für Anaesthesiologie und Transfusionsmedizin der Universität Tübingen