Reduction in tracheal lumen due to endotracheal intubation and its calculated clinical significance

Understanding the mechanisms of ventilator-induced lung injury using animal models

Mechanical ventilation is a life-saving therapy in several clinical situations, promoting gas exchange and providing rest to the respiratory muscles. However, mechanical ventilation may cause hemodynamic instability and pulmonary structural damage, which is known as ventilator-induced lung injury (VILI). The four main injury mechanisms associated with VILI are as follows: barotrauma/volutrauma caused by overstretching the lung tissues; atelectrauma, caused by repeated opening and closing of the alveoli resulting in shear stress; and biotrauma, the resulting biological response to tissue damage, which leads to lung and multi-organ failure. This narrative review elucidates the mechanisms underlying the pathogenesis, progression, and resolution of VILI and discusses the strategies that can mitigate VILI. Different static variables (peak, plateau, and driving pressures, positive end-expiratory pressure, and tidal volume) and dynamic variables (respiratory rate, airflow amplitude, and inspiratory time fraction) can contribute to VILI. Moreover, the potential for lung injury depends on tissue vulnerability, mechanical power (energy applied per unit of time), and the duration of that exposure. According to the current evidence based on models of acute respiratory distress syndrome and VILI, the following strategies are proposed to provide lung protection: keep the lungs partially collapsed (SaO2 > 88%), avoid opening and closing of collapsed alveoli, and gently ventilate aerated regions while keeping collapsed and consolidated areas at rest. Additional mechanisms, such as subject-ventilator asynchrony, cumulative power, and intensity, as well as the damaging threshold (stress-strain level at which tidal damage is initiated), are under experimental investigation and may enhance the understanding of VILI.

Retrospective computed tomography analysis of endotracheal tube constriction and mispositioning in cats and dogs

OBJECTIVE

To discover the prevalence of endotracheal tube (ETT) constriction and rostral and caudal mispositioning in anaesthetized cats and dogs, and to identify associated risk factors.

STUDY DESIGN

Retrospective analysis.

ANIMALS

A total of 146 cats and 670 dogs.

METHODS

Computed tomography images of the head/neck/thorax from orotracheally intubated cats and dogs were visually assessed for constriction or mispositioning of the ETT. If constriction was present, measurements of the cross-sectional area (CSA) of the ETT lumen at constricted and un-constricted locations were compared. Location and cause of constriction were noted and the expected increase in resistance to gas flow was calculated. Animal information was collected from clinical records. Normality of continuous variables was assessed via the Shapiro-Wilk test. Chi-square tests examined associations between variables. Kendall's tau-b test was performed between measured ETT size and degree of constriction.

RESULTS

The ETT extended rostrally beyond incisors in 52% of cases; the connector was within the oral cavity in 19% of cases. The ETT extended beyond the first rib in 25.5% of cases. The prevalence of ETT constriction was 22.7%. Median reduction in CSA was 7.68% (0.14-64.19%). Median increase in resistance assuming laminar and turbulent flow was 16.5% (0.3-680%) and 21% (0.3-1200%), respectively. The most common cause of constriction was the presence of a radiotherapy mouth gag. Significant associations existed between presence of constriction and rostral mispositioning, and caudal mispositioning and extreme brachycephaly. Increased severity of constriction was more likely in smaller ETT.

CONCLUSIONS AND CLINICAL RELEVANCE

Constriction and mispositioning of ETT occurred very commonly in this population. Checking the ETT within the oral cavity for constriction and mispositioning is recommended. Radiotherapy mouth gags increase the risk of ETT compression. Smaller ETT are at greater risk of severe constriction. Brachycephalic dogs are at particular risk of caudal mispositioning.

Pediatric Ventilation Liberation: A Survey of International Practice Among 555 Pediatric Intensivists

Pediatric ventilation liberation has limited evidence, likely resulting in wide practice variation. To inform future work, practice patterns must first be described.

Effect of Protocolized Weaning and Spontaneous Breathing Trial vs Conventional Weaning on Duration of Mechanical Ventilation: A Randomized Controlled Trial

Background

Identifying ventilated patients ready for extubation is a challenge for clinicians. Premature extubation increases risks of reintubation while delayed weaning increases complications of prolonged ventilation. We compared the duration of mechanical ventilation (MV) and extubation failure in children extubated using a weaning protocol based on pressure support spontaneous breathing trial (PS SBT) vs those extubated after nonprotocolized physician-directed weaning.

Patients and methods

A prospective randomized controlled trial was conducted in the pediatric intensive care unit of a tertiary care hospital in children ventilated for ≥24 hours. All eligible patients underwent daily screening and were randomized once found fit. The intervention group underwent PS SBT of 2 hours duration followed by a T-piece trial and extubation. Controls underwent conventional weaning with synchronized intermittent mandatory ventilation mode and a T-piece trial before extubation.

Results

Eighty patients were randomized into two groups of 40 each. About 77.5% of patients passed the PS SBT on the first attempt. No statistical difference was found either in the duration of MV between the two groups [median (interquartile range) in days: 4.77 (2.89, 9.46) in controls and 4.94 (2.23, 6.35) in cases, p = 0.62] or in the rate of extubation failure (13% and 10.5%, p = 1). Mortality was found to be significantly higher in the reintubated patients compared to those not reintubated in both groups (p = 0.002 in cases and 0.005 in controls).

Conclusion

Weaning using PS SBT-based protocol though did not shorten the duration of MV, it was found to be safe for assessing extubation readiness and did not increase extubation failure (CTRI no—CTRI/2018/04/013270).

How to cite this article

Kishore R, Jhamb U. Effect of Protocolized Weaning and Spontaneous Breathing Trial vs Conventional Weaning on Duration of Mechanical Ventilation: A Randomized Controlled Trial. Indian J Crit Care Med 2021;25(9):1059–1065.

Reliability of peak expiratory flow percentage compared to endoscopic grading in subglottic stenosis

OBJECTIVE

To determine the reliability of pulmonary function testing compared to endoscopic grading in the assessment of subglottic stenosis.

METHODS

Consecutively treated patients with subglottic stenosis at a tertiary care specialty hospital from 2009 to 2019 were identified. Two fellowship-trained laryngologists and two otolaryngologists blinded to clinical history reviewed laryngo tracheoscopic examinations and assessed the degree of stenosis using the Cotton-Myer grading system (% stenosis). Nine full flow-volume loops were performed at the time of each exam.

RESULTS

The endoscopic images of 45 subjects were graded for degree of stenosis and the spirometry data were analyzed. The kappa values for Cotton-Myer grade overall was 0.37, grade I was -0.103, grade II was 0.052, and grade III was 0.045. The overall intraclass correlation of the physician grading of estimated percent obstruction (% stenosis) was 0.712 (P < .01) whereas the overall intraclass correlation for PEF% was 0.96 (P < .01). Within each Cotton-Myer grade, the intraclass correlation for % stenosis was 0.45 (P = .02) for grade I, 0.06 (P = .30) for grade II, and 0.16 (P = .03) for grade III. The intraclass correlation for PEF% for grade I was 0.97 (P < .01), grade II was 0.92 (P < .01), and grade III was 0.96 (P < .01).

CONCLUSION

Cotton-Myer grading and estimating percent obstruction (% stenosis) for adult subglottic stenosis showed poor reliability as an assessment tool compared to the excellent intraclass correlation seen with pulmonary function tests within each Cotton-Myer grade subgroup. We recommend pulmonary function testing, specifically PEF% because it is a normalized value, for the assessment and management of subglottic stenosis.

LEVEL OF EVIDENCE

4.

A Respiratory Therapist-Driven Pathway Improves Timeliness of Extubation Readiness Assessment in a Single PICU

OBJECTIVES

Our smart aim was to decrease the time between when a mechanically ventilated patient was eligible for and when they underwent their first extubation readiness test (delta time) by 50% within 3 months through the development and implementation of a respiratory therapist-driven extubation readiness test pathway.

DESIGN

Quality improvement project.

SETTING

Single, tertiary care, 24-bed, academic PICU.

PATIENTS

Pediatric patients admitted to the PICU and requiring mechanical ventilation for a primary pulmonary process.

INTERVENTIONS

We developed an extubation readiness test pathway that consisted of an eligibility screen and a standard testing process. Patients were screened every 3 hours. Upon passing the screen and being cleared by a prescriber, a test was initiated. No clinical management was dictated to prescribers.

MEASUREMENTS AND MAIN RESULTS

The preintervention and intervention cohorts included 109 and 43 mechanical ventilation courses, respectively. The mean delta time decreased from 33.77 hours to 2.92 hours after pathway implementation (p = 0.000). The medical length of stay decreased from 196.6 to 177.2 hours (p = 0.05). There were no statistically significant changes in duration of mechanical ventilation until first extubation (112.9 vs 122.3 hr; p = 0.651) and 48-hour extubation failure rate (16.5% vs 4.8%; p = 0.056). The sensitivity and positive predictive value for the extubation readiness test were 89.5% and 94.4%, respectively. The mean for all process compliance measures was 91.5%.

CONCLUSIONS

A respiratory therapist-driven extubation readiness test pathway can be safely implemented in a large, academic PICU. The pathway resulted in earlier extubation readiness testing without increasing key balancing measures-the duration of mechanical ventilation, PICU length of stay, or the extubation failure rate.

The basics of respiratory mechanics: ventilator-derived parameters

Mechanical ventilation is a life-support system used to maintain adequate lung function in patients who are critically ill or undergoing general anesthesia. The benefits and harms of mechanical ventilation depend not only on the operator's setting of the machine (input), but also on their interpretation of ventilator-derived parameters (outputs), which should guide ventilator strategies. Once the inputs-tidal volume (V_T), positive end-expiratory pressure (PEEP), respiratory rate (RR), and inspiratory airflow (V')-have been adjusted, the following outputs should be measured: intrinsic PEEP, peak (Ppeak) and plateau (Pplat) pressures, driving pressure (ΔP), transpulmonary pressure (P_L), mechanical energy, mechanical power, and intensity. During assisted mechanical ventilation, in addition to these parameters, the pressure generated 100 ms after onset of inspiratory effort (P_0.1) and the pressure-time product per minute (PTP/min) should also be evaluated. The aforementioned parameters should be seen as a set of outputs, all of which need to be strictly monitored at bedside in order to develop a personalized, case-by-case approach to mechanical ventilation. Additionally, more clinical research to evaluate the safe thresholds of each parameter in injured and uninjured lungs is required.

Work of breathing for cuffed and uncuffed pediatric endotracheal tubes in an in vitro lung model setting

BACKGROUND

Over the last decade, cuffed endotracheal tubes are increasingly used in pediatric anesthesia and also in pediatric intensive care. However, the smaller inner diameter of cuffed endotracheal tubes and, implicitly, the increased endotracheal tube resistance is still a matter of debate.

AIMS

This in vitro study investigated work of breathing and inspiratory airway pressures in cuffed and uncuffed endotracheal tubes and the impact of pressure support ventilation and automatic tube compensation.

METHODS

In 5 simulated neonatal and pediatric lung models, the Active Servo Lung 5000 and an intensive care ventilator were used to quantify the differences in work of breathing under spontaneous breathing (with and without pressure support ventilation and automatic tube compensation) between cuffed and uncuffed endotracheal tubes. Additionally, differences in inspiratory airway pressures, measured either proximal or distal of the endotracheal tube, between cuffed and uncuffed endotracheal tubes under mechanical ventilation were investigated.

RESULTS

Work of breathing was overall 10.27% [95% confidence interval 9.01-11.94] higher with cuffed than with uncuffed endotracheal tubes and was dramatically reduced by 34.19% [95% confidence interval 31.61-35.25] with the application of pressure support. Automatic tube compensation almost diminished work of breathing differences between the 2 endotracheal tube types in nearly all pediatric lung models. Peak inspiratory and mean airway pressures measured at the proximal endotracheal tube end revealed significantly higher values in cuffed than in uncuffed endotracheal tubes. However, these differences measured at the distal end of the endotracheal tube became minimal.

CONCLUSION

This in vitro study confirmed significant differences in work of breathing and inspiratory pressures between cuffed and uncuffed endotracheal tubes. Work of breathing, however, is almost neutralized by pressure support ventilation with automatic tube compensation and distal inspiratory airway pressures that, from a clinical perspective, are not significantly increased.

Factors Associated With Unplanned Extubation in Children: A Case–Control Study

Purpose:

Although several studies assess unplanned extubation (UE) in children, few have addressed determinants of UE and factors associated with reintubation in a case-controlled manner. We aimed to identify the risk factors and outcomes associated with UE in a pediatric intensive care unit.

Methods:

Cases of UE were randomly matched with control patients at a ratio of 1:4 for age, severity of illness, and admission diagnosis. For cases and controls, we also collected data associated with UE events, reintubation, and outcomes.

Results:

We analyzed 94 UE patients (0.75 UE per 100 intubation days) and found no differences in demographics between the 2 groups. Logistic regression revealed that patient agitation (odds ratio [OR]: 2.44; 95% confidence interval [CI]: 1.28-4.65), continuous sedation infusion (OR: 3.27; 95% CI: 1.70-6.29), night shifts (OR: 9.16; 95% CI: 4.25-19.72), in-charge nurse experience <2 years (OR: 2.38; 95% CI: 1.13-4.99), and oxygenation index (OI) >5 (OR: 76.9; 95% CI: 16.79-352.47) were associated with UE. Risk factors for reintubation after UE included prior level of sedation (COMFORT score < 27; OR: 7.93; 95% CI: 2.30-27.29), copious secretion (OR: 11.88; 95% CI: 2.20-64.05), and OI > 5 (OR: 9.32; 95% CI: 2.45-35.48).

Conclusions:

This case–control study showed that both patient- and nurse-associated risk factors were related to UE. Risk factors associated with reintubation included lower levels of consciousness, copious secretions, and higher OI. Further evidence-based studies, including a larger sample size, are warranted to identify predisposing factors in UEs.

Impact of endotracheal tube shortening on work of breathing in neonatal and pediatric in vitro lung models

BACKGROUND

Work of breathing accounts for a significant proportion of total oxygen consumption in neonates and infants. Endotracheal tube inner diameter and length significantly affect airflow resistance and thus work of breathing. While endotracheal tube shortening reduces endotracheal tube resistance, the impact on work of breathing in mechanically ventilated neonates and infants remains unknown.

AIM

The objective of this in vitro study was to quantify the effect of endotracheal tube shortening on work of breathing in simulated pediatric lung settings. We hypothesized that endotracheal tube shortening significantly reduces work of breathing.

METHODS

We used the Active-Servo-Lung 5000 to simulate different clinical scenarios in mechanically ventilated infants and neonates under spontaneous breathing with and without pressure support. Endotracheal tube size, lung resistance, and compliance, as well as respiratory settings such as respiratory rate and tidal volume were weight and age adapted for each lung model. Work of breathing was measured before and after maximal endotracheal tube shortening and the reduction of the daily energy demand calculated.

RESULTS

Tube shortening with and without pressure support decreased work of breathing to a maximum of 10.1% and 8.1%, respectively. As a result, the calculated reduction of total daily energy demand by endotracheal tube shortening was between 0.002% and 0.02%.

CONCLUSION

In this in vitro lung model, endotracheal tube shortening had minimal effects on work of breathing. Moreover, the calculated percentage reduction of the total daily energy demand after endotracheal tube shortening was minimal.

miR-322 stabilizes MEK1 expression to inhibit RAF/MEK/ERK pathway activation in cartilage

Cartilage originates from mesenchymal cell condensations that differentiate into chondrocytes of transient growth plate cartilage or permanent cartilage of the articular joint surface and trachea. MicroRNAs fine-tune the activation of entire signaling networks and thereby modulate complex cellular responses, but so far only limited data are available on miRNAs that regulate cartilage development. Here, we characterize a miRNA that promotes the biosynthesis of a key component in the RAF/MEK/ERK pathway in cartilage. Specifically, by transcriptome profiling we identified miR-322 to be upregulated during chondrocyte differentiation. Among the various miR-322 target genes in the RAF/MEK/ERK pathway, only Mek1 was identified as a regulated target in chondrocytes. Surprisingly, an increased concentration of miR-322 stabilizes Mek1 mRNA to raise protein levels and dampen ERK1/2 phosphorylation, while cartilage-specific inactivation of miR322 in mice linked the loss of miR-322 to decreased MEK1 levels and to increased RAF/MEK/ERK pathway activation. Such mice died perinatally due to tracheal growth restriction and respiratory failure. Hence, a single miRNA can stimulate the production of an inhibitory component of a central signaling pathway to impair cartilage development.

Micropatterned Endotracheal Tubes Reduce Secretion-Related Lumen Occlusion

Tracheal intubation disrupts physiological homeostasis of secretion production and clearance, resulting in secretion accumulation within endotracheal tubes (ETTs). Novel in vitro and in vivo models were developed to specifically recapitulate the clinical manifestations of ETT occlusion. The novel Sharklet™ micropatterned ETT was evaluated, using these models, for the ability to reduce the accumulation of both bacterial biofilm and airway mucus compared to a standard care ETT. Novel ETTs with micropattern on the inner and outer surfaces were placed adjacent to standard care ETTs in in vitro biofilm and airway patency (AP) models. The primary outcome for the biofilm model was to compare commercially-available ETTs (standard care and silver-coated) to micropatterned for quantity of biofilm accumulation. The AP model's primary outcome was to evaluate accumulation of artificial airway mucus. A 24-h ovine mechanical ventilation model evaluated the primary outcome of relative quantity of airway secretion accumulation in the ETTs tested. The secondary outcome was measuring the effect of secretion accumulation in the ETTs on airway resistance. Micropatterned ETTs significantly reduced biofilm by 71% (p = 0.016) compared to smooth ETTs. Moreover, micropatterned ETTs reduced lumen occlusion, in the AP model, as measured by cross-sectional area, in distal (85%, p = 0.005), middle (84%, p = 0.001) and proximal (81%, p = 0.002) sections compared to standard care ETTs. Micropatterned ETTs reduced the volume of secretion accumulation in a sheep model of occlusion by 61% (p < 0.001) after 24 h of mechanical ventilation. Importantly, micropatterned ETTs reduced the rise in ventilation peak inspiratory pressures over time by as much as 49% (p = 0.005) compared to standard care ETTs. Micropatterned ETTs, demonstrated here to reduce bacterial contamination and mucus occlusion, will have the capacity to limit complications occurring during mechanical ventilation and ultimately improve patient care.

Tracheal tube obstruction in mechanically ventilated patients assessed by high-resolution computed tomography

BACKGROUND

Tracheal intubation compromises mucus clearance and secretions accumulate inside the tracheal tube (TT). The aim of this study was to evaluate with a novel methodology TT luminal obstruction in critically ill patients.

METHODS

This was a three-phase study: (1) the authors collected 20 TTs at extubation. High-resolution computed tomography (CT) was performed to determine cross-sectional area (CSA) and mucus distribution within the TT; (2) five TTs partially filled with silicone were used to correlate high-resolution CT results and increased airflow resistance; and (3) 20 chest CT scans of intubated patients were reviewed for detection of secretions in ventilated patients' TT.

RESULTS

Postextubation TTs showed a maximum CSA reduction of (mean±SD) 24.9±3.9% (range 3.3 to 71.2%) after a median intubation of 4.5 (interquartile range 2.5 to 6.5) days. CSA progressively decreased from oral to lung end of used TTs. The luminal volume of air was different between used and new TTs for all internal diameters (P<0.01 for new vs. used TTs for all studied internal diameters). The relationship between pressure drop and increasing airflow rates was nonlinear and depended on minimum CSA available to ventilation. Weak correlation was found between TT occlusion and days of intubation (R²=0.352, P=0.006). With standard clinical chest CT scans, 6 of 20 TTs showed measurable secretions with a CSA reduction of 24.0±3.9%.

CONCLUSIONS

TT luminal narrowing is a common finding and correlates with increased airflow resistance. The authors propose high-resolution CT as a novel technique to visualize and quantify secretions collected within the TT lumen.

Tracheal stenosis following percutaneous dilatational tracheostomy using the single tapered dilator: an MRI study

Despite widespread adoption of percutaneous dilatational tracheostomy within the critical care setting, there is still uncertainty regarding long-term complications, particularly in relation to missed or subclinical tracheal stenosis. In this study, all patients underwent tracheostomy using a single tapered dilator ≥ three months prior to enrollment and were evaluated using magnetic resonance imaging, spirometry and questionnaire. Tracheal area was recorded and deemed to be stenotic if a reduction of ≥10% was found. Fifty patients underwent magnetic resonance imaging and 49 attended for interview. Five patients were diagnosed with tracheal stenosis-none were symptomatic. Six of the 50 tracheostomies were technically difficult. Spirometry was not predictive of stenosis. A post critical care exercise tolerance of less than 100 metres was found in four tracheal stenosis patients. The prevalence of subclinical tracheal stenosis following percutaneous tracheostomy is low, with limited clinical significance. No patients required corrective surgery for tracheal stenosis. Routine airway follow-up in asymptomatic patients appears to be unwarranted.

Tracheal Size Variability Is Associated With Sex

Purpose:

Whereas selection of endotracheal tube (ETT) size in pediatric patients benefits from predictive nomograms, adult ETT sizing is relatively arbitrary. We sought to determine associations between cervical tracheal cross-sectional area (CTCSA) and clinical variables.

Methods:

One hundred thirty-two consecutive patients undergoing noncontrasted chest computed tomography (CT) at a single tertiary care institution from January 2010 to June 2011 were reviewed. Patients with improper CT technique, endotracheal intubation, and pulmonary/tracheal pathology were excluded. Tracheal luminal diameters in anteroposterior (D1) and transverse (D2) were measured 2 cm inferior to the cricoid and used to determine CTCSA = π*D1*D2*¼. The demographic variables of age, height, weight, and body mass index (BMI) were tested for association with CTCSA by Spearman correlation. Wilcoxon rank-sum test was used to compare CTCSA by race and sex. Multivariate linear regression was performed including all clinical variables.

Results:

There were 91 patients who met inclusion criteria. There was no correlation between age, weight, or BMI and CTCSA. There was a significant positive correlation between patient height and CTCSA ( P = .001, R = 0.35); however, this was confounded by sex. Female patients had significantly smaller CTCSA (mean = 241 mm2) compared to male patients (mean = 349 mm2, P < .001). Multivariate linear regression stratified by sex revealed that height is correlated with CTCSA only in males ( P = .028). Males also had more variability in CTCSA (SD 118.6) compared to females (SD 65.5).

Conclusion:

Our data suggest that selection of ETT size in male patients should include height as a predictive factor. For female patients, it may be appropriate to select a uniformly smaller diameter ETT size.

Evaluation of plethysmography for diagnosis of postintubation tracheal stenosis

OBJECTIVES

Post-intubation tracheal stenosis is a serious disease caused by cuff pressure during tracheal intubation. The reported prevalence of post-intubation tracheal stenosis is 10%-22% in intubated patients. Invasive diagnostic methods based on fiberoptic or rigid bronchoscopy are suggested for initial assessment and treatment. This study aimed to evaluate plethysmography as an alternative diagnostic tool for post-intubation tracheal stenosis. We also assessed the relationship between plethysmography and bronchoscopy findings in patients with post-intubation tracheal stenosis.

METHODS

The sample included 30 patients who were admitted to our intensive care unit or surgical ward during the course of a year, and diagnosed with post-intubation tracheal stenosis. All patients underwent plethysmography and rigid bronchoscopy. The relationship between the plethysmography and bronchoscopy findings was examined.

RESULTS

Stricture intensity had the strongest correlation with upper airway resistance (p = 0.001). The relationship of length of stricture to forced expiratory volume in 1 s and maximum expiratory flow 50 and 75 was significant in univariate analysis, and to reserve volume and total lung capacity in multivariate analysis.

CONCLUSION

Significant relationships were found between plethysmography variables and rigid bronchoscopy findings in patients with post-intubation tracheal stenosis, and some formulas were developed to give an alternative estimate of stricture severity, without bronchoscopy.

A spontaneous breathing trial with pressure support overestimates readiness for extubation in children

OBJECTIVE

To evaluate the performance of an extubation readiness test based on a spontaneous breathing trial using pressure support.

DESIGN

Retrospective chart review.

SETTING

Pediatric intensive care unit.

PATIENTS

All infants and children admitted to the pediatric intensive care unit requiring intubation from July 2007 to December 2008 were eligible for this study.

INTERVENTIONS

Routine use of an extubation readiness test using pressure support set according to endotracheal tube size to determine completion of weaning and readiness for extubation.

MEASUREMENTS AND MAIN RESULTS

A total of 755 extubation readiness tests were performed in 538 patients with a pass rate of 83%. Of 500 children who passed the extubation readiness test and were extubated without planned noninvasive ventilation use, the extubation failure rate was 11.2% (5.8% required reintubation). Extubation failure was defined as need for noninvasive ventilation or reintubation within 24 hrs of planned extubation. Logistic regression analysis revealed a significant association between duration of mechanical ventilation and extubation failure. Children ventilated for over 48 hrs had an 18.5% failure rate despite passing an extubation readiness test before extubation and the extubation readiness test was not a significant predictor of extubation success. Most extubation failures were the result of inadequate gas exchange attributable to lower respiratory tract dysfunction.

CONCLUSIONS

A spontaneous breathing trial using pressure support set at higher levels for smaller endotracheal tubes overestimates readiness for extubation in children and contributes to a higher failed extubation rate. The objective data obtained during an extubation readiness test may help to identify patients who will benefit from extubation to noninvasive ventilation.

Endoscopic intubation of exotic companion mammals

Rabbits, guinea pigs, chinchillas and many other small exotic mammals are not intubated routinely, because intubation requires specialized equipment and is difficult to perfect. Using a face mask for these species solely on the basis that they are unable to regurgitate ignores the numerous other benefits of airway control. This article summarizes the many advantages of endotracheal intubation and the various methods of intubation that have been reported. It introduces endoscopic intubation as a method that overcomes many of the difficulties associated with other methods and describes the equipment needed, how to intubate with an endoscope, how to confirm proper endotracheal tube placement, and possible complications. Over-the-endoscope intubation is discussed in detail, as it appears to provide the most versatile and reliable method of intubating exotic companion mammals.

Flow resistance, work of breathing of humidifiers, and endotracheal tubes in the hyperbaric chamber

Humidification of inspired gas is critical in ventilated patients, usually achieved by heat and moisture exchange devices (HMEs). HME and the endotracheal tube (ETT) add airflow resistance. Ventilated patients are sometimes treated in hyperbaric chambers. Increased gas density may increase total airway resistance, peak pressures (PPs), and mechanical work of breathing (WOB). We tested the added WOB imposed by HMEs and various sizes of ETT under hyperbaric conditions. We mechanically ventilated 4 types of HMEs and 3 ETTs at 6 minute ventilation volumes (7-19.5 L/min) in a hyperbaric chamber at pressures of 1 to 6 atmospheres absolute (ATA). Peak pressure increased with increasing chamber pressure with an HME alone, from 2 cm H₂O at 1 ATA to 6 cm H(2)O at 6 ATA. Work of breathing was low at 1 ATA (0.2 J/L) and increased to 1.2 J/L at 6 ATA at minute ventilation = 19.5 L/min. Connecting the HME to an ETT increased PP as a function of peak flow and chamber pressure. Reduction of the ETT diameter (9 > 8 > 7.5 mm) and increase in chamber pressure increased the PP up to 27.7 cm H₂O, resistance to 33.2 cmH₂O*s/L, and WOB to 3.76 J/L at 6 ATA with a 7.5-mm EET. These are much greater than the usually accepted critical peak pressures of 25 cm H₂O and WOB of 1.5 to 2.0 J/L. Endotracheal tubes less than 8 mm produce significant added WOB and airway pressure swings under hyperbaric conditions. The hyperbaric critical care clinician is advised to use the largest possible ETT. The tested HMEs add negligible resistance and WOB in the chamber.

Work of breathing during successful spontaneous breathing trial

OBJECTIVE

The aim of this study was to evaluate the work of breathing (WOB) behavior during a 120-minute successful spontaneous breathing trial (SBT) with T-tube trial, and its predictive value for extubation outcome.

DESIGN

A prospective cohort study.

SETTING

2 medical-surgical intensive care units.

PATIENTS

Fifty-one consecutive patients mechanically ventilated for more than 48 hours after a successful SBT were extubated based on the institutional protocol and followed for the occurrence of postextubation respiratory distress during 48 hours.

MEASUREMENTS AND MAIN RESULTS

All cases were serially monitored during 120 minutes of SBT using the respiratory monitoring system Ventrak 1500 (Medical Novametrix Systems, Wallingford, CT). Successful extubation occurred in 38 (74.5%) of 51 of the sample. Respiratory and hemodynamic parameters, APACHE II score, sex, days on mechanical ventilation, and cause of respiratory failure were unable to predict extubation outcome. The WOB significantly increased during SBT in extubation failure patients (WOB at 1st minute 0.24 +/- 0.06 J/L vs WOB at 120th minute = 0.39 +/- 0.07 J/L; P < .01) when compared to successfully extubated patients (WOB at 1st minute 0.21 +/- 0.08 J/L vs WOB at 120th minute = 0.24 +/- 0.11 J/L; P = .12). The WOB variation was able to predict extubation outcome only after the 90th minute of SBT (extubation failure = 0.35 +/- 0.08 J/L vs extubation success = 0.22 +/- 0.11 J/L; P = .01).

CONCLUSION

An increase in the WOB could predict extubation failure during a T-tube trial of 120 minutes.

Programming pressure support ventilation in pediatric patients in ambulatory surgery with a laryngeal mask airway

BACKGROUND

Anesthesia workstations with pressure support ventilation (PSV) are available, but there are few studies published on how to program flow-triggered PSV using a laryngeal mask airway (LMA) under general anesthesia in pediatric patients.

METHODS

We studied 60 ASA I and II patients, from 2 mo to 14 yr, scheduled for ambulatory surgery under combined general and regional anesthesia with a LMA. Patients were classified according to their body weight as follows: Group A < or =10 kg, Group B 11-20 kg, and Group C >20 kg. All were ventilated in PSV using the following settings: positive end-expiratory pressure of 4 cm H2O, the minimum flow-trigger without provoking auto-triggering, and the minimum level of pressure support to obtain 10 mL/kg of tidal volume.

RESULTS

The flow-trigger most frequently used in our study was 0.4 L/min, ranging from 0.2 to 0.6 L/min. We found no correlation between the flow-trigger setting and the patient's age, weight, compliance, resistance, or respiratory rate. There was a good correlation between the level of pressure support (Group A = 15 cm H2O, Group B = 10 cm H2O and Group C = 9 cm H2O) and age (P < 0.001), weight (P < 0.001), dynamic compliance (P < 0.001), and airway resistances (P < 0.001).

CONCLUSIONS

PSV with a Proseal LMA in outpatient pediatric anesthesia can be programmed simply using the common clinical noninvasive variables studied. However, more studies are needed to estimate the level of pressure support that may be required in other clinical situations (respiratory pathology, endotracheal tubes, or other types of surgeries) or with other anesthesia workstations.

Objective sizing of upper airway stenosis: a quantitative endoscopic approach

OBJECTIVE

In patients with airway stenosis, anatomy of the lesion determines the magnitude of the biomechanical ventilatory disturbance and thus the nature and severity of symptoms. It also gives information about biology, likelihood of response to treatment, and prognosis of laryngotracheal lesions. Accurate airway sizing throughout treatment is therefore central to managing this condition. We developed a method for objective assessment of airway lesions during endoscopy.

METHODS

We used airway simulations to investigate the effects of endoscope tilt and lens distortions on measurement accuracy, devising and validating clinical rules for quantitative airway endoscopy. A calibrator was designed to assess lesion length, location, and cross-section during tracheoscopy.

RESULTS

It proved possible to calculate the length and location of the stenosis using simple mathematics. Cross-section measurements were more than 95% accurate, independent of endoscope tilt and without making assumptions about endoscope optics and visuospatial distortion, for both pediatric and adult airway dimensions. The technique was used to characterize airway lesions in 10 adult patients with an average age of 48 years undergoing therapeutic laryngotracheoscopy. Lesions occurred on average 36 mm below the glottis (range, 21-54 mm) and were 9.3 mm long (5-17 mm). The average pretreatment airway cross-section was 48.3 mm, increasing to 141.1 mm after laser therapy. Two independent observers calculated airway cross-sections, achieving an interobserver concordance of 0.98.

CONCLUSIONS

This method can be used to objectively and precisely determine the anatomy of airway lesions, allowing accurate documentation of lesion characteristics and surgical results, serial monitoring throughout treatment, and comparison of outcomes between different centers.

Pressure-rate products and phase angles in children on minimal support ventilation and after extubation

OBJECTIVE

To compare the pressure-rate products and phase angles of children during minimal support ventilation and after extubation.

DESIGN AND SETTING

Prospective, randomized single-center trial in a pediatric intensive care unit in a tertiary children's hospital.

METHODS

Seventeen endotracheally intubated, mechanically ventilated children were placed on T-piece, T-piece with heliox, continuous positive airway pressure, and pressure support in random order. Esophageal pressure swings, phase angles, respiratory mechanics, and physiological parameters were measured on these modes and after extubation.

MEASUREMENTS AND RESULTS

Pressure-rate product postextubation was significantly higher than on support modes. For each mode and after extubation they were: pressure support 198+/-31, continuous positive airway pressure 237+/-30, T-piece 323+/-47, T-piece/heliox 308+/-61, and extubation 378+/-43 cmH2O/min. Phase angles were significantly higher during T-piece ventilation than pressure support but not did not differ significantly from postextubation.

CONCLUSIONS

Assessment of effort of breathing during even minimal mechanical ventilation may underestimate postextubation effort in children. Postextubation pressure-rate product and hence "effort of breathing" in children is best approximated by T-piece ventilation.

Clinical review: respiratory mechanics in spontaneous and assisted ventilation

Pulmonary disease changes the physiology of the lungs, which manifests as changes in respiratory mechanics. Therefore, measurement of respiratory mechanics allows a clinician to monitor closely the course of pulmonary disease. Here we review the principles of respiratory mechanics and their clinical applications. These principles include compliance, elastance, resistance, impedance, flow, and work of breathing. We discuss these principles in normal conditions and in disease states. As the severity of pulmonary disease increases, mechanical ventilation can become necessary. We discuss the use of pressure–volume curves in assisting with poorly compliant lungs while on mechanical ventilation. In addition, we discuss physiologic parameters that assist with ventilator weaning as the disease process abates.

Endotracheal tube intraluminal diameter narrowing after mechanical ventilation: use of acoustic reflectometry

OBJECTIVE

To quantify the incidence and degree of endotracheal tube intraluminal obstruction after mechanical ventilation and its relation to time of intubation.

DESIGN

Prospective observational study.

SETTING

A 14-bed medical-surgical intensive care unit at a university-affiliated teaching hospital.

PATIENTS

Ninety-four endotracheal tubes used in 80 patients requiring mechanical ventilation for more than 12 h.

INTERVENTIONS AND RESULTS

Acoustic reflectometry was performed in every endotracheal tube after patient extubation to measure its volume reduction. The intraluminal volumes of used endotracheal tubes in mechanically ventilated patients were significantly lower than those of unused tubes of the same size (5.52+/-0.92 ml(3) versus 6.54+/-0.79 ml(3), p<0.05). The mean difference in endotracheal tube segment volumes was 15.2% (range 0-66%). Volume reduction was above 10% in 60.8% of the tubes. In 22% of endotracheal tubes the remaining inner diameter was less than 7 mm. Reduction below this figure was less frequent (9.3%) in tubes 8 mm or more (p<0.05). The percentage of endotracheal tube volume reduction was not associated with the duration of intubation (r=-0.09, p= n.s.) Peak pressure measured before extubation did not predict obstruction (r=0.11, p= n.s.).

CONCLUSIONS

Inadvertent endotracheal tube obstruction was common in patients requiring mechanical ventilation and may be significant as early as at 24 h. Moderate obstruction in endotracheal tube lumens should be suspected in cases of difficulties in weaning, even in patients who were ventilated for less than 1 day.

The future of the cuffed endotracheal tube

It has been traditionally taught that only uncuffed endotracheal tubes (ETTs) should be used for intubation in children younger than 8, or even 10, years old. However, recent literature suggests that the advantages of using uncuffed ETTs in children may be just another myth of paediatric anaesthesia. Using an uncuffed ETT does allow a tube of larger internal diameter to be used, minimizing resistance to airflow and the work of breathing in the patient who is breathing spontaneously. However, this advantage does not hold for ventilated patients, for whom ventilator settings can be adjusted to provide optimal airflow. Longer duration of intubation and a poorly fitted ETT are risk factors for mucosal damage, whether the ETT is cuffed or uncuffed. Furthermore, a properly sized, positioned, and inflated modern (low-pressure, high-volume) cuffed ETT can offer many advantages over an uncuffed ETT, including greater ease of intubation, better control of air leakage, lower rate and better control of flow of anaesthetic gases, and decreased risk of aspiration and infection.