ECG interference suppressed using a harmonic generator.

Non-linear loading of the utilities supply introduced significant electromagnetic field (EMF) interference severe enough to disrupt electrocardiograph (ECG) monitoring and recordings in the new Emergency Department (ED) at the Royal Melbourne Hospital (RMH). This interference was evident even though standard Mu-metal shielding had been installed over the main hospital power feed which runs underneath the department. Investigations revealed that the source of the interference was due to 3rd harmonic currents flowing in the mains cable. This interference was suppressed by introducing third harmonic current into the main power cable in anti-phase to the interfering signal.

Oesophageal techniques for deriving the lung dynamics of quietly breathing sheep.

An airway monitoring system was developed to record the lung dynamics of sheep using oesophageal pressure techniques. These techniques are minimally invasive and do not impede the animal from breathing spontaneously. This study investigates three methods for deriving lung resistance (R) and compliance (C) by evaluating their ability to select regular breaths amongst artefact effected signals. The Multi Linear Regression method, the Modified Mead-Whittenberger (MMW) method and Isovolumetric method were tested against signals with movement and/or swallowing artifacts. Their coefficient of variation (% CV) revealed that the MMW had the least variation with R at 35% CV and C at 25 % CV. The lung dynamics for 14 quietly breathing sheep were R = 1.5 +/- 0.5 cmH2O/l/s, C = 0.20 +/- 0.08 l/cmH2O, tidal volume = 0.25 +/- 0.05 litres and respiratory rate = 26 +/- 6 bpm.

Pre-bypass myocardial ischaemia in patients undergoing coronary artery surgery.

One hundred patients undergoing elective coronary artery surgery were studied to determine the incidence of pre-bypass myocardial ischaemia. Leads II, aVF and V5 of the electrocardiogram (ECG) were recorded at five-minute intervals from arrival in the anaesthetic room until onset of cardiopulmonary bypass. Thirteen patients developed sixteen episodes of significant ST depression on the ECG during the study period. Three patients were diagnosed as having postoperative myocardial infarction by ECG criteria and creatine phosphokinase-MB rise above 80 units. None of these patients had pre-bypass ST depression. Comparisons of these results with similar studies are presented.

EEG monitoring using aperiodic analysis during carotid and open heart surgery.

The Cerebral Electrical Activity (CEA) was monitored by Lifescan in 38 patients undergoing carotid surgery under general or regional anaesthesia and another 20 patients during cardiopulmonary bypass undergoing open heart surgery. This monitor uses aperiodic analysis to process the electroencephalogram. Abrupt changes in CEA were observed when blood flow was disrupted due to surgical intervention or administration of thiopentone. Gradual changes in CEA were due to anaesthesia or hypothermia. In one patient a long period of depressed CEA did not result in a neurological deficit and in another patient a neurological event was undetected by the monitor.

Comparison of the processed electroencephalogram and awake neurological assessment during carotid endarterectomy.

A processed electroencephalogram (EEG) produced by the Lifescan monitor (Neurometrics), was compared to awake neurological assessment for detecting cerebral ischaemia in seventy patients undergoing carotid endarterectomy under cervical plexus block. Of the six patients demonstrating neurological signs on cross-clamping the carotid, five displayed simultaneous EEG changes, four being detected during surgery, and one being detected after reviewing the EEG postoperatively. Another four patients displayed EEG changes indicative of ischaemia but unassociated with neurological signs. A further patient displayed contralateral intraoperative EEG changes. Hypotension resulted in one EEG change and two cases were associated with technical difficulties with the monitor. The presence of false negatives, possible false positives, technical errors and subjective interpretation associated with the processed EEG make it less reliable than awake neurological assessment for the detection of cerebral ischaemia.

The use of aperiodic analysis of the EEG during carotid artery surgery.

A new processed EEG machine, the Lifescan, which uses aperiodic analysis, was used to monitor cerebral activity prospectively in twenty-one patients undergoing carotid artery surgery under general anaesthesia. The machine was easy to apply, use and read. Volatile agents caused a bilateral decrease in high frequency activity. Unilateral changes consistent with cerebral ischaemia at the time of carotid cross-clamping were also seen. One such prolonged change was not associated with neurological deficit. A further patient awoke with neurological deficit without displaying Lifescan evidence of ischaemia. The machine requires further assessment.

Reliability of estimation of pulmonary artery wedge pressures during coronary artery bypass surgery.

The ability to reliably estimate pulmonary artery wedge pressure measurements using the clinical and basic cardiovascular parameters was assessed in fifty-six patients having elective coronary artery bypass surgery. The patients were divided into three groups: Group 1 had normal left ventricular function (22 patients), Group 2 had moderate left ventricular dysfunction (20 patients) and Group 3 had more severe left ventricular dysfunction (14 patients). The percentages of correct pulmonary artery wedge pressure estimations were 55%, 56% and 52% in Groups 1, 2 and 3 respectively. We conclude that pulmonary artery wedge pressure measurements can only be reliably estimated from the clinical situation and basic cardiovascular parameters on about 50% of occasions and that the reliability of the estimation does not vary between groups with different left ventricular performance.

Radial artery cannulation--the influence of method on blood flow after decannulation.

A prospective randomised study of two hundred patients undergoing open-heart surgery was carried out to determine if the method of radial artery cannulation (direct threading or transfixion) had any influence on the incidence of abnormal flow after decannulation. A standard 20-gauge non-tapered teflon-coated cannula was used and the groups were well matched for age, sex, wrist circumference, duration of cannulation and haematoma formation, all of which have been postulated to influence thrombosis rate. We were unable to demonstrate a statistically significant difference between the two methods of cannulation. The overall abnormal flow rate at five days assessed by Doppler ultrasound was low at 5%.