Helium in the adult critical care setting

Evaluation and Prevention of Perioperative Respiratory Failure

Respiratory failure is a common perioperative complication. The risk of respiratory failure can be reduced with effective preoperative evaluation, preventative measures, and knowledge of evidence-based management techniques. Effective preoperative screening methods include ARISCAT scoring, OSA screening, and the LAS VEGAS score (including the ASA physical status score). Evaluation by the six-minute walk test and a routine pulmonary physical exam has been shown to be effective at predicting postoperative pulmonary complications, whereas evidence on the predictive power of pulmonary function tests and chest radiography has been inconclusive. Preoperative smoking cessation and lung expansion maneuvers have been shown to decrease the risk of pulmonary complications postoperatively. Intraoperative management techniques that decrease the pulmonary complication risk include neuromuscular blockade reversal with sugammadex, limiting surgical times to less than 3 h when possible, lung-protective ventilation techniques, and multimodal analgesia to decrease opioid usage. In the immediate postoperative period, providers should be prepared to quickly treat bronchospasm, hypoventilation, and upper airway obstruction. For post-surgical patients who remain in the hospital, the risk of pulmonary complications can be decreased with lung expansion techniques, adequate analgesia, automated continuous postoperative ward monitoring, non-invasive ventilatory support, and early mobilization. This article was written to analyze the available literature on this topic in order to learn and practice the prevention of perioperative respiratory failure when caring for patients on a daily basis.

Gaseous mediators: an updated review on the effects of helium beyond blowing up balloons

Noble gases, although supposed to be chemically inert, mediate numerous physiological and cellular effects, leading to protection against ischaemia-reperfusion injury in different organs. Clinically, the noble gas helium is used in treatment of airway obstruction and ventilation disorders in children and adults. In addition, studies from recent years in cells, isolated tissues, animals and finally humans show that helium has profound biological effects: helium applied before, during or after an ischaemic event reduced cellular damage, known as "organ conditioning", in some tissue, e.g. the myocardium. Although extensive research has been performed, the exact molecular mechanisms behind these organ-protective effects of helium are yet not completely understood. In addition, there are significant differences of protective effects in different organs and animal models. A translation of experimental findings to the clinical situation has yet not been shown.

A Multicenter Randomized Trial Assessing the Efficacy of Helium/Oxygen in Severe Exacerbations of Chronic Obstructive Pulmonary Disease

RATIONALE

During noninvasive ventilation (NIV) for chronic obstructive pulmonary disease (COPD) exacerbations, helium/oxygen (heliox) reduces the work of breathing and hypercapnia more than air/O₂, but its impact on clinical outcomes remains unknown.

OBJECTIVES

To determine whether continuous administration of heliox for 72 hours, during and in-between NIV sessions, was superior to air/O₂ in reducing NIV failure (25-15%) in severe hypercapnic COPD exacerbations.

METHODS

This was a prospective, randomized, open-label trial in 16 intensive care units (ICUs) and 6 countries. Inclusion criteria were COPD exacerbations with Pa_CO2 ≥ 45 mm Hg, pH ≤ 7.35, and at least one of the following: respiratory rate ≥ 25/min, Pa_O2 ≤ 50 mm Hg, and oxygen saturation (arterial [Sa_O2] or measured by pulse oximetry [Sp_O2]) ≤ 90%. A 6-month follow-up was performed.

MEASUREMENTS AND MAIN RESULTS

The primary endpoint was NIV failure (intubation or death without intubation in the ICU). The secondary endpoints were physiological parameters, duration of ventilation, duration of ICU and hospital stay, 6-month recurrence, and rehospitalization rates. The trial was stopped prematurely (445 randomized patients) because of a low global failure rate (NIV failure: air/O₂ 14.5% [n = 32]; heliox 14.7% [n = 33]; P = 0.97, and time to NIV failure: heliox group 93 hours [n = 33], air/O₂ group 52 hours [n = 32]; P = 0.12). Respiratory rate, pH, Pa_CO2, and encephalopathy score improved significantly faster with heliox. ICU stay was comparable between the groups. In patients intubated after NIV failed, patients on heliox had a shorter ventilation duration (7.4 ± 7.6 d vs. 13.6 ± 12.6 d; P = 0.02) and a shorter ICU stay (15.8 ± 10.9 d vs. 26.7 ± 21.0 d; P = 0.01). No difference was observed in ICU and 6-month mortality.

CONCLUSIONS

Heliox improves respiratory acidosis, encephalopathy, and the respiratory rate more quickly than air/O₂ but does not prevent NIV failure. Overall, the rate of NIV failure was low. Clinical trial registered with www.clinicaltrials.gov (NCT 01155310).

Exam 1 Questions

Which of the following is the most common form of incomplete spinal cord injury?A.

Central cord syndrome

B.

Cauda equina syndrome

C.

Anterior spinal cord syndrome

D.

Posterior spinal cord syndrome

E.

Brown-Sequard lesion

A 64-year-old male with a history of chronic alcohol abuse and congestive heart failure is currently recovering from excision of a large right shoulder lesion suspicious for melanoma. Postoperatively, he is experiencing bleeding and oozing from his surgical site that has persisted despite suture repair and direct pressure for an extended period of time. His labs are drawn, and are as follows: platelets 141 × 10³/mL, INR 1.2, fibrinogen 90 mg/dL. Which of the following blood products should be administered next?A.

Fresh frozen plasma

B.

Cryoprecipitate

C.

Prothrombin complex concentrate

D.

Recombinant activated factor VII

E.

Aminocaproic acid

A 75-year-old, 90 kg male with a history of peripheral vascular disease, coronary artery disease, and epilepsy following a recent cerebral infarction presents to the emergency department after having three witnessed seizures at home. He was intubated at the scene by the paramedics, and received 8 mg of intravenous lorazepam and 1 g of phenytoin. While you are evaluating him, he has another generalized tonic-clonic seizure, and the nurse asks if you would like to initiate a continuous propofol infusion. His blood pressure is 94/42 mmHg, and he is having numerous premature ventricular contractions (PVCs) on the electrocardiographic monitor. He has no history of platelet or liver dysfunction. Which of the following should be performed next?A.

Complete the phenytoin load to attain 20 mg/kg, then start propofol infusion

B.

Complete the phenytoin load to attain 20 mg/kg only

C.

Administer valproate, 30 mg/kg over 10 min, as well as midazolam 0.2 mg/kg

D.

Start immediate midazolam infusion at 2 mg/kg/h

E.

Give a 1 L normal saline bolus, and start a norepinephrine infusion to normalize blood pressure

A 38-year-old male is brought to the emergency department after a motor vehicle accident. He is found to have significant ecchymoses on his chest and face, with multiple apparent rib fractures. He is in mild respiratory distress, with an oxygen saturation of 89% on room air, and hypotensive, with a systolic blood pressure of 88 mmHg. He has absent breath sounds on the right side. There is currently a delay in obtain a bedside portable chest x-ray. Which of the following should be performed next?A.

28-French chest tube placement

B.

16-French chest tube placement

C.

Obtain computed tomography (CT) of the chest

D.

Administer 30 cc/kg crystalloid

E.

Obtain urgent cardiothoracic surgery consult

Stress ulcer prophylaxis is often undertaken to prevent clinically important upper gastrointestinal (GI) bleeding. Which of the following factors puts patients at highest risk for such bleeding episodes?A.

Respiratory failure

B.

History of alcohol abuse

C.

NPO status

D.

Diverticulitis

E.

All of the above

In an intact heart, the Frank-Starling mechanism describes contractility increases in responses to:A.

Decreased preload

B.

Increased afterload

C.

Decreased left ventricular end-diastolic pressure

D.

Increased left ventricular end-diastolic volume

E.

Increased pulmonary vascular resistance

A 68-year-old female with a history of hyperlipidemia, hypothyroidism, and gastric cancer on total parenteral nutrition is currently in the ICU following a small traumatic subdural hemorrhage. On hospital day 5, the patient begins to spike fevers that persist despite broad spectrum antibiotic coverage with vancomycin and piperacillin-tazobactam. She is otherwise hemodynamically stable. The lab calls you to notify you that multiple sets of blood cultures display budding yeast forms and pseudohyphae. Which of the following should be administered next?A.

Fluconazole

B.

Posaconazole

C.

Anidulafungin

D.

Caspofungin

E.

Amphotericin B

A 56-year-old male with a past medical history of hypertension, hyperlipidemia, and morbid obesity is currently intubated in the ICU following a left middle cerebral artery infarct. The respiratory therapist alerts you the fact that the patient has become markedly dysynchronous with the ventilator, including breath holding episodes, breath stacking, and resisting ventilator-delivered breaths. A variety of pressure- and volume-regulated ventilator modes have been attempted without improvement, as well as boluses of both fentanyl and midazolam. The most recent arterial blood gas is as follows: pH 7.19, PaCO₂ 78 mmHg, PaO₂ 61 mmHg. The patient is now hypotensive to 91/66 mmHg with sinus tachycardia at 117 beats/min. A recent bedside chest x-ray shows no consolidation or pneumothorax. Which of the following should be performed next?A.

Prone the patient

B.

Administer nitric oxide at 10 parts per million

C.

Administer 10 mg of cisatracurium

D.

Administer a mixture of 60% helium/40% oxygen

E.

Administer a continuous infusion of phenobarbital

Compared to lactulose for the treatment of hepatic encephalopathy, polyethylene glycol (PEG) has been shown to:A.

Decrease in-hospital mortality

B.

More rapidly improve symptoms

C.

Increase the rate of gastrointestinal complications

D.

Increase the incidence of major electrolyte abnormalities

E.

None of the above

Which of the following neurologic insults is the least likely to cause central (non-infectious) fever in the ICU?A.

Intracranial neoplasm

B.

Intraventricular hemorrhage

C.

Normal pressure hydrocephalus

D.

Subarachnoid hemorrhage

E.

Traumatic brain injury

A 57-year-old male with a history of epilepsy and medication noncompliance is admitted to a small community hospital after a brief tonic-clonic seizure. A non-contrast head CT on admission is normal. On the second hospital day, the patient begins to complain of severe substernal chest pressure, and an urgent bedside EKG shows evidence of an acute inferior myocardial infarction (MI). The nearest percutaneous coronary intervention (PCI) capable center is approximately 150 min away by the fastest transport method available. Which of the following is the most appropriate next step in this patient’s care?A.

Arrange for transport to the closest PCI center with anticipated balloon time within 30 min of arrival

B.

Prepare to administer fibrinolytic therapy

C.

Consult cardiothoracic surgery for possible coronary artery bypass grafting (CABG)

D.

Place the patient on a continuous nitroglycerine infusion and administer aspirin, clopidogrel, and heparin

E.

Await serum cardiac biomarkers and repeat EKG in 1 h

A 62-year-old male with unknown past medical history who recently immigrated from El Salvador is currently in the stroke unit after suffering from an acute left middle cerebral artery infarction. The patient is aphasic; his wife states that he been in his usual state of health lately, and denies any recent weakness, dizziness, chest pain, cough, shortness of breath, or fevers. On reviewing this patient’s belongings, the nurse discovers a bottle of isoniazid, as well as paperwork demonstrating a positive quantiferon gold test performed at a local clinic approximately 3 weeks ago. He does not appear to be on any other medications. A bedside portable chest x-ray is performed, which preliminarily appears normal. Which of the following should be performed next?A.

Move the patient to a negative pressure isolation room, continue isoniazid

B.

Isolate the patient, continue isoniazid, add rifampin

C.

Isolate the patient, continue isoniazid, add rifampin and pyrazinamide

D.

Isolate the patient, continue isoniazid, add rifampin, pyrazinamide and ethambutol

E.

None of the above

A 56-year-old, 70 kg female patient in oliguric renal failure would be expected to have a daily urine output of:A.

No more than 50 mL

B.

No more than 400 mL

C.

No more than 800 mL

D.

Less than 70 mL/h

E.

Less than 35 mL/h

A 37-year-old female with a history of epilepsy is admitted to the ICU with status epilepticus. She required several doses of lorazepam in the emergency department in addition to fosphenytoin, intubation, and a continuous propofol infusion. There was concern for aspiration in the prehospital setting. Approximately 3 days after being admitted to the hospital, her respiratory status has worsened; she is increasingly hypoxic, and her chest x-ray demonstrates diffuse bilateral interstitial infiltrates. The patient is afebrile with minimal secretions. Her most recent arterial blood gas is as follows: pH 7.21, PaO₂ 107 mmHg, PCO₂ 55 mmHg, 100% FiO₂, and a positive end-expiratory pressure (PEEP) of 8 cm H₂O. According to the Berlin criteria, how would you categorize this patient’s acute respiratory distress syndrome (ARDS)?A.

Acute lung injury (ALI)

B.

Mild ARDS

C.

Moderate ARDS

D.

Severe ARDS

E.

None of the above

An 80-year-old male presents to the emergency department with multiple episodes of bright red blood per rectum. He is on aspirin and clopidogrel for a history of coronary artery disease and a previous transient ischemic attack. He underwent aortic graft surgery for repair of an abdominal aortic aneurysm 2 years ago. A complete blood count and coagulation profile are all within normal limits. His vital signs are as follows: blood pressure 102/58 mmHg, heart rate 98 beats/min, respiratory rate 18 breaths/min, oxygen saturation 98% on room air, and temperature 98.3 °F. Which of the following is the next best step in the care of this patient?A.

Transfuse platelets, fresh frozen plasma, and recombinant factor VIIa

B.

Consult gastroenterology for emergent upper endoscopy

C.

Consult gastroenterology for emergent colonoscopy

D.

CT angiogram of the abdomen and pelvis

E.

Expectant management with fluids and blood transfusions

A thrombus in which of the following veins would not be considered a deep vein thrombosis (DVT)?A.

Popliteal vein

B.

Soleal vein

C.

Femoral vein

D.

Gastrocnemius vein

E.

Greater saphenous vein

After partial resection of the pituitary stalk, secretion of which of the following hormones will be most affected?A.

Oxytocin

B.

Adrenocorticotrophic hormone

C.

Melanocyte-stimulating hormone

D.

Thyroid-stimulating hormone

E.

All will be equally affected

A 58-year-old female with a history of hypertension, rheumatoid arthritis, metastatic ovarian cancer, and bilateral deep venous thrombosis status post recent inferior vena cava filter placement presents to the emergency department with right flank pain. She states the pain began approximately 1 h ago when bending down to pick something off the floor, and that it is constant and severe in nature. She denies dysuria or hematuria. Her vital signs are as follows: blood pressure 108/62 mmHg, heart rate 121 beats/min, respiratory rate 20 breaths/min, oxygen saturation 99% on room air, and temperature 99.6 °F. A CT scan of the abdomen is obtained (see Image 1). Which of the following is the next best step in this patient’s management?A.

Administer vancomycin and cefepime, and draw two sets of blood cultures

B.

Urgent vascular surgery consult

C.

Immediately place the patient on her left side

D.

Rapid sequence intubation with mechanical ventilation

E.

Perform bedside diagnostic peritoneal lavage

High-flow oxygen therapy and other inhaled therapies in intensive care units

In this Series paper, we review the current evidence for the use of high-flow oxygen therapy, inhaled gases, and aerosols in the care of critically ill patients. The available evidence supports the use of high-flow nasal cannulae for selected patients with acute hypoxaemic respiratory failure. Heliox might prevent intubation or improve gas flow in mechanically ventilated patients with severe asthma. Additionally, it might improve the delivery of aerosolised bronchodilators in obstructive lung disease in general. Inhaled nitric oxide might improve outcomes in a subset of patients with postoperative pulmonary hypertension who had cardiac surgery; however, it has not been shown to provide long-term benefit in patients with acute respiratory distress syndrome (ARDS). Inhaled prostacyclins, similar to inhaled nitric oxide, are not recommended for routine use in patients with ARDS, but can be used to improve oxygenation in patients who are not adequately stabilised with traditional therapies. Aerosolised bronchodilators are useful in mechanically ventilated patients with asthma and chronic obstructive pulmonary disease, but are not recommended for those with ARDS. Use of aerosolised antibiotics for ventilator-associated pneumonia and ventilator-associated tracheobronchitis shows promise, but the delivered dose can be highly variable if proper attention is not paid to the delivery method.

Helium preconditioning protects against neonatal hypoxia-ischemia via nitric oxide mediated up-regulation of antioxidases in a rat model

This study aimed to investigate the role of nitric oxide (NO) in the neuroprotective effects of helium preconditioning (He-PC) in a neonatal hypoxia/ischemia (HI) rat model. Seven-day old rat pups were divided into normal control group, He-PC group, HI group, He-PC+HI group, L-NAME+HI group and L-NAME+He-PC+HI group. HI was induced by exposure to 80% oxygen for 90 min. He-PC was conducted with 70% helium-30% oxygen for three 5-min periods. Three hours after He-PC, animals in control group and He-PC group were sacrificed, and the brain was collected for the detection of NO content. At 24h after HI, animals in control group, HI group, He-PC+HI group, and L-NAME+He-PC+HI group were sacrificed, and the brain was collected for detection of infarct ratio, antioxidases (SOD, HO-1 and Nrf2), DNA binding activity of Nrf2 and TUNEL staining. Three weeks later, the neurological function and brain atrophy were determined. Results showed pretreatment with L-NAME alone failed to exert protective effect on HI. He-PC significantly increased NO content, reduced the brain infarct area, increased anti-oxidases expression and DNA binding activity of Nrf2, decreased the apoptotic cells, and improved the neurological function and brain atrophy. In addition, this protection was markedly inhibited by L-NAME (a non-selective NOS inhibitor). These findings suggest that the He-PC may induce NO production to activate Nrf2, exerting neuroprotective effect on neonatal HI.

Effects of a helium/oxygen mixture on individuals' lung function and metabolic cost during submaximal exercise for participants with obstructive lung diseases

BACKGROUND

Helium/oxygen therapies have been studied as a means to reduce the symptoms of obstructive lung diseases with inconclusive results in clinical trials. To better understand this variability in results, an exploratory physiological study was performed comparing the effects of helium/oxygen mixture (78%/22%) to that of medical air.

METHODS

The gas mixtures were administered to healthy, asthmatic, and chronic obstructive pulmonary disease (COPD) participants, both moderate and severe (6 participants in each disease group, a total of 30); at rest and during submaximal cycling exercise with equivalent work rates. Measurements of ventilatory parameters, forced spirometry, and ergospirometry were obtained.

RESULTS

There was no statistical difference in ventilatory and cardiac responses to breathing helium/oxygen during submaximal exercise. For asthmatics, but not for the COPD participants, there was a statistically significant benefit in reduced metabolic cost, determined through measurement of oxygen uptake, for the same exercise work rate. However, the individual data show that there were a mixture of responders and nonresponders to helium/oxygen in all of the groups.

CONCLUSION

The inconsistent response to helium/oxygen between individuals is perhaps the key drawback to the more effective and widespread use of helium/oxygen to increase exercise capacity and for other therapeutic applications.

Mechanical ventilation for severe asthma

Acute exacerbations of asthma can lead to respiratory failure requiring ventilatory assistance. Noninvasive ventilation may prevent the need for endotracheal intubation in selected patients. For patients who are intubated and undergo mechanical ventilation, a strategy that prioritizes avoidance of ventilator-related complications over correction of hypercapnia was first proposed 30 years ago and has become the preferred approach. Excessive pulmonary hyperinflation is a major cause of hypotension and barotrauma. An appreciation of the key determinants of hyperinflation is essential to rational ventilator management. Standard therapy for patients with asthma undergoing mechanical ventilation consists of inhaled bronchodilators, corticosteroids, and drugs used to facilitate controlled hypoventilation. Nonconventional interventions such as heliox, general anesthesia, bronchoscopy, and extracorporeal life support have also been advocated for patients with fulminant asthma but are rarely necessary. Immediate mortality for patients who are mechanically ventilated for acute severe asthma is very low and is often associated with out-of-hospital cardiorespiratory arrest before intubation. However, patients who have been intubated for severe asthma are at increased risk for death from subsequent exacerbations and must be managed accordingly in the outpatient setting.

Using helium-oxygen to improve regional deposition of inhaled particles: mechanical principles

BACKGROUND

Helium-oxygen has been used for decades as a respiratory therapy conjointly with aerosols. It has also been shown under some conditions to be a means to provide more peripheral, deeper, particle deposition for inhalation therapies. Furthermore, we can also consider deposition along parallel paths that are quite different, especially in a heterogeneous pathological lung. It is in this context that it is hypothesized that helium-oxygen can improve regional deposition, leading to more homogeneous deposition by increasing deposition in ventilation-deficient lung regions.

METHODS

Analytical models of inertial impaction, sedimentation, and diffusion are examined to illustrate the importance of gas property values on deposition distribution through both fluid mechanics- and particle mechanics-based mechanisms. Also considered are in vitro results from a bench model for a heterogeneously obstructed lung. In vivo results from three-dimensional (3D) imaging techniques provide visual examples of changes in particle deposition patterns in asthmatics that are further analyzed using computational fluid dynamics (CFD).

RESULTS AND CONCLUSIONS

Based on analytical modeling, it is shown that deeper particle deposition is expected when breathing helium-oxygen, as compared with breathing air. A bench model has shown that more homogeneous ventilation distribution is possible breathing helium-oxygen in the presence of heterogeneous obstructions representative of central airway obstructions. 3D imaging of asthmatics has confirmed that aerosol delivery with a helium-oxygen carrier gas results in deeper and more homogeneous deposition distributions. CFD results are consistent with the in vivo imaging and suggest that the mechanics of gas particle interaction are the source of the differences seen in deposition patterns. However, intersubject variability in response to breathing helium-oxygen is expected, and an example of a nonresponder is shown where regional deposition is not significantly changed.

Methods for evaluation of helium/oxygen delivery through non-rebreather facemasks

BACKGROUND

Inhalation of low-density helium/oxygen mixtures has been used both to lower the airway resistance and work of breathing of patients with obstructive lung disease and to transport pharmaceutical aerosols to obstructed lung regions. However, recent clinical investigations have highlighted the potential for entrainment of room air to dilute helium/oxygen mixtures delivered through non-rebreather facemasks, thereby increasing the density of the inhaled gas mixture and limiting intended therapeutic effects. This article describes the development of benchtop methods using face models for evaluating delivery of helium/oxygen mixtures through facemasks.

METHODS

Four face models were used: a flat plate, a glass head manikin, and two face manikins normally used in life support training. A mechanical test lung and ventilator were employed to simulate spontaneous breathing during delivery of 78/22 %vol helium/oxygen through non-rebreather facemasks. Based on comparison of inhaled helium concentrations with available clinical data, one face model was selected for measurements made during delivery of 78/22 or 65/35 %vol helium/oxygen through three different masks as tidal volume varied between 500 and 750 ml, respiratory rate between 14 and 30 breaths/min, the inspiratory/expiratory ratio between 1/2 and 1/1, and the supply gas flow rate between 4 and 15 l/min. Inhaled helium concentrations were measured both with a thermal conductivity analyzer and using a novel flow resistance method.

RESULTS

Face models borrowed from life support training provided reasonably good agreement with available clinical data. After normalizing for the concentration of helium in the supply gas, no difference was noted in the extent of room air entrainment when delivering 78/22 versus 65/35 %vol helium/oxygen. For a given mask fitted to the face in a reproducible manner, delivered helium concentrations were primarily determined by the ratio of supply gas flow rate to simulated patient minute ventilation, with the inspiratory/expiratory ratio playing a secondary role. However, the functional dependence of helium concentration on these two ratios depended on the mask design.

CONCLUSIONS

Large differences in mask performance were identified. With continued refinement, the availability of reliable benchtop methods is expected to assist in the development and selection of patient interfaces for delivery of helium/oxygen and other medical gases.

Bench and mathematical modeling of the effects of breathing a helium/oxygen mixture on expiratory time constants in the presence of heterogeneous airway obstructions

Background

Expiratory time constants are used to quantify emptying of the lung as a whole, and emptying of individual lung compartments. Breathing low-density helium/oxygen mixtures may modify regional time constants so as to redistribute ventilation, potentially reducing gas trapping and hyperinflation for patients with obstructive lung disease. In the present work, bench and mathematical models of the lung were used to study the influence of heterogeneous patterns of obstruction on compartmental and whole-lung time constants.

Methods

A two-compartment mechanical test lung was used with the resistance in one compartment held constant, and a series of increasing resistances placed in the opposite compartment. Measurements were made over a range of lung compliances during ventilation with air or with a 78/22% mixture of helium/oxygen. The resistance imposed by the breathing circuit was assessed for both gases. Experimental results were compared with predictions of a mathematical model applied to the test lung and breathing circuit. In addition, compartmental and whole-lung time constants were compared with those reported by the ventilator.

Results

Time constants were greater for larger minute ventilation, and were reduced by substituting helium/oxygen in place of air. Notably, where time constants were long due to high lung compliance (i.e. low elasticity), helium/oxygen improved expiratory flow even for a low level of resistance representative of healthy, adult airways. In such circumstances, the resistance imposed by the external breathing circuit was significant. Mathematical predictions were in agreement with experimental results. Time constants reported by the ventilator were well-correlated with those determined for the whole-lung and for the low-resistance compartment, but poorly correlated with time constants determined for the high-resistance compartment.

Conclusions

It was concluded that breathing a low-density gas mixture, such as helium/oxygen, can improve expiratory flow from an obstructed lung compartment, but that such improvements will not necessarily affect time constants measured by the ventilator. Further research is required to determine if alternative measurements made at the ventilator level are predictive of regional changes in ventilation. It is anticipated that such efforts will be aided by continued development of mathematical models to include pertinent physiological and pathophysiological phenomena that are difficult to reproduce in mechanical test systems.

Difficult Airway Society Guidelines for the management of tracheal extubation

Tracheal extubation is a high-risk phase of anaesthesia. The majority of problems that occur during extubation and emergence are of a minor nature, but a small and significant number may result in injury or death. The need for a strategy incorporating extubation is mentioned in several international airway management guidelines, but the subject is not discussed in detail, and the emphasis has been on extubation of the patient with a difficult airway. The Difficult Airway Society has developed guidelines for the safe management of tracheal extubation in adult peri-operative practice. The guidelines discuss the problems arising during extubation and recovery and promote a strategic, stepwise approach to extubation. They emphasise the importance of planning and preparation, and include practical techniques for use in clinical practice and recommendations for post-extubation care.