Identification of an optimal CPR chest compression protocol.

In this study, we used a high-fidelity integrated computational model of the respiratory and cardiovascular systems to investigate cardiopulmonary resuscitation (CPR) after cardiac arrest in a virtual healthy subject. For the purpose of this work, a newly developed thoracic model has been integrated to the current model, to study the influence of external chest compressions upon the arrested circulation during CPR. We evaluated the chest compression (CC) parameters, namely, end compression force, compression rate, and duty cycle to optimize the coronary perfusion pressure and the systolic blood pressure, using a genetic algorithm. While the sternal displacement associated with the CC force agreed with the ERC guidelines, the CC rate and duty cycle were respectively higher and lower than the ones recommended by the ERC guidelines. The effect of these CC parameters on cardiac output (CO) were also assessed. The end compression force was the parameter with the largest impact on CO, while the compression rate and duty cycle scarcely influence it.Relevance- Our results may aid in understanding the underlying pathophysiology of cardiac arrest and help guide research into the refinement of CPR strategies, without sacrificing animals or conducting clinical trials, which are difficult to undertake in crisis scenarios.

Computer simulation clarifies mechanisms of carbon dioxide clearance during apnoea.

BACKGROUND: Apnoeic oxygenation can come close to matching the oxygen demands of the apnoeic patient but does not facilitate carbon dioxide (CO2) elimination, potentially resulting in dangerous hypercapnia. Numerous studies have shown that high-flow nasal oxygen administration prevents hypoxaemia, and appears to reduce the rate of increase of arterial CO2 partial pressure (PaCO2), but evidence is lacking to explain these effects. METHODS: We extended a high-fidelity computational simulation of cardiopulmonary physiology to include modules allowing variable effects of: (a) cardiogenic oscillations affecting intrathoracic gas spaces, (b) gas mixing within the anatomical dead space, (c) insufflation into the trachea or above the glottis, and (d) pharyngeal pressure oscillation. We validated this model by reproducing the methods and results of five clinical studies on apnoeic oxygenation. RESULTS: Simulated outputs best matched clinical data for model selection of parameters reflecting: (a) significant effects of cardiogenic oscillations on alveoli, both in terms of strength of the effect (4.5 cm H2O) and percentage of alveoli affected (60%), (b) augmented gas mixing within the anatomical dead space, and (c) pharyngeal pressure oscillations between 0 and 2 cm H2O at 70 Hz. CONCLUSIONS: Cardiogenic oscillations, dead space gas mixing, and micro-ventilation induced by pharyngeal pressure variations appear to be important mechanisms that combine to facilitate the clearance of CO2 during apnoea. Evolution of high-flow oxygen insufflation devices should take advantage of these insights, potentially improving apnoeic gas exchange.

Primary blast lung injury simulator: a new computerised model.

Mathematical modelling and computational simulation are becoming increasingly important tools in many fields of medicine where in vivo studies are expensive, difficult or impractical. This is particularly the case with primary blast lung injury, and in this paper, we give a brief overview of mathematical models before describing how we generated our blast lung injury simulator and describe some early results of its use.

Validity and reliability of an objective structured assessment tool for performance of ultrasound-guided regional anaesthesia.

BACKGROUND: We examined the validity and reliability of the previously developed criterion-referenced assessment checklist (AC) and global rating scale (GRS) to assess performance in ultrasound-guided regional anaesthesia (UGRA). METHODS: Twenty-one anaesthetists' single, real-time UGRA procedures (total: 21 blocks) were assessed using a 22-item AC and a 9-item GRS scored on 3-point and 5-point Likert scales, respectively. We used one-way analysis of variance to compare the assessment scores between three groups (Group 1: ≤30 blocks in the preceding year; Group 2: 31-100; and Group 3: >100). The concurrent validity was evaluated using Pearson's correlation (r). We calculated Type A intra-class correlation coefficient using an absolute-agreement definition in two-way random effects model, and inter-rater reliability using an absolute agreement between raters. The inter-item consistency was assessed by Cronbach's α. RESULTS: The greater UGRA experience in the preceding year was associated with better AC [F (2, 18) 12.01; P<0.001] and GRS [F (2, 18) 7.44; P=0.004] scores. There was a strong correlation between the mean AC and GRS scores [r=0.73 (P<0.001)], and a strong inter-item consistency for AC (α=0.94) and GRS (α=0.83). The intra-class correlation coefficient (95% confidence interval) and inter-rater reliability (95% confidence interval) for AC were 0.96 (0.95-0.96) and 0.91 (0.88-0.95), respectively, and 0.93 (0.90-0.94) and 0.80 (0.74-0.86) for GRS. CONCLUSIONS: Both assessments differentiated between individuals who had performed fewer (≤30) and many (>100) blocks in the preceding year, supporting construct validity. It also established concurrent validity and overall reliability. We recommend that both tools can be used in UGRA assessment.

Peripheral nerve injury arising in anaesthesia practice.

In this article, we will discuss the pathophysiology of peripheral nerve injury in anaesthetic practice, including factors which increase the susceptibility of nerves to damage. We will describe a practical and evidence-based approach to the management of suspected peripheral nerve injury and will go on to discuss major nerve injury patterns relating to intra-operative positioning and to peripheral nerve blockade. We will review the evidence surrounding particular strategies to reduce the incidence of peripheral nerve injury during nerve blockade, including nerve localisation methods, timing of blocks, needle techniques and design, injection pressure-monitoring and local anaesthetic and adjunct choice.

Spinal cord injury arising in anaesthesia practice.

Spinal cord injury arising during anaesthetic practice is a rare event, but one that carries a significant burden in terms of morbidity and mortality. In this article, we will review the pathophysiology of spinal cord injury. We will then discuss injuries relating to patient position, spinal cord hypoperfusion and neuraxial techniques. The most serious causes of spinal cord injury - vertebral canal haematoma, spinal epidural abscess, meningitis and adhesive arachnoiditis - will be discussed in turn. For each condition, we draw attention to practical, evidence-based measures clinicians can undertake to reduce their incidence, or mitigate their severity. Finally, we will discuss transient neurological symptoms. Some cases of spinal cord injury during anaesthesia can be ascribed to anaesthesia itself, arising as a direct consequence of its conduct. The injury to a spinal nerve root by inaccurate and/or incautious needling during spinal anaesthesia is an obvious example. But in many cases, spinal cord injury during anaesthesia is not caused by, related to, or even associated with, the conduct of the anaesthetic. Surgical factors, whether direct (e.g. spinal nerve root damage due to incorrect pedicle screw placement) or indirect (e.g. cord ischaemia following aortic surgery) are responsible for a significant proportion of spinal cord injuries that occur concurrently with the delivery of regional or general anaesthesia.

Cohort study of preoperative blood pressure and risk of 30-day mortality after elective non-cardiac surgery.

BACKGROUND: Preoperative blood pressure (BP) thresholds associated with increased postoperative mortality remain unclear. We investigated the relationship between preoperative BP and 30-day mortality after elective non-cardiac surgery. METHODS: We performed a cohort study of primary care data from the UK Clinical Practice Research Datalink (2004-13). Parsimonious and fully adjusted multivariable logistic regression models, including restricted cubic splines for numerical systolic and diastolic BP, for 30-day mortality were constructed. The full model included 29 perioperative risk factors, including age, sex, comorbidities, medications, and surgical risk scale. Sensitivity analyses were conducted for age (>65 vs <65 years old) and the timing of BP measurement. RESULTS: A total of 251 567 adults were included, with 589 (0.23%) deaths within 30 days of surgery. After adjustment for all risk factors, preoperative low BP was consistently associated with statistically significant increases in the odds ratio (OR) of postoperative mortality. Statistically significant risk thresholds started at a preoperative systolic pressure of 119 mm Hg (adjusted OR 1.02 [95% confidence interval (CI) 1.01-1.02]) compared with the reference (120 mm Hg) and diastolic pressure of 63 mm Hg [OR 1.24 (95% CI 1.03-1.49)] compared with the reference (80 mm Hg). As BP decreased, the OR of mortality risk increased. Subgroup analysis demonstrated that the risk associated with low BP was confined to the elderly. Adjusted analyses identified that diastolic hypertension was associated with increased postoperative mortality in the whole cohort. CONCLUSIONS: In this large observational study we identified a significant dose-dependent association between low preoperative BP values and increased postoperative mortality in the elderly. In the whole population, elevated diastolic, not systolic, BP was associated with increased mortality.

Primary blast lung injury - a review.

Bomb or explosion-blast injuries are likely to be increasingly encountered as terrorist activity increases and pre-hospital medical care improves. We therefore reviewed the epidemiology, pathophysiology and treatment of primary blast lung injury. In addition to contemporary military publications and expert recommendation, an EMBASE and MEDLINE search of English speaking journals was undertaken using the medical subject headings (MeSHs) 'blast injury' and 'lung injury'. Review articles, retrospective case series, and controlled animal modelling studies published since 2000 were evaluated. 6-11% of military casualties in recent conflicts have suffered primary blast lung injury but the incidence increases to more than 90% in terrorist attacks occurring in enclosed spaces such as trains. The majority of victims require mechanical ventilation and intensive care management. Specific therapies do not exist and treatment is supportive utilizing current best practice. Understanding the consequences and supportive therapies available to treat primary blast lung injury are important for anaesthetists.

High PEEP in acute respiratory distress syndrome: quantitative evaluation between improved arterial oxygenation and decreased oxygen delivery.

BACKGROUND: Positive end-expiratory pressure (PEEP) is widely used to improve oxygenation and prevent alveolar collapse in mechanically ventilated patients with the acute respiratory distress syndrome (ARDS). Although PEEP improves arterial oxygenation predictably, high-PEEP strategies have demonstrated equivocal improvements in ARDS-related mortality. The effect of PEEP on tissue oxygen delivery is poorly understood and is difficult to quantify or investigate in the clinical environment. METHODS: We investigated the effects of PEEP on tissue oxygen delivery in ARDS using a new, high-fidelity, computational model with highly integrated respiratory and cardiovascular systems. The model was configured to replicate published clinical trial data on the responses of 12 individual ARDS patients to changes in PEEP. These virtual patients were subjected to increasing PEEP levels during a lung-protective ventilation strategy (0-20 cm H2O). Measured variables included arterial oxygenation, cardiac output, peripheral oxygen delivery, and alveolar strain. RESULTS: As PEEP increased, tissue oxygen delivery decreased in all subjects (mean reduction of 25% at 20 cm H2O PEEP), despite an increase in arterial oxygen tension (mean increase 6.7 kPa at 20 cm H2O PEEP). Changes in arterial oxygenation and tissue oxygen delivery differed between subjects but showed a consistent pattern. Static and dynamic alveolar strain decreased in all patients as PEEP increased. CONCLUSIONS: Incremental PEEP in ARDS appears to protect alveoli and improve arterial oxygenation, but also appears to impair tissue oxygen delivery significantly because of reduced cardiac output. We propose that this trade-off may explain the poor improvements in mortality associated with high-PEEP ventilation strategies.

Retention of laryngoscopy skills in medical students: a randomised, cross-over study of the Macintosh, A.P. Advance(™) , C-MAC(®) and Airtraq(®) laryngoscopes.

In addition to being effective and easy to learn how to use, the ideal laryngoscope should be associated with minimal reduction in skill performance during gaps in practice over time. We compared the time taken to intubate the trachea of a manikin by novice medical students immediately after training, and then after 1 month, with no intervening practice. We designed a two-period, four-group, randomised, cross-over trial to compare the Macintosh, Venner(™) A.P. Advance(™) with difficult airway blade, C-MAC(®) with D-Blade and Airtraq(®) with wireless video-viewer. A bougie was used to aid intubation with the Macintosh and the C-MAC. After training, there was no significant difference in median (IQR [range]) intubation time using the videolaryngoscopes compared with the Macintosh, which took 30 (26.5-35 [12-118])s. One month later, the intubation time was longer using the C-MAC (41 (29.5-52 [20-119])s; p = 0.002) and A.P. Advance (40 (28.5-57.5 [21-107])s; p = 0.0003)m compared with the Macintosh (27 (21-29 [16-90])s); there was no difference using the Airtraq (27 (20.5-32.5 [15-94])s; p = 0.258) compared with the Macintosh. While skill acquisition after a brief period of learning and practice was equal for each laryngoscope, performance levels differed after 1 month without practice. In particular, the consistency of performance using the C-MAC and A.P. Advance was worse compared with the Macintosh and the Airtraq. While the clinical significance of this is doubtful, we believe that reliable and consistent performance at laryngoscopy is desirable; for the devices that we tested, this requires regular practice.

Apnoeic oxygenation in pregnancy: a modelling investigation.

Recent studies have shown that nasal oxygen delivery can prolong the time to desaturation during apnoea in the non-pregnant population. We investigated the benefits of apnoeic oxygenation during rapid sequence induction in the obstetric population using computational modelling. We used the Nottingham Physiology Simulator, and pre-oxygenated seven models of pregnancy for 3 min using Fi O2 1.0, before inducing apnoea. We found that increasing Fi O2 at the open glottis increased the time to desaturation, extending the time taken for Sa O2 to reach 40% from 4.5 min to 58 min in the average parturient model (not in labour). Our study suggests that a small increase in time to desaturation could be achieved at Fi O2 0.4-0.6, which could be delivered by standard nasal cannulae. The greatest increases in time to desaturation were seen at Fi O2 1.0, which could be delivered by high-flow nasal cannulae under ideal conditions.

Clinical follow-up does not improve survival after resection of stage I-III colorectal cancer: A cohort study.

INTRODUCTION: The benefit of clinical follow-up alongside CT & CEA in detecting recurrent colorectal cancer (CRC) remains unclear. Despite this, clinical review remains part of most surveillance protocols. This study assessed the efficacy of clinical follow-up in addition to CT/CEA in detecting disease recurrence. METHODS: Patients undergoing surgery for CRC at a single centre between 2009 and 2011 were identified. Follow-up included clinical review, CT and CEA for 5 years. The primary endpoint of the study was method of detection of recurrence. Secondary endpoints included detection of surgically treatable recurrence, compliance with follow-up, disease free survival and overall survival. RESULTS: 118 patients with stage I-III CRC were included. Only 68.9% of scheduled follow-up events were performed (76.6% clinical reviews, 76.2% CT scans and 60.4% CEA tests). At median follow-up of 36 months, 26 patients had developed recurrence (median DFS 45.8 months). 17 patients (14.7%) had died (median OS 49.3 months). Sensitivity and specificity of follow up modality in detecting recurrence were; CT (92.3%, 100%), CEA (57.7%, 100%), clinical review (23.0%, 27.2%). Addition of clinical review did not identify any disease recurrence that was not detected by scheduled CT. Eight patients (30.7%) had surgically treatable recurrence - all were identified by scheduled CT. CONCLUSION: The addition of CEA testing and clinical review to scheduled CT scanning offered no benefit in the detection of recurrent disease. Clinical review could be removed from follow-up protocols without any reduction in the detection of recurrent cancer.

Cadaveric study of movement of an unstable atlanto-axial (C1/C2) cervical segment during laryngoscopy and intubation using the Airtraq(®) , Macintosh and McCoy laryngoscopes.

Concern that laryngoscopy and intubation might create or exacerbate a spinal cord injury has generated extensive research into cervical spinal movement during laryngoscopy. We performed a randomised trial on six cadavers, using three different laryngoscopes, before and after creating a type-2 odontoid peg fracture. Our primary outcome measure was the change in the space available for the spinal cord at the C1/2 segment measured by cinefluoroscopy. Tracheal intubation was performed using a minimal view of the glottis, a bougie, and manual in-line stabilisation. In a cadaveric model of type-2 odontoid fracture, the space available for the cord was preserved in maximum flexion and extension, and changed little on laryngoscopy and intubation.

Evaluation of the physiological properties of ventilatory ratio in a computational cardiopulmonary model and its clinical application in an acute respiratory distress syndrome population.

BACKGROUND: Owing to complexities of measuring dead space, ventilatory failure is difficult to quantify in critical care. A simple, novel index called ventilatory ratio (VR) can quantify ventilatory efficiency at the bedside. The study objectives were to evaluate physiological properties of VR and examine its clinical applicability in acute respiratory distress syndrome (ARDS) patients. METHODS: A validated computational model of cardiopulmonary physiology [Nottingham Physiology Simulator (NPS)] was used to evaluate VR ex vivo in three virtual patients with varying degrees of gas exchange defects. Arterial P(CO2) and mixed expired P(CO2) were obtained from the simulator while either dead space or CO2 production was altered in isolation. VR and deadspace fraction was calculated using these values. A retrospective analysis of a previously presented prospective ARDS database was then used to evaluate the clinical utility of VR. Basic characteristics of VR and its association with mortality were examined. RESULTS: The NPS showed that VR behaved in an intuitive manner as would be predicted by its physiological properties. When CO2 production was constant, there was strong positive correlation between dead space and VR (modified Pearson's r 0.98, P<0.01). The ARDS database had a mean VR of 1.47 (standard deviation 0.58). Non-survivors had a significantly higher VR compared with survivors [1.70 vs 1.34, mean difference 0.35, 95% confidence interval (CI) 0.16-0.56, P<0.01]. VR was an independent predictor of mortality (odds ratio 3.05, CI 1.35-6.91, P<0.01). CONCLUSIONS: VR is influenced by dead space and CO2 production. In ARDS, high VR was associated with increased mortality.

Responsiveness of the EQ-5D health-related quality-of-life instrument in assessing low back pain.

OBJECTIVE: To compare the responsiveness of the EuroQol five-dimensional questionnaire (EQ-5D) generic quality-of-life instrument with that of specific instruments-the Brief Pain Inventory (BPI) and the Oswestry Disability Index (ODI)-in assessing low back pain. METHODS: Data were obtained from a group of patients receiving epidural steroid injections. We assessed responsiveness by using correlation, by estimating standardized response means, by receiver operating characteristic curve analysis, and by comparing the minimum clinically important differences peculiar to each of the instruments. RESULTS: ODI, BPI, and EQ-5D index scores, and changes in scores, were found to be correlated. Estimated standardized response means and receiver operating characteristic curve analysis suggested lower responsiveness for the EQ-5D index score. Clinically significant categories of mild, moderate, and severe BPI pain intensity translated into progressively and significantly lower mean EQ-5D index scores. An increase or a decrease in severity level reported on any of the five EQ-5D dimensions was associated with significant changes (with appropriate signs) in the condition-specific scores. No change in severity in any EQ-5D dimension was associated with no change in the specific scores. Significant changes in the EQ-5D index scores were associated with clinically important changes in the ODI and BPI scores. Correlation between index scores and responses on EQ-5D's visual analogue scale was only moderate. CONCLUSIONS: The EQ-5D index is less responsive than instruments specific to pain measurement, although it is capable of indicating clinically important changes. The lower responsiveness arises from EQ-5D's more limited gradation of severity and its multidimensionality.