Anaesthesia and Intensive Care

The purpose of reclassifying breathing systems is to enhance understanding by relating structure to function. A previous classification¹ which appears to fulfill this objective is updated to include recent developments in a newly named group of systems called “displacement afferent reservoir” (DAR) breathing systems. A classification table allows for quick comparison and shows that, of the non-absorber systems, the DAR group has the best characteristics of fresh gas utilization.

A single, lightweight, valveless, non-absorber breathing system, which functions with near maximal efficiency (hence the name Maxima) in spontaneous and controlled ventilation is described. It may be classified as an enclosed efferent afferent reservoir (EEAR) breathing system and is characterised by the selective elimination of alveolar gas in all modes of ventilation. A functional description explains how this is achieved in the system in spontaneous and controlled ventilation.

The potential for error which arises in combination systems for different switch positions with different modes of ventilation is avoided, as it is a single universal system. With no moving parts, mechanical reliability is guaranteed. The apparatus deadspace is negligible making the system usable even in neonates.

Finally, the characteristics of the theoretical ideal non-absorber circuit are discussed as this illustrates the objective of the circuit design and important physical principles surrounding its function.

The Maxima is a new universal breathing system, which, despite its being valveless, functions with near maximal efficiency in spontaneous and controlled ventilation.

It was compared to the Bain system in controlled ventilation by adjusting fresh gas flows (V̇F) to achieve an endtidal CO₂ of 5% in 40 patients aged from one to seventy-six years. They received a combined regional and general anaesthetic technique. The overall V̇F requirement of the Maxima system was: weight in kg x 35 ml.min⁻¹ .kg⁻¹ + 1160 ml.min⁻¹; the Bain system required V̇F of: weight in kg x 42 ml.min-1 .kg-1 + 2070 ml.min⁻¹. The mean V̇F requirement for the Bain system was greater by 45%. Of these forty patients, the results from the twenty-four adult patients weighing 50 kg or more gave the respective V̇F requirements for the Maxima and Bain systems of 52 and 71 ml.min⁻¹ .kg⁻¹.

In a separate study using twenty-nine patients, the fractional utilization of fresh gas (FU) (or VAe/VF where VAe = effective alveolar ventilation) in the Maxima and Bain systems was shown to be 0.94 and 0.71 respectively.

An enclosed efferent, afferent reservoir breathing system (Maxima, Life Air Pty Ltd), being valveless, was compared to a simple afferent reservoir system (Humphrey ADE, A mode), having a valve, by assessing fresh gas flow (V̇F) requirements, with respect to ventilation (V̇E), that prevents rebreathing in volunteers, awake and breathing spontaneously. The results are recorded in terms of the quotient V̇F/V̇E associated with the onset of rebreathing. At the same time four clinical methods for assessing rebreathing were evaluated from the perspective of suitability for practical application during anaesthesia.

No significant difference was found in the VF requirements between the ADE and Maxima breathing systems with respective values obtained for the quotient VF/VE of 0.80 and 0.77.

Our findings showed that the method of sampling carbon dioxide (CO₂) at the outflow of the efferent limb of the patient connector (eliminated CO₂ method) in both afferent reservoir systems provided the highest flow rate and most reliable indication for detecting potential or actual rebreathing, when attempting to minimize VF. It provides no additional deadspace or resistance to gas flow, and has the advantage of being the only qualitative method that is reliable for the purpose of detecting the onset of rebreathing.

The triggering system in pressure support ventilation needs to respond rapidly, especially in neonates. The aim of this study was to compare the effects of flow-triggered and pressure-triggered pressure support ventilation on neonatal mechanical ventilation using an animal model. Respiratory flow, airway pressure, oesophageal pressure, and diaphragmatic electromyogram were measured during pressure support ventilation in five anaesthetized rabbits. The animals were connected to a VIPBIRD (Bird, U.S.A.) (CPAP mode, pressure support ventilation, 5 cm H₂O) and PEEP 0 cm H₂O). Flow-triggering sensitivity was set at 0.21/min, 0.51/min, 1.011/min, or 1.51/min. Pressure-triggering sensitivity was set at −1.0 cm H2_O. Shorter trigger delay and longer pressure support time were observed in flow-triggering. There was also less diaphragmatic activity in flow-triggering as evidenced by the amplitude of integrated diaphragmatic electromyogram and negative deflection of oesophageal pressure. The findings suggest that flow-triggering will prove superior to pressure-triggering in pressure support ventilation for neonates.

A prospective observational study was performed to assess the reliability of fibreoptic oximetric catheters and to identify the incidence and causes of jugular bulb oxygen desaturation in patients with acute closed head injury. There were twenty- five patients (30± 16 years) with GCS ≤8 in this study.

Jugular bulb oximetry, mean arterial pressure, intracranial pressure, end-tidal CO₂ and pulse oximetry were monitored continuously. Catheter calibration against a laboratory oximeter was performed post insertion and thereafter eight-hourly. Cerebral venous desaturation was defined as a jugular bulb oxygen saturation <55% of >10 minutes duration. There was a poor correlation for the first in vivo calibration (r² = 0.602, P<0.001, n=25). Thereafter a close correlation between jugular bulb catheter and oximetry values was demonstrated (r² = 0.868, P<0.001, n=205). Forty-two episodes of jugular bulb oxygen desaturation of 88 minutes mean duration (range 10 to 555) were observed. 83% occurred within 48 hours following injury. Hypocapnia was associated in 45% of episodes; hypoperfusion in 22%; raised ICP in 9% and a combination of the above in 24%.

Validation with a laboratory oximeter is essential prior to continuous jugular bulb oximetry. Sustained episodes of cerebral venous desaturation are frequent within the first 48 hours following acute head injury. Factors such as hypocapnia and cerebral hypoperfusion that primarily reduce cerebral blood flow are predominant.

Monitoring of gastric intramucosal pH (pH_i) is advocated in critical illness to detect tissue acidosis due to regional hypoperfusion. However, the number of patients who may benefit from such monitoring remains unclear and the relationship between low pH_i and outcome requires further definition. Sixty consecutive patients with Acute Physiology and Chronic Health Evaluation (APACHE II) scores <30 were studied throughout ICU stay to investigate the incidence of low pH_i (<7.32 for > 1 hour), its relationship to outcome, and temporally associated clinical events. pH_i was measured 2 to 6 hours post-ICU, admission and 8-hourly thereafter. Forty-four patients (73%) exhibited low pH_i Fourteen patients died in ICU with 13 deaths occurring in the low pH_i group (P=0.05). length of ICU stay was greater in the low pH_i group (P=0.02). The development of low pH_i was temporally associated with maximal sepsis score, weaning from assisted ventilation and commencement of enteral feeding.

This study examines the feasibility of using Quality-Adjusted Life Years (QALYs) to assess patient outcome and the economic justification of treatment in an Intensive Care Unit (ICU).

248 patients were followed for three years after admission. Survival and quality of life for each patient was evaluated. Outcome for each patient was quantified in discounted Quality-Adjusted Life Years (dQALYs). The economic justification of treatment was evaluated by comparing the total and marginal cost per dQALY for this patient group with the published cost per QALY for other medical interventions.

150 patients were alive after three years. Quality of life for most longterm survivors was good. Patient outcome (QALYs) was greatest for asthma and trauma patients, and least for cardiogenic pulmonary oedema. The tentative estimated cost- effectiveness of treatment varied from AUD $297 per QALY for asthma to AUD $2323 per QALY for patients with pulmonary oedema. This compares favourably with many preventative and non-acute medical treatments.

Although the methodology is developmental, the measurement of patient outcome using QALYs appears to be feasible in a general hospital ICU.

Cystic fibrosis is an autosomal recessive disorder which affects one in 2500 live births. It is a multisystem disease and has a variety of presentations. The major clinical features affect the gastrointestinal and respiratory tracts. Severe respiratory disease, diabetes and gastroesophageal reflux are common features of concern to anaesthetists. Improved care of young patients has allowed many to survive into adulthood. Lung transplantation has significantly improved the outlook for many patients. At Alfred Hospital, 74 patients with cystic fibrosis underwent 149 procedures from January 1978 to January 1994, with a mortality of 0.6% (95% CI 0.4%-0.8%). This retrospective cohort study describes the anaesthetic management and perioperative care of these patients. Most of the anaesthetics were for procedures related to cystic fibrosis but 12% were for unrelated conditions. Cystic fibrosis related procedures include diagnostic, venous access, enteral feeding procedures, treatment of complications of cystic fibrosis and lung transplantation. Despite extremely poor respiratory function, these patients can be managed with acceptably low postoperative mortality (1%). Pre- and postoperative care must be directed towards optimal clearance of viscous respiratory secretions. Procedures need to be planned so that optimal care can be given by each member of the team caring for cystic fibrosis patients.

A randomized double-blind study was conducted in 83 women scheduled for elective caesarean section to determine the efficacy of EMLA and lignocaine infiltration for epidural insertion. The patients were randomly allocated to one of three groups: Group A (EM/LIG) received EMLA and intradermal and subdermal 1% lignocaine infiltration, Group B (EM/SAL) EMLA and saline while Group C (PL/LIG) received placebo cream and 1% lignocaine.

Assessments using a 100 mm pain score were performed on skin infiltration and after subsequent insertion of a 16 gauge Tuohy needle into the supraspinous ligament. Skin changes under the applied cream and nursing rating of patients’ response were also noted. Statistical analyses were performed using Kruskal-Wallis and Fisher's Exact Tests.

Groups were comparable for age, weight, parity and EMLA application time (interquartile range 105-150 minutes) There were significantly higher pain scores for skin infiltration in Group C (PL/LIG) (P<0.01) and for epidural needle insertion in Group B (EM/SAL) (P<0.05).

We concluded that in this patient population, the application of EMLA cream for at least 90 minutes plus 1% lignocaine infiltration optimized patient comfort for epidural insertion.