Discontinuation of Mechanical Ventilation

A Predictive Model for Prolonged Mechanical Ventilation After Triple-Branched Stent Graft for Acute Type A Aortic Dissection

INTRODUCTION

The aim of this study is to develop a model for predicting the risk of prolonged mechanical ventilation (PMV) following surgical repair of acute type A aortic dissection (AAAD).

METHODS

We retrospectively collected clinical data from 381 patients with AAAD who underwent emergency surgery. Clinical features variables for predicting postoperative PMV were selected through univariate analysis, least absolute shrinkage and selection operator regression analysis, and multivariate logistic regression analysis. A risk prediction model was established using a nomogram. The model's accuracy and reliability were evaluated using the area under the curve of the receiver operating characteristic curve and the calibration curve. Internal validation of the model was performed using bootstrap resampling. The clinical applicability of the model was assessed using decision curve analysis and clinical impact curve.

RESULTS

Among the 381 patients, 199 patients (52.2%) experienced postoperative PMV. The predictive model exhibited good discriminative ability (area under the curve = 0.827, 95% confidence interval: 0.786-0.868, P < 0.05). The calibration curve confirmed that the predicted outcomes of the model closely approximated the ideal curve, indicating agreement between the predicted and actual results (with an average absolute error of 0.01 based on 1000 bootstrap resampling). The decision curve analysis curve demonstrated that the model has significant clinical value.

CONCLUSIONS

The nomogram model established in this study can be used to predict the risk of postoperative PMV in patients with AAAD. It serves as a practical tool to assist clinicians in adjusting treatment strategies promptly and implementing targeted therapeutic measures.

APACHE II score cannot predict successful weaning from prolonged mechanical ventilation

At least 5% of all intensive care unit patients require prolonged respiratory support. Multiple factors have been suggested as possible predictors of successful respiratory weaning so far. We sought to verify whether the Acute Physiology and Chronic Health Evaluation II (APACHE II) can predict freedom from prolonged mechanical ventilation (PMV) in patients treated in a regional weaning centre. The study group comprised 130 consecutive patients (age; median (interquartile range): 71 (62–77) years), hospitalized between 1 January 2012, and 31 December 2013. APACHE II score was assessed based on the worst values taken during the first 24 hours after admission. Glasgow coma scale was excluded from calculations due to the likely influence of sedative agents. The outcome was defined as freedom from mechanical ventilation, with or without tracheostomy on discharge. Among survivors (n = 115), 88.2% were successfully liberated from mechanical ventilation and 60.9% from tracheostomy. APACHE II failed to predict freedom from mechanical ventilation (area under the receiver–operating characteristic curve [AUROC] = 0.534; 95% confidence interval [CI]: 0.439–0.628; p = 0.65) and tracheostomy tube removal (AUROC = 0.527; 95% CI: 0.431–0.621; p = 0.63). Weaning outcome was unrelated to the aetiology of respiratory failure on admission (p = 0.41). APACHE II cannot predict weaning outcome in patients requiring PMV.

Bortezomib partially protects the rat diaphragm from ventilator-induced diaphragm dysfunction

OBJECTIVE

Controlled mechanical ventilation leads to diaphragmatic contractile dysfunction and atrophy. Since proteolysis is enhanced in the diaphragm during controlled mechanical ventilation, we examined whether the administration of a proteasome inhibitor, bortezomib, would have a protective effect against ventilator-induced diaphragm dysfunction.

DESIGN

Randomized, controlled experiment.

SETTINGS

Basic science animal laboratory.

INTERVENTIONS

Anesthetized rats were submitted for 24 hrs to controlled mechanical ventilation while receiving 0.05 mg/kg bortezomib or saline. Control rats were acutely anesthetized.

MEASUREMENTS AND MAIN RESULTS

After 24 hrs, diaphragm force production was significantly lower in mechanically ventilated animals receiving an injection of saline compared to control animals (-36%, p<.001). Importantly, administration of bortezomib improved the diaphragmatic force compared to mechanically ventilated animals receiving an injection of saline (+15%, p<.01), but force did not return to control levels. Compared to control animals, diaphragm cross-sectional area of the type IIx/b fibers was significantly decreased by 28% in mechanically ventilated animals receiving an injection of saline (p<.01) and by 16% in mechanically ventilated animals receiving an injection of bortezomib (p<.05). Diaphragmatic calpain activity was significantly increased in mechanically ventilated animals receiving an injection of saline (+52%, p<.05) and in mechanically ventilated animals receiving an injection of bortezomib (+36%, p<.05). Caspase-3 activity was increased after controlled mechanical ventilation with saline by 55% (p<.05), while it remained similar to control animals in mechanically ventilated animals receiving an injection of bortezomib. Diaphragm 20S proteasome activity was slightly increased in both ventilated groups, and the amount of ubiquitinated proteins was significantly and similarly enhanced in mechanically ventilated animals receiving an injection of saline and mechanically ventilated animals receiving an injection of bortezomib.

CONCLUSIONS

These data show that the administration of bortezomib partially protects the diaphragm from controlled mechanical ventilation-induced diaphragm contractile dysfunction without preventing atrophy. The fact that calpain activity was still increased after bortezomib treatment may explain the persistence of atrophy. Part of bortezomib effects might have been due to its ability to inhibit caspase-3 in this model.

Intermittent spontaneous breathing protects the rat diaphragm from mechanical ventilation effects

OBJECTIVE

Short-term mechanical ventilation has been proven to reduce diaphragm force and fiber dimensions. We hypothesized that intermittent spontaneous breathing during the course of mechanical ventilation would minimize the effects of mechanical ventilation on diaphragm force and expression levels of transcription factors (MyoD and myogenin).

DESIGN

Randomized, controlled experiment.

SETTING

Animal basic science laboratory.

SUBJECTS

Male Wistar rats, weighing 350-500 g.

INTERVENTIONS

Anesthetized and tracheotomized rats were submitted to either 24 hrs of spontaneous breathing (SB, n = 5), 24 hrs of continuous controlled mechanical ventilation (CMV, n = 7), or controlled mechanical ventilation with intermittent spontaneous breathing: 60 mins every 5 hrs of mechanical ventilation repeated four times (ISB60, n = 8) or 5 mins every 5 hrs 55 mins of mechanical ventilation repeated four times (SB5, n = 9). They were compared with control animals free from intervention (C, n = 5).

MEASUREMENTS AND MAIN RESULTS

The profile of the diaphragm force-frequency curve of the controls and SB group was significantly different from that of the ISB and CMV groups; especially, the mean asymptotic force was less in the ISB and CMV compared with controls and SB. CMV resulted in a significant decrease in the diaphragm type I (-26%, p < .05 vs. C) and type IIx/b (-39%, p < .005 vs. C and SB) cross-sectional area, whereas this was not observed in the ISB groups. Diaphragm MyoD protein expression was significantly decreased after ISB60 (-35%, p < .0001 vs. C and SB) and even more after CMV (-73%, p < .0001 vs. others). The same pattern was observed with myogenin protein levels. Positive relationships between diaphragm MyoD and myogenin protein levels and diaphragm force were observed.

CONCLUSIONS

The data demonstrated that intermittent spontaneous breathing during the course of mechanical ventilation may minimize the deleterious effect of controlled mechanical ventilation on diaphragm force, fiber dimensions, and expression of transcription factors.

Mechanical ventilation-induced oxidative stress in the diaphragm

Prolonged mechanical ventilation (MV) results in oxidative damage in the diaphragm; however, it is unclear whether this MV-induced oxidative injury occurs rapidly or develops slowly over time. Furthermore, it is unknown whether both soluble (cytosolic) and insoluble (myofibrillar) proteins are equally susceptible to oxidation during MV. These experiments tested two hypotheses: 1). MV-induced oxidative injury in the diaphragm occurs within the first 6 h after the initiation of MV; and 2). MV is associated with oxidative modification of both soluble and insoluble proteins. Adult Sprague-Dawley rats were randomly divided into one of seven experimental groups: 1) control (n = 8); 2) 3-h MV (n = 8); 3). 6-h MV (n = 6); 4). 18-h MV (n = 8); 5). 3-h anesthesia-spontaneous breathing (n = 8); 6). 6-h anesthesia-spontaneous breathing (n = 6); and 7). 18-h anesthesia-spontaneous breathing (n = 8). Markers of oxidative injury in the diaphragm included the measurement of reactive (protein) carbonyl derivatives (RCD) and total lipid hydroperoxides. Three hours of MV did not result in oxidative injury in the diaphragm. In contrast, both 6 and 18 h of MV promoted oxidative injury in the diaphragm, as indicated by increases in both protein RCD and lipid hydroperoxides. Electrophoretic separation of soluble and insoluble proteins indicated that the MV-induced accumulation of RCD was limited to insoluble proteins with molecular masses of approximately 200, 120, 80, and 40 kDa. We conclude that MV results in a rapid onset of oxidative injury in the diaphragm and that insoluble proteins are primary targets of MV-induced protein oxidation.

Detrimental effects of short-term mechanical ventilation on diaphragm function and IGF-I mRNA in rats

OBJECTIVES

Because respiratory muscle weakness appears to play an important role in weaning from mechanical ventilation, we developed an animal model of mechanical ventilation with appropriate controls in order to determine whether 24 h of mechanical ventilation already affected diaphragmatic function.

DESIGN AND INTERVENTIONS

Fifty-two male Wistar rats were randomized into three groups: a non-anesthetized control group (C, n=10), an anesthetized spontaneously breathing group (SB, n=9 out of 26), and an anesthetized and mechanically ventilated group (MV, n=12 out of 16).

RESULTS

After 24 h, in vitro diaphragmatic force was decreased in SB group but even more so in MV group (i.e., 80 Hz: -15% in SB, P<0.005 vs C and -34% in MV group, P<0.005 vs C and SB). This was associated with a significant decrease in the diaphragm type I and type IIa dimensions in the SB group, which was more pronounced in the MV group. Interestingly, diaphragm IGF-I mRNA was decreased in the SB group (-14%, P<0.05 vs C), but more so in MV group (-29%, P<0.001 vs C and P<0.01 vs SB). Moreover, there was a significant correlation between diaphragm force and IGF-I mRNA (at 80 Hz r=0.51, P=0.0056).

CONCLUSIONS

We conclude that 24 h of mechanical ventilation in rats, independently of anesthesia, already significantly reduced diaphragm force, fiber dimensions, and its IGF-I mRNA levels.

Mechanical ventilation results in progressive contractile dysfunction in the diaphragm

These experiments tested the hypothesis that a relatively short duration of controlled mechanical ventilation (MV) will impair diaphragmatic maximal specific force generation (specific P(o)) and that this force deficit will be exacerbated with increased time on the ventilator. To test this postulate, adult Sprague-Dawley rats were randomly divided into one of six experimental groups: 1) control (n = 12); 2) 12 h of MV (n = 4); 3) 18 h of MV (n = 4); 4) 18 h of anesthesia and spontaneous breathing (n = 4); 5) 24 h of MV (n = 7); and 6) 24 h of anesthesia and spontaneous breathing (n = 4). MV animals were anesthetized, tracheostomized, and ventilated with room air. Animals in the control group were acutely anesthetized but were not exposed to MV. Animals in two spontaneous breathing groups were anesthetized and breathed spontaneously for either 18 or 24 h. No differences (P > 0.05) existed in diaphragmatic specific P(o) between control and the two spontaneous breathing groups. In contrast, compared with control, all durations of MV resulted in a reduction (P < 0.05) in diaphragmatic specific tension at stimulation frequencies ranging from 15 to 160 Hz. Furthermore, the MV-induced decrease in diaphragmatic specific P(o) was time dependent, with specific P(o) being approximately 18 and approximately 46% lower (P < 0.05) in animals mechanically ventilated for 12 and 24 h, respectively. These data support the hypothesis that relatively short-term MV impairs diaphragmatic contractile function and that the magnitude of MV-induced force deficit increases with time on the ventilator.

Monitoring changes in lung air and liquid volumes with electrical impedance tomography

Electrical impedance tomography (EIT) uses electrical measurements at electrodes placed around the thorax to image changes in the conductivity distribution within the thorax. This technique is well suited to studying pulmonary function because the movement of air, blood, and extravascular fluid induces significant conductivity changes within the thorax. We conducted three experimental protocols in a total of 19 dogs to assess the accuracy with which EIT can quantify changes in the volumes of both gas and fluid in the lungs. In the first protocol, lung volume increments from 50 to 1,000 ml were applied with a large syringe. EIT measured these volume changes with an average error of 27 +/- 6 ml. In the second protocol, EIT measurements were made at end expiration and end inspiration during regular ventilation with tidal volume ranging from 100 to 1,000 ml. The average error in the EIT estimates of tidal volume was 90 +/- 43 ml. In the third protocol, lung liquid volume was measured by instilling 5% albumin solution into a lung lobe in increments ranging from 10 to 100 ml. EIT measured these volume changes with an average error of 10 +/- 10 ml and was also able to detect into which lobe the fluid had been instilled. These results indicate that EIT can noninvasively measure changes in the volumes of both gas and fluid in the lungs with clinically useful accuracy.

Weaning and extubation in the intensive care unit. Clinical or index-driven approach?

OBJECTIVE

To assess the outcome of a clinical judgement-based approach to weaning and extubation and to adduce the predictive accuracy of various mechanical respiratory indices measured in parallel.

DESIGN

Prospective study.

SETTING

Multidisciplinary intensive care unit at a university teaching hospital.

PATIENTS

163 consecutive mechanically ventilated patients, excluding tracheotomy, for weaning trial and extubation.

INTERVENTIONS

Using bedside clinical assessment, aided by arterial gas analysis, patients were weaned from mechanical ventilation to spontaneous ventilation via to continuous positive airway pressure (CPAP) circuit (with pressure support) of a microprocessor-controlled ventilator. Extubation occurred from the CPAP circuit at 7 cmH2O pressure support, fractional inspired oxygen (FIO2) < or = 0.5 and CPAP level of < or = 5 cmH2O, such that the partial pressure of oxygen in arterial blood (PaO2) was > or = 65 mmHg. Before extubation, observation for a 1-h (T0 and T60) trial period allowed measurement of vital capacity (VC), expired minute volume (VE), respiratory rate/tidal volume (f/VT) and maximal inspiratory pressure (MIP) using a one-way valve technique over 25 s.

MEASUREMENTS AND MAIN RESULTS

Over 7 months, 163 patients (62 females and 101 males; mean (SD) age 64(15) years) were considered. There were 91 surgical (18 with chronic obstructive pulmonary disease; COPD) and 72 medical (28 with COPD) patients. Ventilation was for > or = 1 day (median 5 days, range 1-31) in 115 [group I; APACHE II score 23(8)] and < or = 1 day in 48 [Group II; APACHE II score 17(6)]. Three patients (all Group I: 2 surgical, 1 medical) were reintubated within 24 h, an overall extubation failure rate of 1.8%. In group I, at T0, PaO2/FIO2 was 238(65), f/VT 50(26), MIP 44(21) cmH2O, VE 10.6(3.7) l/min, VC 13(5) ml/kg. Cardiorespiratory variables did not change significantly in either group, T0 to T60. For prediction of reintubation (n = 163), only VE (threshold > 10 l/min) and f/VT (threshold > 100) demonstrated moderate sensitivity and specificity at T60: 67 and 52% and 33 and 94%, respectively.

CONCLUSIONS

Bedside clinical judgement of weaning and extubation produces satisfactory outcomes. As a routine, mechanical predictive indices have limited utility.

Neuromuscular disorders associated with failure to wean from the ventilator

OBJECTIVE

To determine, by retrospective chart analysis, the frequency, type and significance of neuromuscular disorders in patients whose clinical features suggested a neuromuscular cause of failure to wean.

BACKGROUND

Failure to wean is a common and difficult problem in critical care units. While a neuromuscular cause may be suspected in some patients, the frequency and type has not been determined utilizing comprehensive electrophysiological studies of limbs and the respiratory system. Such knowledge may aid in patient management and prognosis.

METHODS

The clinical setting was a critical care/trauma centre that admits 1500 patients per year, approximately 500 being on ventilators for longer than five days. We analyzed the hospital charts of 40 patients admitted to the unit during three years, whose respiratory assessment suggested a neuromuscular cause for failure to wean from the ventilator. To investigate this possibility, we performed electrophysiological studies of the limbs and also of the respiratory system by phrenic nerve conduction and needle electromyography of the chest wall and diaphragm. The results were compared to 25 healthy controls.

RESULTS

38 of 40 patients (95%) had a neuromuscular disorder: 25--critical illness polyneuropathy, 2--Guillain-Barré syndrome, 4--diabetic and critical illness polyneuropathy, 2--uremic and critical illness polyneuropathy, 10--an abnormality of central drive, 5--unilateral phrenic nerve palsy, 3--a neuromuscular transmission defect, and 5--a primary myopathy. Fifteen (38%) had a combination of disorders. Patients with more severe polyneuropathy took longer to wean, a mean of 136 versus 52 days (p = 0.007). The severity of the polyneuropathy had no effect on mortality.

CONCLUSIONS

Electrophysiological studies of limbs and the respiratory system are together valuable in confirming the presence, and identifying the specific type of neuromuscular cause for difficulty in weaning from the ventilator. This information is important in patient management and prognosis.

Unplanned extubation. Predictors of successful termination of mechanical ventilatory support

BACKGROUND

Unplanned extubation (self-extubation or accidental extubation) occurs commonly in mechanically ventilated patients, and many patients do not receive mechanical ventilation indefinitely. Unfortunately, weaning parameters are often unavailable in the setting of unplanned extubation, and it would be useful to define pre-extubation respiratory and ventilatory parameters that predict which patients require reintubation.

METHODS

The medical records of all patients who experienced unplanned extubation for the 2-year period of July 1989 to July 1991 were reviewed. Pre-extubation values of respiratory rate, tidal volume (VT), fraction of inspired oxygen (FIo2), PEEP, ventilatory mode, and ventilator-delivered minute volume (VVE, ventilator rate multiplied by set VT) were recorded. In addition, the following data were obtained: age, gender, respiratory failure diagnosis, duration of intubation, amount, and type of sedative agents in the 24 h before extubation. Comparisons of these values among patients who ultimately required reintubation and those who were not reintubated were made using the Mann-Whitney U two-sample test.

RESULTS

During this period, there were 23 unplanned extubations involving 22 patients. Reintubation was required for 18 episodes of unplanned extubation, but was not required for 5 episodes. There were no significant differences between the two groups for any of the parameters except VVE and FIo2. The mean pre-extubation FIo2 of the reintubated group (0.49) was significantly higher than that of the patients who were not reintubated (0.35) (p = 0.021); all of the patients who remained extubated were receiving an FIo2 < or = 0.40. The VVE was also higher in the reintubated group (9.73 L/min) than in the patients who were not reintubated (1.40 L/min); all patients who remained extubated were receiving < or = 7.0 L/min of ventilator-delivered minute ventilation.

CONCLUSIONS

Reintubation after unplanned extubation should not be considered mandatory. Patients who require reintubation have significantly higher preextubation FIo2 and ventilatory requirements than patients who remain extubated.

Difficult weaning from mechanical ventilation

Difficult weaning is fortunately a rare occurrence in mechanically ventilated patients in ICU. When faced with this problem, a vast number of factors must be carefully considered simultaneously: physiological adjustment, technical problems (tubing, circuit resistances, . . .) [13]. The most promising approach to difficult weaning to date centers on the respiratory muscle function which represents the most common factor allowing weaning success or failure.

Successful weaning from mechanical ventilation. Strategies to avoid failure

The majority of patients who require mechanical ventilation can be weaned quickly from the ventilator. Patients with severe lung disease and multisystem disease often require a more prolonged, gradual reduction of ventilatory support. Identification of patients who are ready to be weaned, correction of common problems that occur in weaning, and use of a standard approach during weaning trials can improve the likelihood of successful discontinuation of mechanical ventilation.