Effect of Intraoperative High Positive End-Expiratory Pressure (PEEP) With Recruitment Maneuvers vs Low PEEP on Postoperative Pulmonary Complications in Obese Patients: A Randomized Clinical Trial

IMPORTANCE

An intraoperative higher level of positive end-expiratory positive pressure (PEEP) with alveolar recruitment maneuvers improves respiratory function in obese patients undergoing surgery, but the effect on clinical outcomes is uncertain.

OBJECTIVE

To determine whether a higher level of PEEP with alveolar recruitment maneuvers decreases postoperative pulmonary complications in obese patients undergoing surgery compared with a lower level of PEEP.

DESIGN, SETTING, AND PARTICIPANTS

Randomized clinical trial of 2013 adults with body mass indices of 35 or greater and substantial risk for postoperative pulmonary complications who were undergoing noncardiac, nonneurological surgery under general anesthesia. The trial was conducted at 77 sites in 23 countries from July 2014-February 2018; final follow-up: May 2018.

INTERVENTIONS

Patients were randomized to the high level of PEEP group (n = 989), consisting of a PEEP level of 12 cm H2O with alveolar recruitment maneuvers (a stepwise increase of tidal volume and eventually PEEP) or to the low level of PEEP group (n = 987), consisting of a PEEP level of 4 cm H2O. All patients received volume-controlled ventilation with a tidal volume of 7 mL/kg of predicted body weight.

MAIN OUTCOMES AND MEASURES

The primary outcome was a composite of pulmonary complications within the first 5 postoperative days, including respiratory failure, acute respiratory distress syndrome, bronchospasm, new pulmonary infiltrates, pulmonary infection, aspiration pneumonitis, pleural effusion, atelectasis, cardiopulmonary edema, and pneumothorax. Among the 9 prespecified secondary outcomes, 3 were intraoperative complications, including hypoxemia (oxygen desaturation with Spo2 ≤92% for >1 minute).

RESULTS

Among 2013 adults who were randomized, 1976 (98.2%) completed the trial (mean age, 48.8 years; 1381 [69.9%] women; 1778 [90.1%] underwent abdominal operations). In the intention-to-treat analysis, the primary outcome occurred in 211 of 989 patients (21.3%) in the high level of PEEP group compared with 233 of 987 patients (23.6%) in the low level of PEEP group (difference, -2.3% [95% CI, -5.9% to 1.4%]; risk ratio, 0.93 [95% CI, 0.83 to 1.04]; P = .23). Among the 9 prespecified secondary outcomes, 6 were not significantly different between the high and low level of PEEP groups, and 3 were significantly different, including fewer patients with hypoxemia (5.0% in the high level of PEEP group vs 13.6% in the low level of PEEP group; difference, -8.6% [95% CI, -11.1% to 6.1%]; P < .001).

CONCLUSIONS AND RELEVANCE

Among obese patients undergoing surgery under general anesthesia, an intraoperative mechanical ventilation strategy with a higher level of PEEP and alveolar recruitment maneuvers, compared with a strategy with a lower level of PEEP, did not reduce postoperative pulmonary complications.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02148692.

Protective ventilation with high versus low positive end-expiratory pressure during one-lung ventilation for thoracic surgery (PROTHOR): study protocol for a randomized controlled trial

BACKGROUND

Postoperative pulmonary complications (PPC) may result in longer duration of in-hospital stay and even mortality. Both thoracic surgery and intraoperative mechanical ventilation settings add considerably to the risk of PPC. It is unclear if one-lung ventilation (OLV) for thoracic surgery with a strategy of intraoperative high positive end-expiratory pressure (PEEP) and recruitment maneuvers (RM) reduces PPC, compared to low PEEP without RM.

METHODS

PROTHOR is an international, multicenter, randomized, controlled, assessor-blinded, two-arm trial initiated by investigators of the PROtective VEntilation NETwork. In total, 2378 patients will be randomly assigned to one of two different intraoperative mechanical ventilation strategies. Investigators screen patients aged 18 years or older, scheduled for open thoracic or video-assisted thoracoscopic surgery under general anesthesia requiring OLV, with a maximal body mass index of 35 kg/m2, and a planned duration of surgery of more than 60 min. Further, the expected duration of OLV shall be longer than two-lung ventilation, and lung separation is planned with a double lumen tube. Patients will be randomly assigned to PEEP of 10 cmH2O with lung RM, or PEEP of 5 cmH2O without RM. During two-lung ventilation tidal volume is set at 7 mL/kg predicted body weight and, during OLV, it will be decreased to 5 mL/kg. The occurrence of PPC will be recorded as a collapsed composite of single adverse pulmonary events and represents the primary endpoint.

DISCUSSION

PROTHOR is the first randomized controlled trial in patients undergoing thoracic surgery with OLV that is adequately powered to compare the effects of intraoperative high PEEP with RM versus low PEEP without RM on PPC. The results of the PROTHOR trial will support anesthesiologists in their decision to set intraoperative PEEP during protective ventilation for OLV in thoracic surgery.

TRIAL REGISTRATION

The trial was registered in clinicaltrials.gov ( NCT02963025 ) on 15 November 2016.

Respiratory complications of anaesthesia

Postoperative pulmonary complications are a major determinant of outcome for patients and consume huge resources within hospital, particularly in critical care. Prediction and anticipation of postoperative pulmonary complications are vital for patient selection and, in some cases, for informed patient consent. Being able to assess the likelihood of postoperative pulmonary complications also allows research into methods to reduce them by allowing allocation of patients to the appropriate arms of research trials. Some patients have pre-operative characteristics or belong to patient groups such as those with chronic obstructive pulmonary disease or obstructive sleep apnoea, where techniques and evidence-based guidance to avoid or reduce complications are becoming established. Intra-operative ventilation and the use of lung-protective ventilation may be helpful during major surgery, but studies looking at reduced tidal volumes, recruitment and levels of positive end-expiratory pressure, have this far only led to a degree of consensus in terms of tidal volume, although parameters that predispose to postoperative pulmonary complications are becoming clearer. Optimal postoperative care in terms of analgesia, positioning, physiotherapy and mobilisation is another developing area. Techniques such as continuous positive airways pressure, non-invasive ventilation and high-flow nasal humidified oxygen appear to show some benefit, but the exact roles, pressures and timings of each are currently being explored. Much remains to be researched and developed into evidence-based practice.

Association between night-time surgery and occurrence of intraoperative adverse events and postoperative pulmonary complications

BACKGROUND

The aim of this post hoc analysis of a large cohort study was to evaluate the association between night-time surgery and the occurrence of intraoperative adverse events (AEs) and postoperative pulmonary complications (PPCs).

METHODS

LAS VEGAS (Local Assessment of Ventilatory Management During General Anesthesia for Surgery) was a prospective international 1-week study that enrolled adult patients undergoing surgical procedures with general anaesthesia and mechanical ventilation in 146 hospitals across 29 countries. Surgeries were defined as occurring during 'daytime' when induction of anaesthesia was between 8:00 AM and 7:59 PM, and as 'night-time' when induction was between 8:00 PM and 7:59 AM.

RESULTS

Of 9861 included patients, 555 (5.6%) underwent surgery during night-time. The proportion of patients who developed intraoperative AEs was higher during night-time surgery in unmatched (43.6% vs 34.1%; P<0.001) and propensity-matched analyses (43.7% vs 36.8%; P=0.029). PPCs also occurred more often in patients who underwent night-time surgery (14% vs 10%; P=0.004) in an unmatched cohort analysis, although not in a propensity-matched analysis (13.8% vs 11.8%; P=0.39). In a multivariable regression model, including patient characteristics and types of surgery and anaesthesia, night-time surgery was independently associated with a higher incidence of intraoperative AEs (odds ratio: 1.44; 95% confidence interval: 1.09-1.90; P=0.01), but not with a higher incidence of PPCs (odds ratio: 1.32; 95% confidence interval: 0.89-1.90; P=0.15).

CONCLUSIONS

Intraoperative adverse events and postoperative pulmonary complications occurred more often in patients undergoing night-time surgery. Imbalances in patients' clinical characteristics, types of surgery, and intraoperative management at night-time partially explained the higher incidence of postoperative pulmonary complications, but not the higher incidence of adverse events.

CLINICAL TRIAL REGISTRATION

NCT01601223.

The feasibility of using 'bring your own device' (BYOD) technology for electronic data capture in multicentre medical audit and research

Large-scale audit and research projects demand robust, efficient systems for accurate data collection, handling and analysis. We utilised a multiplatform 'bring your own device' (BYOD) electronic data collection app to capture observational audit data on theatre efficiency across seven hospital Trusts in South Yorkshire in June-August 2013. None of the participating hospitals had a dedicated information governance policy for bring your own device. Data were collected by 17 investigators for 392 individual theatre lists, capturing 14,148 individual data points, 12, 852 (91%) of which were transmitted to a central database on the day of collection without any loss of data. BYOD technology enabled accurate collection of a large volume of secure data across multiple NHS organisations over a short period of time. Bring your own device technology provides a method for collecting real-time audit, research and quality improvement data within healthcare systems without compromising patient data protection.

Patients with faecal peritonitis admitted to European intensive care units: an epidemiological survey of the GenOSept cohort

INTRODUCTION

Faecal peritonitis (FP) is a common cause of sepsis and admission to the intensive care unit (ICU). The Genetics of Sepsis and Septic Shock in Europe (GenOSept) project is investigating the influence of genetic variation on the host response and outcomes in a large cohort of patients with sepsis admitted to ICUs across Europe. Here we report an epidemiological survey of the subset of patients with FP.

OBJECTIVES

To define the clinical characteristics, outcomes and risk factors for mortality in patients with FP admitted to ICUs across Europe.

METHODS

Data was extracted from electronic case report forms. Phenotypic data was recorded using a detailed, quality-assured clinical database. The primary outcome measure was 6-month mortality. Patients were followed for 6 months. Kaplan-Meier analysis was used to determine mortality rates. Cox proportional hazards regression analysis was employed to identify independent risk factors for mortality.

RESULTS

Data for 977 FP patients admitted to 102 centres across 16 countries between 29 September 2005 and 5 January 2011 was extracted. The median age was 69.2 years (IQR 58.3-77.1), with a male preponderance (54.3%). The most common causes of FP were perforated diverticular disease (32.1%) and surgical anastomotic breakdown (31.1%). The ICU mortality rate at 28 days was 19.1%, increasing to 31.6% at 6 months. The cause of FP, pre-existing co-morbidities and time from estimated onset of symptoms to surgery did not impact on survival. The strongest independent risk factors associated with an increased rate of death at 6 months included age, higher APACHE II score, acute renal and cardiovascular dysfunction within 1 week of admission to ICU, hypothermia, lower haematocrit and bradycardia on day 1 of ICU stay.

CONCLUSIONS

In this large cohort of patients admitted to European ICUs with FP the 6 month mortality was 31.6%. The most consistent predictors of mortality across all time points were increased age, development of acute renal dysfunction during the first week of admission, lower haematocrit and hypothermia on day 1 of ICU admission.

Intelligent model-based advisory system for the management of ventilated intensive care patients: Hybrid blood gas patient model

Arterial blood gas (ABG) analyses are essential for assessing the acid-base status and guiding the adjustment of mechanical ventilation in critically ill patients. Conventional ABG sampling requires repeated arterial punctures or the insertion of an arterial catheter causing pain, haemorrhage and thrombosis to the patients. Less invasive and non-invasive blood gas analysers, with a technology still in transition, have offered some promise in the recent years. SOPAVent (Simulation of Patients under Artificial Ventilation) is a five compartment blood gas model which captures the basic features of respiratory physiology and gas exchange in the human lungs. It uses ventilator settings and routinely monitored physiological parameters as inputs to produce steady-state estimates of the patient's ABG. This paper overviews the original SOPAVent model and presents an improved data-driven hybrid model that is patient-specific and gives continuous and totally non-invasive ABG predictions. The model has been comprehensively tested in simulations and validated using recorded measurements of ABG and ventilator parameters from ICU patients.

Intra-abdominal pressure measurement: validation of intragastric pressure as a measure of intra-abdominal pressure

BACKGROUND

The diagnosis of abdominal compartment syndrome depends upon the demonstration of an elevated intra-abdominal pressure (IAP). Direct measures of IAP are impractical in the critical care unit; intravesical pressure (IVP) and intragastric pressure (IGP) should represent acceptable surrogate measures. IVP is the preferred measure of IAP in critical care. We considered that IGP represents a practical alternative. The objective of this preliminary study was to observe the relationship between IGP and IAP.

METHODS

After Institutional Ethics Board approval, 29 patients having elective laparoscopic surgery were recruited. IAP was measured directly via the abdominal trochar. This was compared with IGP measured via a commercial balloon catheter placed into the stomach.

RESULTS

Measured IGP was always more positive than IAP; both showed linear correlation (r2>0.9). When IGP was calibrated against IAP, an estimated difference between the IGP and IAP of+/-2.5 mm Hg for 95% of the measurements was seen.

CONCLUSIONS

The study demonstrates the strength of the relationship between IGP and IAP in normal individuals. Application of IGP measurement in the critical care patient is necessary to demonstrate its suitability for continuous IAP assessment.

International Programme for Resource Use in Critical Care (IPOC)--a methodology and initial results of cost and provision in four European countries

BACKGROUND

A standardized top-down costing method is not currently available internationally. An internally validated method developed in the UK was modified for use in critical care in different countries. Costs could then be compared using the World Health Organization's Purchasing Power Parities (WHO PPPs).

METHODS

This was an observational, retrospective, cross-sectional, multicentre study set in four European countries: France, UK, Germany and Hungary. A total of 329 adult intensive care units (ICUs) participated in the study.

RESULTS

The costs are reported in international dollars ($) derived from the WHO PPP programme. The results show significant differences in resource use and costs of ICUs over the four countries. On the basis of the sum of the means for the major components, the average cost per patient day in UK hospitals was $1512, in French hospitals $934, in German hospitals $726 and in Hungarian hospitals $280.

CONCLUSIONS

The reasons for such differences are poorly understood but warrant further investigation. This information will allow us to better adjust our measures of international ICU costs.

The use of iloprost in early pregnancy in patients with pulmonary arterial hypertension

In patients with pulmonary hypertension, pregnancy is associated with a high risk of maternal death. Such patients are counselled to avoid pregnancy, or if it occurs, are offered early interruption. Some patients, however, decide to continue with their pregnancy and others may present with symptoms for the first time whilst pregnant. Pulmonary vasodilator therapy provides a treatment option for these high-risk patients. The present study describes three patients with pulmonary arterial hypertension of various aetiologies who were treated with the prostacyclin analogue iloprost during pregnancy, and the post-partum period. Nebulised iloprost commenced as early as 8 weeks of gestation and patients were admitted to hospital between 24-36 weeks of gestation. All pregnancies were completed with a duration of between 25-36 weeks and all deliveries were by caesarean section under local anaesthetic. All patients delivered children free from congenital abnormalities, and there was no post-partum maternal or infant mortality. In conclusion, although pregnancy is strongly advised against in those with pulmonary hypertension, the current authors have achieved a successful outcome for mother and foetus with a multidisciplinary approach and targeted pulmonary vascular therapy.

Performance of breathing filters under wet conditions: a laboratory evaluation

INTRODUCTION

Heat and moisture exchangers in combination with a bacterial and viral filter (HMEF) are widely used during general anaesthesia. Excess patient secretions occluding the HMEF have been responsible for previous case reports of airway obstruction. A previous study suggested that differences in HMEF design might contribute to filter obstruction under wet conditions.

METHODS

We tested 14 types of HMEF under wet conditions to establish which design features contributed to HMEF obstruction. Incremental amounts of saline were added to each filter. The pressure across the filter was measured with an air flow of 60 litre min(-1).

RESULTS

We observed that saline added to the filter was often not easily visible to the casual observer. This concealment volume varied between filters. Ceramic hydrophobic pleated-membrane filters did not absorb saline and their resistance did not change. The composite filter where the moisture exchange component was either polyurethane foam or cellulose absorbed saline and contributed to a rise in resistance of 70-480% with the higher value more typical of the cellulose-paper-based HMEF.

CONCLUSION

The ideal HMEF for use during general anaesthesia should prevent the passage of viral, bacterial and prion material, should provide this filtration performance even under wet conditions, should supplement humidification of the inspired air and anaesthetic gases and should not increase respiratory work. We have identified large variations in HMEF performance under wet conditions. Users should be aware of performance variation in HMEFs and use a filter suited to the intended application.

Adaptive ventilator FiO2 advisor: use of non-invasive estimations of shunt

A non-invasive and simple method of parameter estimation has been developed for the model-based decision support of the artificial ventilation in intensive care units. The parameter concerned was the respiratory shunt. Originally, the shunt had to be estimated using a numerical algorithm, which was slow and unreliable. The estimation process also required the knowledge of other parameters, whose values could only be obtained using invasive monitoring equipment. In this paper, the respiratory index is used for the shunt estimation. A linear regression model and a non-linear adaptive neuro-fuzzy inference system (ANFIS) model were used to describe the relationship between the respiratory index and the shunt. The shunts estimated using these models were then used to calculate the fractional inspired oxygen needed to attain the target arterial oxygen level of the model patient. The advisor also utilises population median values of the cardiac index and oxygen consumption index. This alleviates the need for invasive monitoring. In a simulation study, the mean squared error of the control using the ANFIS model was 0.75 kPa2 compared to 2.06 kPa2 using the linear regression model. Therefore, the performance of the FiO2 advisor was better when the shunt was estimated using the non-linear ANFIS model.

The cost of a hospital ward in Europe: is there a methodology available to accurately measure the costs?

Costing health care services has become a major requirement due to an increase in demand for health care and technological advances. Several studies have been published describing the computation of the costs of hospital wards. The objective of this article is to examine the methodologies utilised to try to describe the basic components of a standardised method, which could be applied throughout Europe. Cost measurement however is a complex matter and a lack of clarity exists in the terminology and the cost concepts utilised. The methods discussed in this review make it evident that there is a lack of standardized methodologies for the determination of accurate costs of hospital wards. A standardized costing methodology would facilitate comparisons, encourage economic evaluation within the ward and hence assist in the decision-making process with regard to the efficient allocation of resources.

Sleep disruption in critically ill patients--pharmacological considerations

Sleep disturbances are common in critically ill patients and contribute to morbidity. Environmental factors, patient care activities and acute illness are all potential causes of disrupted sleep. Additionally, it is important to consider drug therapy as a contributing factor to this adverse experience, which patients perceive as particularly stressful. Sedative and analgesic combinations used to facilitate mechanical ventilation are among the most sleep disruptive drugs. Cardiovascular, gastric protection, anti-asthma, anti-infective, antidepressant and anticonvulsant drugs have also been reported to cause a variety of sleep disorders. Withdrawal reactions to prescribed and occasionally recreational drugs should also be considered as possible triggers for sleep disruption. Tricyclic antidepressants and benzodiazepines are commonly prescribed in the treatment of sleep disorders, but have problems with decreasing slow wave and rapid eye movement sleep phases. Newer non-benzodiazepine hypnotics offer little practical advantage. Melatonin and atypical antipsychotics require further investigation before their routine use can be recommended.

Rule-base derivation for intensive care ventilator control using ANFIS

In recent years, much research has been done on the use of fuzzy systems in medicine. The fuzzy rule-bases have usually been derived after extensive discussion with the clinical experts. This takes a lot of time from the clinical experts and the knowledge engineers. This paper presents the use of the adaptive neuro-fuzzy inference system (ANFIS) in rule-base derivation for ventilator control. The change of the inspired fraction of oxygen (FiO(2)) advised by eight clinical experts responding to 71 clinical scenarios was recorded. ANFIS and a multilayer perceptron (MLP) were then used to model the relationship between the inputs (the arterial oxygen tension (PaO(2)), FiO(2) and the positive end-expiratory pressure (PEEP) level) and the change in FiO(2) suggested. Compared to a previous fuzzy advisor (FAVeM), both the ANFIS and the MLP were found to correlate with the clinicians' decision better (correlation coefficient of 0.694 and 0.701, respectively compared to 0.630). A formerly developed model-based radial basis network advisor (RBN-MB) was used for comparison. Closed-loop simulations showed that the ANFIS, MLP and the RBN-MB's performance were comparable to the clinicians' performance (correlation coefficients of 0.852, 0.962 and 0.787, respectively). The FAVeM's performance differed from the clinicians' performance (correlation coefficient of 0.332) but the resulting PaO(2) was still within safety limits.

Hyperpolarized 3-helium MR imaging of the lungs: testing the concept of a central production facility

The aim of this study was to test the feasibility of a central production facility with distribution network for implementation of hyperpolarized 3-helium MRI. The 3-helium was hyperpolarized to 50-65% using a large-scale production facility based at a university in Germany. Using a specially designed transport box, containing a permanent low-field shielded magnet and dedicated iron-free glass cells, the hyperpolarized 3-helium gas was transported via airfreight to a university in the UK. At this location, the gas was used to perform in vivo MR experiments in normal volunteers and patients with chronic obstructive lung diseases. Following initial tests, the transport (road-air-road cargo) was successfully arranged on six occasions (approximately once per month). The duration of transport to imaging averaged 18 h (range 16-20 h), which was due mainly to organizational issues such as working times and flight connections. During the course of the project, polarization at imaging increased from 20% to more than 30%. A total of 4 healthy volunteers and 8 patients with chronic obstructive pulmonary disease were imaged. The feasibility of a central production facility for hyperpolarized 3-helium was demonstrated. This should enable a wider distribution of gas for this novel technology without the need for local start-up costs.

Calf compartment pressures in the Lloyd-Davies position: a cause for concern?

Lower limb compartment syndrome is a rare consequence of surgery where the lithotomy position is maintained for several hours. The aim of this study was to observe the effect of the lithotomy position on lower limb compartment pressure and blood flow to the lower limb in surgical patients having colorectal procedures. We prospectively studied 23 patients undergoing colorectal surgery requiring the lithotomy position and recorded lower limb compartment pressure, and the blood pressure in the upper and lower limbs. The lithotomy position led to a significant (p < 0.001) fall in blood pressure to the lower limb from 87 (SD 16) mmHg to 67.9 (SD 12) mmHg and a significant (p < 0.001) rise in lower limb compartment pressure from 13 (SD 7) mmHg to 31 (SD 12) mmHg. These two effects compromise blood flow to the lower limb in long surgical procedures where the lithotomy position is required. Intermittently lowering the legs and restoring blood flow may prevent compartment syndrome developing.

The effect of propofol on airway pressures generated by magnetic stimulation of the phrenic nerves

OBJECTIVES

To assess the effect of propofol on the change in airway pressure produced by diaphragmatic contraction.

DESIGN AND SETTING

Prospective, controlled study in patients anaesthetised with propofol in a university hospital.

PATIENTS AND METHODS

We stimulated the phrenic nerves before and immediately after induction of anaesthesia in 11 subjects, using a pair of 43-mm mean diameter double magnetic coils and measured the change in airway pressure at the mouth (TwPmo) produced by the resulting diaphragmatic contraction. Supramaximality of stimulation was confirmed with electromyogram and pressure measurements. We recorded the change in Resting End Expiratory Position (REEP) using a spirometer. We applied an approximate correction for the effect of lung volume on the amplitude of twitch pressure produced by diaphragmatic contraction.

INTERVENTION

Following the initial stimulations, the patients were anaesthetised with a propofol infusion. Once stable, repeat measurements were made.

MEASUREMENTS AND RESULTS

Following induction, REEP fell by mean 0.3 l standard deviation (SD) 0.2 l. TwPmo fell by mean 14.2% SD 14.0% ( P = 0.01), mean 22.3% SD 11.7% corrected ( P < 0.001). Twitch transdiaphragmatic pressure fell by 18.1% and 20.0% (25.8% and 27.7% corrected) in two further subjects studied with oesophageal and gastric balloon catheters.

CONCLUSION

Propofol does reduce the effectiveness with which diaphragmatic contraction produces changes of pressure in the airway.

Assessment of diaphragm function after stellate ganglion block using magnetic stimulation

Stellate ganglion block is a procedure frequently used for the management of patients with chronic sympathetically mediated pain affecting the arm, neck or head. We studied the effect of stellate ganglion block on ipsilateral phrenic nerve function, and hence diaphragmatic strength, in 11 adult patients with chronic sympathetically mediated pain. Pre- and post-block forced vital capacity (FVC) measurements were recorded using a pneumotachograph and a Magstim nerve stimulator was used to generate pre- and post-block twitch mouth pressures (P(TWM)). This device can be used to stimulate the phrenic nerves and hence the diaphragm. The resulting change in airway pressure was measured at the mouth and has previously been shown to reflect diaphragm strength. There was no statistically significant difference in FVC or P(TWM) pre- or post stellate ganglion block. In conclusion, a stellate ganglion block has no adverse effect on ipsilateral phrenic nerve function or diaphragm strength in healthy adult patients.

Tracheal tube pressure change during magnetic stimulation of the phrenic nerves as an indicator of diaphragm strength on the intensive care unit

Diaphragm strength can be assessed from twitch gastric (TwPgas), twitch oesophageal (TwPoes), and twitch transdiaphragmatic pressure (TwPdi) in response to phrenic nerve stimulation. This requires the passage of balloon catheters, which may be difficult. Changes in pressure measured at the mouth during phrenic nerve stimulation avoid the need for balloon catheters. We hypothesized that pressures measured at the tracheal tube during phrenic stimulation, could also reflect oesophageal pressure change as a result of isolated diaphragmatic contraction and, therefore, reflect diaphragm strength. We aimed to establish the relationship between twitch tracheal tube pressure (TwPet), TwPoes, and TwPdi in patients in the supine and sitting positions. The phrenic nerves were stimulated magnetically bilaterally, in 14 ICU patients while supine and on another occasion while sitting up at 45 degrees. In the sitting position mean TwPoes was 9.1 cm H2O and TwPet 11.3 cm H2O (mean(SD) difference -2.2 (SD 1.5)). In the supine position mean TwPoes was 8.1 cm H2O and TwPet 9.9 cm H2O (mean difference -1.8 (2.2)). The difference between TwPoes and TwPet was less at low twitch amplitude; less than +/- 1 cm H2O below a mean twitch height of 8 cm H2O supine and 10 cm H2O sitting. Sitting TwPet was related to TwPoes r2=0.93 and TwPdi r2=0.65 (P<0.01). Supine TwPet was related to TwPoes r2=0.84 and TwPdi r2=0.83 (P<0.01). The mean within occasion coefficient of variation while sitting was TwPet=13.3%, TwPoes=13.9%, TwPdi=11.2%, and supine TwPet=11.6%, TwPoes=14.6%, TwPdi=11.8%. We conclude that TwPet reflects TwPoes during diaphragmatic stimulation and is worthy of further study to establish its place as a guide to the presence of respiratory muscle strength and fatigue.

Compartment syndrome associated with the Lloyd Davies position. Three case reports and review of the literature

The Lloyd Davies position was developed to facilitate access to the pelvis for gynaecological, urological and colorectal procedures. Previous case reports have demonstrated that prolonged adoption (> 4 h) of this position has been associated with the development of bilateral compartment syndrome of the calves. All three patients reported here suffered severe bilateral calf pain despite the use of thoracic epidurals. All three cases required three-compartment fasciotomies and, 6 months after surgery, were all still severely disabled as a consequence of the compartment syndrome. These case reports stress the dangers of use of the Lloyd Davies position for prolonged procedures and demonstrate that some patients are at risk after relatively short periods (< 3 h). Previous case reports and clinical studies have focused on the effect of limb elevation in stirrups on the arterial pressure in the lower limb. We review the pathophysiology of compartment syndrome and consider factors other than a decrease in arterial pressure that may predispose to compartment syndrome during adoption of the Lloyd Davies position.

Air flow resistance of three heat and moisture exchanging filter designs under wet conditions: implications for patient safety

Heat and moisture exchanging filters (HMEFs) can be blocked by secretions. We have studied HMEF performance under wet conditions to see which particular design features predispose to this complication. Dar Hygrobac-S (composite felt filter and cellulose exchanger), Dar Hygroster (composite pleated ceramic membrane and cellulose exchanger) and Pall BB22-15 (pleated ceramic membrane) HMEFs were tested. Saline retention, saline concealment, and changes in air flow resistance when wet were assessed. The cellulose exchanger in the composite Hygrobac-S and Hygroster retained saline, producing a 'tampon' effect, associated with bi-directional air flow resistances in excess of the international standard of a 5 cm H(2)O pressure drop at 60 litre min(-1) air flow. Furthermore, high air flow resistances occurred before free saline was apparent within the transparent filter housing. The pleat only BB22-15 showed a significant increase in expiratory air flow resistance, but only after the presence of saline was apparent. These data imply that composite HMEFs with cellulose exchangers are more likely to block or cause excessive work of breathing as a result of occult accumulation of patient secretions than pleat only HMEFs.

Intra-operative and post-operative hypercapnia leading to delayed respiratory failure associated with transanal endoscopic microsurgery under general anaesthesia

We present an unusual case of hypercapnia and surgical emphysema during transanal endoscopic microsurgery, which led to delayed post-operative ventilatory failure. The hypercapnia and surgical emphysema were secondary to rectal insufflation with carbon dioxide used to facilitate visualization and resection of a rectal tumour. Despite a return to wakefulness after surgery, the patient's level of consciousness deteriorated in the recovery area as a result of hypercapnia. The PaCO2 rose to 16.8 kPa because of absorption of carbon dioxide from the surgical emphysema. On close examination, surgical emphysema was identified in unusual areas, including the anterior abdominal wall, both loins, both groins and the left thigh. Reventilation was required until these unusual carbon dioxide stores had dissipated. We discuss the need for prolonged post-operative vigilance in patients with surgical emphysema secondary to carbon dioxide insufflation, and the risk of delayed ventilatory failure.

Acute right-to-left inter-atrial shunt; an important cause of profound hypoxia

Three patients presented to our intensive care unit over a 3-yr period with profound hypoxia resulting from acute right-to-left inter-atrial shunt (RLIAS). Patient 1 was a 67-yr-old male with an atrial septal defect who became hypoxic and developed the rare sign of platypnoea following elective repair of an abdominal aortic aneurysm (breathlessness made worse when upright and relieved by lying flat). Patient 2 was a 38-yr-old female who developed platypnoea and hypoxia secondary to a patent foramen ovale (PFO) and pericardial effusion. Patient 3 was a 46-yr-old male with a PFO who developed hypoxia without platypnoea because of multiple pulmonary emboli following right hemicolectomy. These case reports illustrate the need to consider RLIAS as a cause of hypoxia of sudden onset. Early use of bubble contrast echocardiography is indicated.

Respiratory muscle activity in patients with COPD walking to exhaustion with and without pressure support

The function of the diaphragm and other respiratory muscles during exercise in chronic obstructive pulmonary disease (COPD) remains controversial and few data exist regarding respiratory muscle pressure generation in this situation. The inspiratory pressure/time products of the oesophageal and transdiaphragmatic pressure, and the expiratory gastric pressure/time product during exhaustive treadmill walking in 12 patients with severe COPD are reported. The effect of noninvasive positive pressure ventilation during treadmill exercise was also examined in a subgroup of patients (n=6). During free walking, the inspiratory pressure/time products rose early in the walk and then remained level until the patients were forced to stop because of intolerable dyspnoea. In contrast, the expiratory gastric pressure/time product increased progressively throughout the walk. When patients walked the same distance assisted by noninvasive positive pressure ventilation, a substantial reduction was observed in the inspiratory and expiratory pressure/time products throughout the walk. When patients walked with positive pressure ventilation for as long as they could, the pressure/time products observed at exercise cessation were lower than those observed during exercise cessation after free walking. It is concluded that, in severe chronic obstructive pulmonary disease, inspiratory muscle pressure generation does not increase to meet the demands imposed by exhaustive exercise, whereas expiratory muscle pressure generation rises progressively. Inspiratory pressure support was shown to substantially unload all components of the respiratory muscle pump.

Respiratory muscle strength in Cushing's syndrome

The effect of Cushing's syndrome on respiratory muscle strength is unknown. Therefore, we studied 10 consecutive patients with severe Cushing's syndrome. The respiratory muscles were assessed using maximal inspiratory and expiratory mouth pressures (MIP, MEP), maximal sniff transdiaphragmatic pressures (max sniff Pdi), and maximal sniff esophageal pressures (max sniff Pes). Maximal quadricep strength was also assessed. The patients demonstrated an overall mean MIP 92 cm H(2)O, SD 19 (mean 105% of predicted; SD, 23%), mean MEP 134 cm H(2)O, SD 35 (mean 99% of predicted; SD, 25%), mean max sniff Pdi 107 cm H(2)O, SD 12 (mean 78% of predicted; SD, 10%) and mean max sniff Pes of 92 cm H(2)O, SD 11 (mean 92% of predicted; SD, 11%). Quadriceps muscle strength was reduced in all 10 patients: mean 26 kg, SD 9 (mean 49% of predicted strength, SD 21%). Respiratory muscle weakness was not found, despite the presence of severe quadriceps impairment. We conclude that major weakness of the respiratory muscles is not usual in Cushing's syndrome.

Maximum rate of change in oesophageal pressure assessed from unoccluded breaths: an option where mouth occlusion pressure is impractical

The mouth occlusion pressure 100 ms after onset of inspiration (P0.1) is considered a clinically useful measure of the combined output of the respiratory centre and muscle pump. However, theoretical and practical difficulties can arise when using P0.1 in the assessment of patients with severe chronic obstructive pulmonary disease (COPD). It was hypothesized that the maximum rate of change in oesophageal pressure (dPoes,max/dt) may be an alternative to P0.1. To test this hypothesis P0.1 was compared with mean dPoes,max/dt measured from neighbouring unoccluded breaths in five normal subjects during CO2 rebreathing. In all subjects a close correlation was found between both dPoes,max/dt and P0.1 and carbon dioxide tension (PCO2). In six patients with severe COPD performing exhaustive treadmill walks, dPoes,max/dt was found to increase progressively with walking time. Mean dPoes,max/dt at the start was 6.2 cmH2O x 100 ms(-1) and at the finish was 18.7 cmH2O x 100 ms(-1) (p<0.03). In conclusion, the maximum rate of change in oesophageal pressure measured from unoccluded breaths could be an alternative in circumstances where it is not feasible to use measurements of the mouth occlusion pressure 100 ms after onset of inspiration.

Effects of temperature on phrenic nerve and diaphragmatic function during cardiac surgery

We have studied the effects of whole body cooling on phrenic nerve and diaphragmatic function in 26 patients using magnetic stimulation of the phrenic nerves with a pair of Magstim 200 HP stimulator coils during cardiopulmonary bypass. The diaphragmatic electromyogram in response to magnetic pulses was recorded with needle electrodes at two temperatures, approximately 31 degrees C (cold) and approximately 36 degrees C (warm) during the cooling or rewarming phase of hypothermic cardiopulmonary bypass. This 5 degrees C temperature change was associated with clear changes in the evoked electromyographical response of the diaphragm. Median latency between stimulus and electromyographic response was 10.1 (range 8.0-11.8) ms during cold and 8.3 (5.9-10.2) ms during warm stimulation (P < 0.001). Median duration of the muscle compound action potential was prolonged and its amplitude reduced in cold compared with warm stimulations (P < 0.01). These effects were enhanced by application of ice slush to the heart. We conclude that diaphragmatic function may be affected by mild hypothermia after cardiac surgery.

Simulation of cough in man by magnetic stimulation of the thoracic nerve roots

Normal cough requires abdominal muscle contraction. We have previously reported contraction of the abdominal muscles elicited by a single percutaneous magnetic stimulation of the thoracic nerve roots. We hypothesized that paired magnetic twitches could generate sufficient tension in the abdominal muscles to simulate cough. Therefore, six normal subjects were stimulated at the T10 intervertebral level in the seated position. We measured the gastric pressure elicited by paired magnetic stimuli (pTw Pga) with interstimulus intervals in the range of 10 ms (100 Hz) to 999 ms (1 Hz). In the second part of the study we evaluated paired stimuli (at the frequency found to produce the greatest response) using a valve to simulate the function of the glottis; the valve was arranged such that it opened once mouth pressure exceeded a predetermined threshold. Mean pTw Pga during stimulation for the 6 subjects was 74 cm H2O (range, 30-109), and mean peak flow was 209 L/min (range, 128-345 L/min). These values were increased if the subject took a prior inspiration or had previously made a vigorous expiratory effort. Comparable values for a maximal natural cough were 212 cm H2O and 649 L/min. We conclude that paired magnetic thoracic nerve root stimulation produces gastric pressure and expiratory flow of an order of magnitude comparable to a natural cough.

Cervical magnetic stimulation of the phrenic nerves in bilateral diaphragm paralysis

Cervical magnetic stimulation (CMS) produces a greater twitch transdiaphragmatic pressure (TwPdi) than electrical stimulation. This may be because CMS produces rib cage muscle activation, thus producing an inspiratory action independent of the diaphragm. Alternatively, CMS could merely stiffen the rib cage, allowing the diaphragm to act efficiently, by contracting against a stable rib cage. To examine these two hypotheses we studied five patients with isolated bilateral diaphragm paralysis using CMS and bilateral electrical phrenic stimulation (BES). TwPdi, twitch esophageal pressure (TwPes), and twitch gastric pressure (TwPgas) were measured. We also assessed maximal sniff esophageal and transdiaphragmatic pressures (SnPes) (SnPdi), maximal inspiratory and expiratory mouth pressures (MIP) (MEP), and fall in VC on moving from an upright to a supine position. Respiratory muscle strength tests were consistent with bilateral diaphragm paralysis, and the MEPs confirmed normal expiratory muscle function. The patients were able to generate a mean SnPes of -30 cm H2O, mainly because of inspiratory activity of rib cage and neck muscles. However, TwPdi and TwPes during both CMS and BES were close to zero. We conclude that in our patients with diaphragm paralysis caused by neuralgic amyotrophy, CMS stiffens the rib cage but does not have an inspiratory action independent of the diaphragm.

Diaphragm performance during maximal voluntary ventilation in chronic obstructive pulmonary disease

In normal subjects 2 min of maximal voluntary hyperventilation results in failure of tension generation and low-frequency fatigue of the diaphragm. Patients with severe chronic obstructive pulmonary disease (COPD) do not develop diaphragm fatigue during exhaustive treadmill exercise despite excessive inspiratory muscle loading and we hypothesized that they might be relatively resistant to the development of diaphragm fatigue during maximal ventilation. In six patients with severe COPD (mean FEV1 0.671) we therefore loaded the diaphragm using 2 min of maximal isocapnic ventilation (MIV). Initial mean ventilation was 28.6 L/min and diaphragm pressure-time product (PTPdi) 602 cm H2O x s/min; these values were sustained throughout MIV without significant decline. Mean twitch transdiaphragmatic pressure (Tw Pdi) was 19.7 cm H2O 25 min after a control run and 20.5 cm H2O at the same time after MIV [corrected]. Compared with normal subjects previously studied in our laboratory (Hamnegard, C.-H., et al. Eur. Respir. J. 1996;9:241-247) the reduction in PTPdi was disproportionately greater than the reduction in Tw Pdi. We conclude that, unlike normal subjects, 2 min of MIV causes neither failure of diaphragm performance nor low-frequency diaphragm fatigue in patients with severe COPD. It is likely that the diaphragm makes a relatively limited contribution to the generation of maximal levels of ventilation in severe COPD.

Clinical assessment of diaphragm strength by cervical magnetic stimulation of the phrenic nerves

BACKGROUND

Accurate assessment of diaphragm strength can be difficult. Transdiaphragmatic pressure (PDI) measurements during volitional manoeuvres are useful but it may be difficult to ensure maximum patient effort. Magnetic stimulation of the phrenic nerves is easy to perform and the results are reproducible in normal subjects. The purpose of the present study was to evaluate the usefulness of magnetic stimulation of the phrenic nerves in the assessment of diaphragm weakness in patients.

METHODS

Sixty-six patients referred for assessment of respiratory muscle strength and 23 normal subjects were studied. Twitch PDI (TwPDI) following magnetic stimulation of the phrenic nerves and sniffPDI were obtained in all individuals. TWPDI following bilateral electrical stimulation of the phrenic nerves was also obtained in eight patients.

RESULTS

Mean (SD) TwPdi for the normal subjects was 31 (6) cm H2O and 18 (11) cm H2O for the patients. TwPDI and sniffPDI were correlated (r = 0.77). Seven of the 37 patients (19%) with a reduced sniffPDI had a TwPDI within the normal range whereas two of the 32 patients (6%) with a reduced TwPDI had a normal sniffPDI. TwPDI was similar with magnetic and electrical stimulation.

CONCLUSIONS

TwPDI following magnetic stimulation of the phrenic nerves is a clinically useful measurement when assessing diaphragm weakness.

Diaphragm strength in chronic obstructive pulmonary disease

The diaphragm is normally the main inspiratory muscle and diaphragm strength in chronic obstructive pulmonary disease (COPD) is therefore of interest. We assessed diaphragm strength in 20 patients with severe stable COPD (mean FEV1 0.61, mean thoracic gas volume [Vtg] 5.31) and seven normal control subjects, measuring both maximal sniff transdiaphragmatic pressure (sniff Pdi(max)) and twitch transdiaphragmatic pressure (Tw Pdi) elicited by cervical magnetic stimulation (CMS) of the phrenic nerve roots at FRC. Acute-on-chronic hyperinflation was examined in four patients. Mean Tw Pdi in patients and control subjects was 18.5 cm H2O and 25.4 cm H2O, respectively (p < 0.01), and mean sniff Pdi was 81.9 cm H2O and 118 cm H2O, respectively (p < 0.001). Reduction in mean intrathoracic pressures was more marked; twitch esophageal pressure (Tw Pes) was -7.3 cm H2O and -16.3 cm H2O, respectively (p < 0.001) and sniff Pes was -67 cm H2O and -97.8 cm H2O (p < 0.001). During acute-on-chronic hyperinflation there was a linear negative correlation of Tw Pdi with increasing lung volume of 3.5 cm H2O/L. The ability of the diaphragm to generate transdiaphragmatic, and particularly a negative intrathoracic, pressure is reduced in COPD and these changes are exaggerated with acute-on-chronic hyperinflation.

Inspiratory pressure support reduces slowing of inspiratory muscle relaxation rate during exhaustive treadmill walking in severe COPD

When patients with COPD walk to a state of intolerable dyspnea, there is excessive inspiratory muscle loading, as evidenced by slowing of the maximum relaxation rate of the inspiratory muscles, measured from esophageal pressure during a sniff (Sn Pes MRR). In this setting, inspiratory pressure support (IPS) delivered via an orofacial mask increases walking distance and reduces dyspnea, but the mechanism by which this benefit is achieved remains unclear. In this study we compared Sn Pes MRR after equidistant treadmill walking in six men with severe COPD (mean FEV1: 0.6 L, 22% predicted). After the free walk there was a mean slowing of Sn Pes MRR of 41% (p < 0.03). After the IPS-assisted walks, the slowing of Sn Pes MRR was 20% of baseline; this was significantly less than after the free walk (p < 0.05). Four subjects performed shorter walks; after free walks of one third and two thirds of maximum distance, the mean slowing of Sn Pes MRR was 23% and 28%, respectively. We conclude that when patients with COPD walk to exhaustion, IPS reduces slowing of inspiratory muscle MRR, and that this represents a considerable unloading of the inspiratory muscles. The magnitude of the reduction is approximately the same as reducing the distance walked by two thirds.

Bilateral magnetic stimulation of the phrenic nerves from an anterolateral approach

We investigated whether bilateral magnetic stimulation of the phrenic nerves from an anterolateral approach (BAMPS) could combine the reproducibility and ease of use of cervical magnetic stimulation (CMS) with the specificity of bilateral electrical stimulation (BES) and whether it could be used in supine subjects. We placed two double 43-mm coils over the phrenic nerves in the neck. BAMPS produced supramaximal phrenic stimulation by electromyogram (EMG) assessment in six of seven subjects. There was no significant difference in the twitch gastric pressure/twitch esophageal pressure ratio (twitch Pgas/Pes) between BAMPS (1.2) and BES (1.3). Both differed from CMS (0.9, p < 0.001). The effect of a change in posture on twitch transdiaphragmatic pressure (TwPdi) and Pgas/Pes ratio was the same for BAMPS and BES. In normal subjects and patients BAMPS correlated significantly with BES (r = 0.97), maximal sniffs (r = 0.85), and CMS (r = 0.92). The mean difference between BAMPS and BES was 0.3 cm H2O (SD = 2.3). Two-minute maximal isocapnic ventilation produced a 19% fall in TwPdi elicited by BAMPS. BAMPS is easy, well tolerated and can be used in the supine subject. TwPdi and partitioning of Pes and Pgas were very close for BAMPS and BES, suggesting similar specificity for the diaphragm.

Abdominal muscle fatigue after maximal ventilation in humans

Abdominal muscles are the principal muscles of active expiration. To investigate the possibility of abdominal muscle low-frequency fatigue after maximal ventilation in humans, we stimulated the nerve roots supplying the abdominal muscles. We used a magnetic stimulator (Magstim 200) powering a 90-mm circular coil and studied six normal subjects. To assess the optimum level of stimulation and posture, we stimulated at each intervertebral level between T7 and L1 in the prone, supine, and seated positions. At T10, we used increasing power outputs to assess the pressure-power relationship. Care was taken to avoid muscle potentiation. Twitch gastric pressure (Pga) was recorded with a balloon-tipped catheter. Mean (+/-SD) baseline twitch Pga measured with the subjects in the prone position at T10 was 23.5 +/- 5.4 cmH2O. Within-occasion mean twitch Pga coefficient of variation was 4.6 +/- 1.1%. Twitch Pga was measured with the subjects in the prone position with stimulation over T10 before and after 2 min of maximal isocapnic ventilation (MIV). Twenty minutes after MIV, mean twitch Pga fell by 17 +/- 9.1% (P = 0.03) and remained low 90 min after MIV. We conclude that after maximal ventilation in humans there is a reduction of twitch Pga and, therefore, of low-frequency fatigue in abdominal muscles.

Anaesthetic management of labour in two patients with Klippel-Feil syndrome

Two patients with Type I Klippel-Feil syndrome presented at the antenatal clinic. The first patient, who suffered from sleep apnoea, was delivered of a healthy infant by vacuum extraction. The second, who was profoundly deaf and had marked kyphoscoliosis, developed pregnancy-induced hypertension and urinary tract infection and was delivered at 38 weeks by vacuum extraction. In both cases epidural analgesia was employed to allow pain relief during labour. Anaesthetic management of Klippel-Feil syndrome is discussed and the benefits of early anaesthetic assessment and continued involvement of senior anaesthetic and obstetric staff emphasized.