Physiologic comparison of neurally adjusted ventilator assist, proportional assist and pressure support ventilation in critically ill patients.

UNLABELLED: To compare, in a group of difficult to wean critically ill patients, the short-term effects of neurally adjusted ventilator assist (NAVA), proportional assist (PAV+) and pressure support (PSV) ventilation on patient-ventilator interaction. METHODS: Seventeen patients were studied during NAVA, PAV+ and PSV with and without artificial increase in ventilator demands (dead space in 10 and chest load in 7 patients). Prior to challenge addition the level of assist in each of the three modes tested was adjusted to get the same level of patient's effort. RESULTS: Compared to PSV, proportional modes favored tidal volume variability. Patient effort increase after dead space was comparable among the three modes. After chest load, patient effort increased significantly more with NAVA and PSV compared to PAV+. Triggering delay was significantly higher with PAV+. The linear correlation between tidal volume and inspiratory integral of transdiaphragmatic pressure (PTPdi) was weaker with NAVA than with PAV+ and PSV on account of a weaker inspiratory integral of the electrical activity of the diaphragm (∫EAdi)-PTPdi linear correlation during NAVA [median (interquartile range) of r(2), determination of coefficient, 16.2% (1.4-30.9%)]. CONCLUSION: Compared to PSV, proportional modes favored tidal volume variability. The weak ∫EAdi-PTPdi linear relationship during NAVA and poor triggering function during PAV+ may limit the effectiveness of these modes to proportionally assist the inspiratory effort.

Impact of lung ultrasound on clinical decision making in critically ill patients.

PURPOSE: To assess the impact of lung ultrasound (LU) on clinical decision making in mechanically ventilated critically ill patients. METHODS: One hundred and eighty-nine patients took part in this prospective study. The patients were enrolled in the study when LU was requested by the primary physician for (1) unexplained deterioration of arterial blood gases and (2) a suspected pathologic entity [pneumothorax, significant pleural effusion (including parapneumonic effusion, empyema, or hemothorax), unilateral atelectasis (lobar or total), pneumonia and diffuse interstitial syndrome (pulmonary edema)]. RESULTS: Two hundred and fifty-three LU examinations were performed; 108 studies (42.7%) were performed for unexplained deterioration of arterial blood gases, and 145 (57.3%) for a suspected pathologic entity (60 for pneumothorax, 34 for significant pleural effusion, 22 for diffuse interstitial syndrome, 15 for unilateral lobar or total lung atelectasis, and 14 for pneumonia). The net reclassification index was 85.6%, indicating that LU significantly influenced the decision-making process. The management was changed directly as a result of information provided by the LU in 119 out of 253 cases (47%). In 81 cases, the change in patient management involved invasive interventions (chest tube, bronchoscopy, diagnostic thoracentesis/fluid drainage, continuous venous-venous hemofiltration, abdominal decompression, tracheotomy), and in 38 cases, non-invasive (PEEP change/titration, recruitment maneuver, diuretics, physiotherapy, change in bed position, antibiotics initiation/change). In 53 out of 253 cases (21%), LU revealed findings which supported diagnoses not suspected by the primary physician (7 cases of pneumothorax, 9 of significant pleural effusion, 9 of pneumonia, 16 of unilateral atelectasis, and 12 of diffuse interstitial syndrome). CONCLUSION: Our study shows that LU has a significant impact on decision making and therapeutic management.

Effect of albuterol on expiratory resistance in mechanically ventilated patients.

BACKGROUND: In mechanically ventilated patients with COPD, the response of the expiratory resistance of the respiratory system (expiratory R(RS)) to bronchodilators is virtually unknown. OBJECTIVE: To examine the effect of inhaled albuterol on expiratory R(RS), and the correlation of albuterol-induced changes in expiratory R(RS) with end-inspiratory resistance and the expiratory flow-volume relationship. METHODS: We studied 10 mechanically ventilated patients with COPD exacerbation, before and 30 min after administration of albuterol. We obtained flow-volume curves during passive expiration, divided the expired volume into 5 equal volume slices, and then calculated the time constant and dynamic effective deflation compliance of the respiratory system (effective deflation C(RS)) of each slice via regression analysis of the volume-flow and post-occlusion volume-tracheal pressure relationships, respectively. For each slice we calculated expiratory R(RS) as the time constant divided by the effective deflation C(RS). RESULTS: Albuterol significantly decreased the expiratory R(RS) (mean expiratory R(RS) 42.68 ± 17.8 cm H(2)O/L/s vs 38.08 ± 16.1 cm H(2)O/L/s) and increased the rate of lung emptying toward the end of expiration (mean time constant 2.51 ± 1.2 s vs 2.21 ± 1.2 s). No correlation was found between the albuterol-induced changes in expiratory R(RS) and that of end-inspiratory resistance. Only at the end of expiration did albuterol-induced changes in the expiratory flow-volume relationship correlate with changes in expiratory R(RS) in all patients. CONCLUSIONS: In patients with COPD, albuterol significantly decreases expiratory resistance at the end of expiration. In mechanically ventilated patients, neither inspiratory resistance nor the whole expiratory flow-volume curve may be used to evaluate the bronchodilator response of expiratory resistance.

Effect of propofol on breathing stability in adult ICU patients with brain damage.

The aim of the study was to investigate Propofol's effect on breathing stability in brain damage patients, as quantified by the Loop Gain (LG) of the respiratory system (breathing stability increases with decreasing LG). In 11 stable brain damage patients full polysomnography was performed before, during and after propofol sedation, titrated to achieve stage 2 or slow wave sleep. During each period, patients were ventilated with proportional assist ventilation and the % assist was increased in steps, until either periodic breathing (PB) occurred or the highest assist (95%) was achieved. The tidal volume amplification factor (VT(AF)) at the highest assist level reached just before PB occurred was used to calculate LG (LG=1/VT(AF)). In all but one patient, PB was observed. With propofol, the assist level at which PB occurred (73 + or - 19%) was significantly higher, than that before (43 + or - 35%) and after propofol sedation (49 + or - 29%). As a result, with propofol LG (0.49 + or - 0.2) was significantly lower than that before (0.74 + or - 0.2) and after propofol sedation (0.69 + or - 0.2) (p<0.05). We conclude that Propofol decreases LG. Therefore it exerts an overall stabilizing effect on control of breathing.

Effects of relaxation of inspiratory muscles on ventilator pressure during pressure support.

OBJECTIVE: During pressure support ventilation (PS), an abrupt increase in ventilator pressure above the pre-set level is considered to signify expiratory muscle activity. However, relaxation of inspiratory muscles may also cause the same phenomenon, and this hypothesis has not been explored. The aim of this study is to examine the cause of this increase in ventilator pressure, during PS, in critically ill patients. DESIGN: Retrospective study. SETTING: In a university intensive care unit. METHODS: Fifteen patients instrumented with esophageal and gastric balloons, and in whom airway pressure (P (aw)) during PS exhibited an acute increase above the pre-set level towards the end of mechanical inspiration were retrospectively analyzed. For each breath, the time of the rapid increase in P (aw) was identified (t (Paw)) and, using the transdiaphragmatic (P (di)) and gastric (P (ga)) pressure waveforms, related to: (1) the end of neural inspiration (peak P (di)) and (2) the time at which P (ga) started to increase rapidly after the end of neural inspiration indicating expiratory muscle recruitment. RESULTS: The t (Paw) was observed 32+/-34ms after the end of neural inspiration, well before (323+/-182ms) expiratory muscle recruitment (identified in eight patients). There was a significant linear relationship between the rate of rise of P (aw) after t (Paw) and the rates of decline of P (di) and inspiratory flow. CONCLUSION: We conclude that, during PS ventilation, the relaxation of inspiratory muscles accounts for the acute increase in P (aw) above the pre-set level, in addition to the contribution made by the occurrence of expiratory muscle activity.

Non-invasive ventilation in chronic obstructive pulmonary disease patients: helmet versus facial mask.

RATIONALE: The helmet is a new interface with the potential of increasing the success rate of non-invasive ventilation by improving tolerance. OBJECTIVES: To perform a physiological comparison between the helmet and the conventional facial mask in delivering non-invasive ventilation in hypercapnic patients with chronic obstructive pulmonary disease. METHODS: Prospective, controlled, randomized study with cross-over design. In 10 patients we evaluated gas exchange, inspiratory effort, patient-ventilator synchrony and patient tolerance after 30 min of non-invasive ventilation delivered either by helmet or facial mask; both trials were preceded by periods of spontaneous unassisted breathing. MEASUREMENTS: Arterial blood gases, inspiratory effort, duration of diaphragm contraction and ventilator assistance, effort-to-support delays (at the beginning and at the end of inspiration), number of ineffective efforts, and patient comfort. MAIN RESULTS: Non-invasive ventilation improved gas exchange (p<0.05) and inspiratory effort (p<0.01) with both interfaces. The helmet, however, was less efficient than the mask in reducing inspiratory effort (p<0.05) and worsened the patient-ventilator synchrony, as indicated by the longer delays to trigger on (p<0.05) and cycle off (p<0.05) the mechanical assistance and by the number of ineffective efforts (p<0.005). Patient comfort was no different with the two interfaces. CONCLUSIONS: Helmet and facial mask were equally tolerated and both were effective in ameliorating gas exchange and decreasing inspiratory effort. The helmet, however, was less efficient in decreasing inspiratory effort and worsened the patient-ventilator interaction.

Effect of the physical properties of isoflurane, sevoflurane, and desflurane on pulmonary resistance in a laboratory lung model.

BACKGROUND: Airway resistance depends not only on an airway's geometry but also on flow rate, and gas density and viscosity. A recent study showed that at clinically relevant concentrations, the mixtures of volatile agents with air and oxygen and oxygen-nitrogen affected the density of the mixture. The goal of the current study was to investigate the effect of different minimum alveolar concentrations (MACs) of three commonly used volatile agents, isoflurane, sevoflurane, and desflurane, on the measurements of airway resistance. METHODS: A two-chamber fixed-resistance test lung was connected to an anesthesia machine using the volume control mode of ventilation. Pulmonary resistance was calculated at baseline (25% oxygen in air); at 1.0, 1.5, and 2.0 MAC; and also at the same concentrations, 1.2% and 4%, of isoflurane, sevoflurane, and desflurane mixtures with 25% oxygen in air. The analysis of variance test for repeated measures and probabilities for post hoc Tukey and least significant difference tests were used. RESULTS: Isoflurane affected pulmonary resistance only at 2 MAC. Sevoflurane caused a significant increase of pulmonary resistance at 1.5 and 2 MAC, whereas desflurane caused the greatest increase in pulmonary resistance at all MAC values used. At 1.2% concentration, no difference from the baseline resistance was observed, whereas at 4%, the three agents produced similar increases of pulmonary resistance. CONCLUSION: High concentrations of volatile agents in 25% oxygen in air increased the density of the gas mixture and the calculated resistance of a test lung model with fixed resistance.

Respiratory load compensation during mechanical ventilation--proportional assist ventilation with load-adjustable gain factors versus pressure support.

RATIONALE: In mechanically ventilated patients respiratory system impedance may vary from time to time, resulting, with pressure modalities of ventilator support, in changes in the level of assistance. Recently, implementation of a closed-loop adjustment to continuously adapt the level of assistance to changes in respiratory mechanics has been designed to operate with proportional assist ventilation (PAV+). OBJECTIVES: The aim of this study was to assess, in critically ill patients, the short-term steady-state response of respiratory motor output to added mechanical respiratory load during PAV+ and during pressure support (PS). PATIENTS AND INTERVENTIONS: In 10 patients respiratory workload was increased and the pattern of respiratory load compensation was examined during both modes of support. MEASUREMENTS AND RESULTS: Airway and transdiaphragmatic pressures, volume and flow were measured breath by breath. Without load, both modes provided an equal support as indicated by a similar pressure-time product of the diaphragm per breath, per minute and per litre of ventilation. With load, these values were significantly lower (p<0.05) with PAV+ than those with PS (5.1+/-3.7 vs 6.1+/-3.4 cmH2O.s, 120.9+/-77.6 vs 165.6+/-77.5 cmH2O.s/min, and 18.7+/-15.1 vs 24.4+/-16.4 cmH2O.s/l, respectively). Contrary to PS, with PAV+ the ratio of tidal volume (VT) to pressure-time product of the diaphragm per breath (an index of neuroventilatory coupling) remained relatively independent of load. With PAV+ the magnitude of load-induced VT reduction and breathing frequency increase was significantly smaller than that during PS. CONCLUSION: In critically ill patients the short-term respiratory load compensation is more efficient during proportional assist ventilation with adjustable gain factors than during pressure support.

Bedside waveforms interpretation as a tool to identify patient-ventilator asynchronies.

OBJECTIVE: During assisted modes of ventilatory support the ventilatory output is the final expression of the interaction between the ventilator and the patient's controller of breathing. This interaction may lead to patient-ventilator asynchrony, preventing the ventilator from achieving its goals, and may cause patient harm. Flow, volume, and airway pressure signals are significantly affected by patient-ventilator interaction and may serve as a tool to guide the physician to take the appropriate action to improve the synchrony between patient and ventilator. This review discusses the basic waveforms during assisted mechanical ventilation and how their interpretation may influence the management of ventilated patients. The discussion is limited on waveform eye interpretation of the signals without using any intervention which may interrupt the process of mechanical ventilation. DISCUSSION: Flow, volume, and airway pressure may be used to (a) identify the mode of ventilator assistance, triggering delay, ineffective efforts, and autotriggering, (b) estimate qualitatively patient's respiratory efforts, and (c) recognize delayed and premature opening of exhalation valve. These signals may also serve as a tool for gross estimation of respiratory system mechanics and monitor the effects of disease progression and various therapeutic interventions. CONCLUSIONS: Flow, volume, and airway pressure waveforms are valuable real-time tools in identifying various aspects of patient-ventilator interaction.

Patient-ventilator interaction: an overview.

During assisted mechanical ventilation, the total pressure applied to respiratory system is the sum of ventilator and muscle pressure. As a result, the respiratory system is under the influence of two pumps, the ventilator pump (ie, Paw), which is controlled by the physician's brain and the capabilities of the ventilator, and the patient's own respiratory muscle pump (Pmus), which is controlled by the patient's brain. The patient-ventilator interaction is mainly an expression of the function of these two brains, which should be in harmony to promote patient-ventilator synchrony. The achievement of this harmony depends exclusively on the physician, who should be aware that during assisted mechanical ventilation the respiratory system is not a passive structure but reacts to pressure delivered by the ventilator via various feedback systems and, depending on several factors both to the ventilator and patient, may modify the function of the ventilator. Finally, the physician should know that the ventilator imposes significant constraints to the respiratory system, the magnitude of which depends heavily on the triggering variable, the variable that controls the gas delivery and the cycling off criterion.

Determinants of the cuff-leak test: a physiological study.

INTRODUCTION: The cuff-leak test has been proposed as a simple method to predict the occurrence of post-extubation stridor. The test is performed by cuff deflation and measuring the expired tidal volume a few breaths later (VT). The leak is calculated as the difference between VT with and without a deflated cuff. However, because the cuff remains deflated throughout the respiratory cycle a volume of gas may also leak during inspiration and therefore this method (conventional) measures the total leak consisting of an inspiratory and expiratory component. The aims of this physiological study were, first, to examine the effects of various variables on total leak and, second, to compare the total leak with that obtained when the inspiratory component was eliminated, leaving only the expiratory leak. METHODS: In 15 critically ill patients mechanically ventilated on volume control mode, the cuff-leak volume was measured randomly either by the conventional method (Leakconv) or by deflating the cuff at the end of inspiration and measuring the VT of the following expiration (Leakpause). To investigate the effects of respiratory system mechanics and inspiratory flow, cuff-leak volume was studied by using a lung model, varying the cross-sectional area around the endotracheal tube and model mechanics. RESULTS: In patients Leakconv was significantly higher than Leakpause, averaging 188 +/- 159 ml (mean +/- SD) and 61 +/- 75 ml, respectively. In the model study Leakconv increased significantly with decreasing inspiratory flow and model compliance. Leakpause and Leakconv increased slightly with increasing model resistance, the difference being significant only for Leakpause. The difference between Leakconv and Leakpause increased significantly with decreasing inspiratory flow (V'I) and model compliance and increasing cross-sectional area around the tube. CONCLUSION: We conclude that the cross-sectional area around the endotracheal tube is not the only determinant of the cuff-leak test. System compliance and inspiratory flow significantly affect the test, mainly through an effect on the inspiratory component of the total leak. The expiratory component is slightly influenced by respiratory system resistance.

Pattern of lung emptying and expiratory resistance in mechanically ventilated patients with chronic obstructive pulmonary disease.

OBJECTIVES: To study the pattern of lung emptying and expiratory resistance in mechanically ventilated patients with chronic obstructive pulmonary disease (COPD). DESIGN: A prospective physiological study. SETTING: A 12-bed Intensive Care Unit. PATIENTS: Ten patients with acute exacerbation of COPD. INTERVENTIONS: At three levels of positive end-expiratory pressure (PEEP, 0, 5 and 10 cm H(2)O) tracheal (Ptr) and airway pressures, flow (V') and volume (V) were continuously recorded during volume control ventilation and airway occlusions at different time of expiration. MEASUREMENTS AND RESULTS: V-V' curves during passive expiration were obtained, expired volume was divided into five equal volume slices and the time constant (tau) and dynamic deflation compliance (Crs(dyn)) of each slice was calculated by regression analysis of V-V' and post-occlusion V-Ptr relationships, respectively. In each volume slice the existence or not of flow limitation was examined by comparing V-V' curves with and without decreasing Ptr. For a given slice total expiratory resistance was calculated as tau/Crs(dyn), whereas expiratory resistance (Rrs) and time constant (tau(rs)) of the respiratory system were subsequently estimated taken into consideration the presence of flow limitation. At zero PEEP, tau(rs) increased significantly toward the end of expiration due to an increase in Rrs. PEEP significantly decreased Rrs at the end of expiration and resulted in a faster and relatively constant rate of lung emptying. CONCLUSIONS: Patients with COPD exhibit a decrease in the rate of lung emptying toward the end of expiration due to an increase in Rrs. PEEP decreases Rrs, resulting in a faster and uniform rate of lung emptying.

Metastatic liver disease and fulminant hepatic failure: presentation of a case and review of the literature.

Although liver metastases are commonly found in cancer patients, fulminant hepatic failure (FHF) secondary to diffuse liver infiltration is rare. Furthermore, clinical presentation and laboratory findings are obscure and far from being pathognomonic for the disease. We report a case of a patient who died in the intensive care unit of our hospital from multiple organ failure syndrome secondary to FHF, as a result of liver infiltration from poorly differentiated small cell lung carcinoma. We also present the current knowledge about the clinical picture, laboratory findings and physical history of neoplastic liver-metastasis-induced FHF.

Effects of the flow waveform method of triggering and cycling on patient-ventilator interaction during pressure support.

OBJECTIVE: To examine patient-ventilator interaction during pressure support ventilation in critically ill patients when they were ventilated: (1) by a new system (Vision, Respironics) which uses the flow waveform as a method of triggering and cycling; and (2) by a new generation ventilator (Evita 4, Drager) which uses the traditional flow triggering (2 l/min) and cycling criterion (25% of peak flow). DESIGN: Prospective clinical and bench study. METHODS: Twelve mechanically ventilated patients were studied at three levels of pressure support, applied randomly with both ventilators. The two systems of triggering were further studied at controlled levels of dynamic hyperinflation and respiratory drive using an active lung model. RESULTS: Patients' breathing patterns, respiratory effort, and arterial blood gases were not affected by the type of ventilator. The flow waveform method of triggering was more sensitive to patient effort than the flow triggering, resulting in less ineffective effort but a greater number of auto-triggerings. At controlled levels of dynamic hyperinflation and inspiratory effort the simulated patient effort needed to trigger the ventilator was considerably less with the flow waveform method of triggering than that with the flow triggering. The flow waveform method of cycling resulted in mechanical breaths with similar characteristics to those that used the traditional flow criterion of breath termination. CONCLUSIONS: We conclude that the flow waveform method of triggering improves the ventilator function and decreases the patient effort during the triggering phase. This system is highly sensitive, but under certain circumstances may be unstable.

Effects of theophylline on ventilatory poststimulus potentiation in patients with brain damage.

Patients with brain damage, in contrast to normal subjects, exhibit a significant ventilatory undershoot when brief hypocapnic hypoxia is terminated abruptly by hyperoxia. This has been attributed to an impairment of activation of short-term potentiation, a brain stem mechanism promoting breathing stability. We hypothesized that in these patients theophylline, a drug that stabilizes breathing, may affect short-term potentiation. Eight stable patients with brain damage and 10 normal adults were studied. Activation of short-term potentiation was examined by brief exposure to hypoxia followed by hyperoxia after pretreatment with placebo or theophylline. Both in patients and normal subjects at the end of hypoxia ventilation increased to a similar magnitude with and without theophylline. In normal subjects independent of pretreatment, when hypoxia was terminated abruptly by hyperoxia, ventilation declined slowly to baseline without an undershoot, indicating activation of short-term potentiation. In patients with placebo, ventilation upon switching to hyperoxia exhibited a significant undershoot. This undershoot was significantly attenuated by theophylline, although compared with normal subjects, a slight hypoventilation was observed. We conclude that in patients with brain damage, theophylline largely prevents the hyperoxic drop of ventilation, presumably by affecting the activation of short-term potentiation. This may underlie the beneficial effect of theophylline on breathing stability.