Plasma catecholamines and oxygen consumption during weaning from mechanical ventilation.

Previous studies on oxygen consumption (VO2) during weaning from mechanical ventilation assumed that an increase in VO2 (delta VO2) reflected oxygen consumption by respiratory muscles (VO2RESP), and proposed delta VO2 as a weaning predictor. We measured VO2 CO2 production (VCO2) and plasma catecholamines in 20 short-term ventilated patients during weaning by SIMV and CPAP. delta VO2 as a percentage of VO2 during spontaneous ventilation (delta VO2%) ranged from 4.8% to 41.5%. VCO2 also increased and correlated with VO2. Plasma adrenaline and noradrenaline increased significantly to levels known to produce considerable increases in metabolic rate. Mean arterial pressure and heart rate concomitantly increased, but spontaneous minute ventilation decreased. Thus, since the increased plasma catecholamines are calorigenic, the assumption that delta VO2 represents VO2RESP is incorrect. Although mean delta VO2% of successfully weaned patients was significantly less than that of failure-to-wean patients, the wide scatter of individual values in the latter group excludes delta VO2% as an accurate weaning predictor.

Gastric emptying following brain injury: effects of choice of sedation and intracranial pressure.

OBJECTIVE: To compare the effects of opioid and non-opioid sedation on gastric emptying. DESIGN: Prospective, randomized trial. SETTING: University teaching hospital ICU. PATIENTS: 21 brain injured patients requiring sedation, mechanical ventilation and intracranial pressure (ICP) monitoring for > 24 h. INTERVENTIONS: Patients were randomized to receive infusions of either morphine plus midazolam (M), or propofol (P). Gastric emptying was assessed by the paracetamol absorption technique and by residual volumes following a 200 ml test feed. MEASUREMENTS AND RESULTS: Pre-sedation Glasgow Coma Score, mean ICP and the presence of bowel sounds were noted. Plasma concentrations of paracetamol were measured over 3 h following a 1 g gastric dose. There were no differences in median peak paracetamol concentration (M, 18.5 versus P, 20.8 mg/l), median time to peak concentration (M, 20 versus P, 25 min), median area under the concentration-time curve (AUC), or in the median residual volumes at 1 h (M, 14 versus P, 10.5 ml) and 2 h (M, 5 versus P, 3 ml). In patients with ICP > 20 mmHg, paracetamol concentrations were lower (p < 0.05), and AUC after 30 min was lower (165 mg.min/l versus 411 mg.min/l, p = 0.023). Mean ICP was correlated with AUC (Kendall rank p = 0.027). Gastric emptying did not correlate with initial Glasgow Coma Score or presence of bowel sounds. CONCLUSIONS: Gastric emptying is not improved in patients with brain injury by avoiding morphine (1-8 mg/h) in the sedative regimen. Intracranial hypertension is associated with reduced gastric emptying.

Superimposed inspiratory work of the Siemens Servo 300 ventilator during continuous positive airway pressure.

OBJECTIVE: To compare the superimposed inspired work of breathing (SIW) of the Siemens Servo 300 ventilator with the Siemens Servo 900 C ventilator. DESIGN: Comparisons made at continuous positive airway pressure (CPAP) levels of 0, 4, and 8 cmH2O, and at trigger sensitivities of -1 and -2 cmH2O, and flow triggering. SETTING: General intensive care unit in a University teaching hospital. PATIENTS: 7 patients receiving CPAP. At all levels of CPAP, the SIW was significantly less with the Siemens Servo 300 ventilator as compared to the Siemens Servo 900 C ventilator despite similar trigger sensitivities. No significant difference was found in the SIW of the Servo 300 ventilator when comparing trigger sensitivities of -1 cmH2O, -2 cmH2O, and flow triggering. Different levels of CPAP had no effect on SIW. CONCLUSIONS: The Siemens Servo 300 ventilator entails less superimposed inspiratory work of breathing than the Siemens Servo 900 C ventilator.

Translation and calibration of a Chinese version of the Sickness Impact Profile for use in Hong Kong.

Because of the lack of suitable generic health status measures in Hong Kong that reflect the value structure and culture of the society, we have translated and calibrated the Sickness Impact Profile, which was originally developed in the United States. Translation consisted of a sequence of forward translations into Chinese, back translations into English and, when there was a loss of meaning or conceptual equivalence, retranslation into Chinese. Sixty Hong Kong Chinese people, including health professionals, patients, and members of the public were then recruited to determine item and dimension weights for the Hong Kong population. Individual ratings were averaged to obtain a consensus value for each item. Within-category reliability analysis for scores varied from 0.70 to 0.92 (Cronbach's alpha values) and overall internal consistency was 0.98. There were only small differences between this version and the original American English version in the ratings given to individual questions (Pearson's product moment correlation coefficient, r=0.8). We have developed a Chinese translation of the Sickness Impact Profile, which can now be used to evaluate health outcomes in Hong Kong and to compare outcomes with studies in other populations where the Sickness Impact Profile was used.

Difficult weaning from mechanical ventilation.

Weaning from mechanical ventilation may be influenced by factors relating to equipment, techniques and procedures. Criteria to initiate weaning and predictors of weaning outcome are generally unreliable, but mechanical work of breathing, the tidal volume: frequency ratio and the inspiratory pressure: maximal inspiratory pressure ratio may anticipate those likely to fail weaning. The optimal weaning ventilatory mode is not known, but intermittent mandatory ventilation, pressure support ventilation, and continuous positive pressure ventilation are the most commonly used. The resistances of individual components of breathing circuits are extremely important. Blow-by heated humidifiers and ventilators which compensate for the impedances of their inspiratory demand valves impose clinically acceptable spontaneous breathing loads. Close monitoring, adequate respiratory muscle rest, attention to mineral deficiencies, nutrition and pulmonary hygiene are also important parts of the weaning process.

The state of health and provision of health care in Hong Kong.

In 1997, Hong Kong became a Special Administrative Region of the People's Republic of China. The new government took control of a Western style health-care system and a population whose health is comparable to the rest of the developed world. This article describes the existing health-care system, the state of public health and some of the challenges and opportunities facing the new administration.

Defining adult respiratory distress syndrome.

Adult respiratory distress syndrome is a continuum of severe, evolving, diffuse lung injury, often complicated by multiorgan failure. Its pathophysiology is unclear and its treatment is empirical. It is diagnosed clinically using criteria that have no consensus and do not address underlying mechanisms.

Ventilation--matching man, mode and machine.

Mechanical ventilation has a number of clinical applications but is used principally to maintain gas exchange in anaesthesia and cardiorespiratory failure. This paper will discuss the clinical applications of ventilators and various modes of ventilation.

Newer modes of mechanical ventilatory support.

Recent modes of ventilatory support aim to facilitate weaning and minimise the physiological disadvantages of intermittent positive pressure ventilation (IPPV). Intermittent mandatory ventilation (IMV) allows the patient to breathe spontaneously in between ventilator breaths. Mandatory minute volume ventilation (MMV) ensures that the patient always receives a preset minute volume, made up of both spontaneous and ventilator breaths. Pressure supported (assisted) respiration is augmentation of a spontaneous breath up to a preset pressure level, and is different from 'triggering', which is a patient-initiated ventilator breath. Other modes or refinements of IPPV include high frequency ventilation, expiratory retard, differential lung ventilation, inversed ratio ventilation, 'sighs', varied inspiratory flow waveforms and extracorporeal membrane oxygenation. While these techniques have useful applications in selective situations, IPPV remains the mainstay of managing respiratory failure for most patients.

Radial artery cannulation.

The status of radial artery cannulation is reviewed. The technique of percutaneous puncture, equipment in use, and known complications are discussed. Recommendations are made for precautionary measures to reduce the complication rate. It is concluded that radial artery cannulation is a safe and useful procedure in certain anaesthetic and intensive care situations.

Anaesthesia and clinical pharmacology: related disciplines.

The disciplines of anaesthesia and clinical pharmacology are related in as much as both have a mutual interest in the safe and conservative use of drugs in man. Anaesthetists have effectively been practising rational therapeutics for many years and they have much to offer academic departments of clinical pharmacology in terms of teaching and research programmes. Similarly, the clinical pharmacologist can usefully contribute to anaesthetic training and research programmes in relation to aspects such as education, precise analytical methods, data-handling and clinical trial techniques. The inter-relationship of the two disciplines can be of mutual advantage and has the potential to provide a stronger basis for sound therapeutic practice.

Increases in cuff volume and pressure in red rubber endotracheal tubes during anaesthesia.

Increases in endotracheal tube cell volume and pressure during anaesthesia have been reported to be due to the diffusion of nitrous oxide into the cuff. This study compared cuff volume and pressure changes in anaesthetized intubated patients who were ventilated with those allowed to breath spontaneously. The cuffs of Magill red rubber endotracheal tubes were inflated with either air or a nitrous oxide-oxygen mixture. Serial pressure and volume recordings confirmed that both parameters increased when the cuff was inflated with air. The increase in cuff pressure was however, greater during intermittent positive pressure ventilation than for spontaneous respiration. There were no significant changes when the cuff was inflated with the nitrous oxide-oxygen mixture.

The effect of H2 antagonists on duration of action of suxamethonium in the parturient.

Fifteen patients who underwent elective Caesarean section were given sodium citrate, either alone or combined with oral cimetidine or ranitidine, as antacid premedication. No significant differences in time to 25 or 50% recovery of the first twitch of a train-of-four after administration of suxamethonium were found between any of the groups. It is concluded that H2-antagonists do not significantly affect the duration of action of suxamethonium in parturients, in contrast to the findings of a study in nonpregnant patients.

Anaesthesia for bullectomy. Use of propofol, high frequency jet ventilation and extradural blockade.

A patient with a single large bulla occupying 50% of the right hemithorax was anaesthetised successfully with a combination of techniques not previously described. The technique consisted of extradural analgesia and intravenous infusion of propofol, while ventilation was maintained with high frequency jet ventilation through a single lumen tracheal tube. The advantages of this technique are a reduced risk of barotrauma, good operating conditions and good analgesia.

Intravenous N-acetylcysteine for paracetamol poisoning.

Intravenously administered N-acetylcysteine was used to treat 11 patients at risk of hepatic damage from paracetamol poisoning. Complete protection against liver failure was seen in all cases without any adverse effects. The efficacy and safety of intravenously administered N-acetylcysteine make it the treatment of choice in paracetamol poisoning.

A multiple-purpose automatic pressure regulator.

The Gairdner Hospital Automatic Pressure Regulator was developed to deliver a constant pre-set gas pressure for the continuous inflation of infusion pump cuffs or pneumatic tourniquets. It has been found to be economical, safe, reliable and easy to use. The device facilitates both continuous flushing of catheters and rapid intravenous infusions especially during emergency situations. It may also be used in other clinical situations where a continuous gas supply at a pre-set constant pressure is required.

Inspiratory work imposed by demand valve ventilator circuits.

The inspiratory work (WI) imposed by three commonly used demand valve ventilator circuits was studied using a lung model to simulate spontaneous ventilation. The CPU-1 and Engstrom Erica circuits recorded WI of 379 mJ/l and 190 mJ/l respectively. A negative WI of -32 mJ/l was recorded for the Servo 900C, denoting that the circuit performed work on the lung. The demand valves recorded a time delay between inspiratory effort and onset of gas flow, of 300 ms (CPU-1), 190 ms (Servo 900c) and 160 ms (Engstrom Erica). Both the Servo 900C and Engstrom Erica demand valves were able to generate a high inspiratory gas flow response, but the CPU-1 lacked such a flow compensation. Expiratory work was also greatest with the CPU-1 (156 mJ/l) with 141 mJ/l and 90 mJ/l recorded for the Servo 900C and Engstrom Erica. Of the three ventilators studied, the Servo 900C appears to be the ventilator circuit of choice for spontaneous ventilation.

Vecuronium: an anthropometric comparison.

This study set out to determine if there was any resistance to vecuronium in Nepalese studied in Nepal compared with Nepalese, Chinese and European patients studied in Hong Kong. The four groups, each of 10 male and 10 female patients, were intubated 60 s after administration of 0.1 mg.kg-1 vecuronium. The Nepalese patients in Nepal had significantly less satisfactory intubating conditions (p = 0.002). Similarly, male patients had significantly less satisfactory conditions than female patients (p = 0.004). Some anthropometric measurements were significantly different between the patients in Nepal and those in Hong Kong. There were also sex-related anthropometric differences. It is suggested that differences in response to vecuronium could be explained by differences in distribution volume and muscle mass.

Resistance of humidifiers, and inspiratory work imposed by a ventilator-humidifier circuit.

The pressures and resistances of a bubble humidifier (Bennett Cascade) and a blow-by humidifier (Fisher and Paykel) were measured and computed at gas flow rates from 4.5 to 100 litre min-1. Pressures increased with flows, with the Bennett pressures being greater at all flows. The resistance of the Fisher-Paykel increased with flows, but remained less than that of the Bennett. An inverse resistance-flow relationship was seen with the Bennett up to a flow of 35 litre min-1. The work of breathing through a Servo 900C ventilator-humidifier circuit was computed, using a lung model. Work was performed by the Servo 900C on the lung, especially with the Fisher-Paykel circuit. The Bennett circuit required considerably greater (3.7 times more) inspiratory work. Thus the Bennett Cascade humidifier may present an unacceptable inspiratory load during spontaneous breathing.

The haemodynamic effects of propofol and thiopentone for induction of caesarean section.

Forty Chinese women for elective caesarean section received either propofol 2 mg.kg-1 or thiopentone 4 mg.kg-1 for induction of general anaesthesia. Systolic, mean and diastolic arterial pressures and heart rate were recorded non-invasively every minute for ten minutes. Post-induction arterial pressures were similar to pre-induction values with no differences between thiopentone and propofol. Following intubation, the rise in systolic arterial pressure was greater in the thiopentone group, 32.1 mmHg (SD 23.7) compared with the propofol group, 17.4 mmHg (SD 23.8), (P less than 0.05). In the thiopentone group, arterial pressures were slower in returning to baseline values. Heart rate was initially elevated in both groups to the same degree. At caesarean section, induction with propofol causes less variation in arterial pressure than thiopentone. Hypotension is probably prevented by the coincident stimulus of rapid sequence induction. Neonatal Apgar scores were similar between the two groups.