Gordon B. Drummond

Abdominal muscle activity and intraabdominal pressure after upper abdominal surgery

To examine the influence of abdominal muscle activity on intraabdominal pressure, the integrated surface EMG from upper abdominal muscle (EAB) was related to changes in intragastric pressure (PGA) in six patients after upper abdominal surgery. A similar respiratory pattern of EMG activity was observed in all subjects. EAB increased rapidly at the onset of expiration, and thereafter more slozoly throughout expiration. At the onset of inspiration EAB decreased abruptly, and was small during inspiration. Changes in intragastric pressures were closely related to changes in EAB. In fizle patients PGA and EAB waveforms were almost identical over the respiratory cycle. In one subject a biphasic change in PGA during inspiration was observed, suggesting the influence of other respiratory muscles. Abdominal muscle action results in changes in intraabdominal pressure previously attributed to diaphragmatic impairment.

Activity of lower intercostal and abdominal muscle after upper abdominal surgery.

The decrease in end-expiratory lung volume after upper abdominal surgery has been attributed, in part, to reflex spasm of the abdominal muscles. To examine the influence of abdominal surgery on abdominal muscle tone, electro-myographic (EMG) activity of abdominal and lower intercostal muscle was compared before operation with that at 3 hr and at 24 hr after operation in 18 healthy patients undergoing elective gastric or biliary surgery. After operation, EMG activity increased markedly and showed a phasic pattern of activity associated with respiration in most patients. This was characterized by a progressive increase in EMG activity during expiration with an abrupt decrease at the onset of inspiration. We conclude that increased expiratory activity in abdominal and lower intercostal muscle may be responsible for the decrease in lung volumes that occurs after upper abdominal surgery.

Validation of a new non-invasive automatic monitor of respiratory rate for postoperative subjects

BACKGROUND Respiratory rate is an important measurement in patient care but frequently poorly assessed. We set out to develop a simple non-invasive device to reliably measure respiratory movements and estimate respiratory rate, in clinical circumstances. METHODS Respiratory movement was detected with an encapsulated tri-axial accelerometer (Orient speck) and the data transmitted wirelessly to a computer for analysis. We studied subjects after gynaecological surgery who received opioid analgesia, and compared the derived signal with a signal from nasal cannula using directly matched breaths and within the same 5 min epoch. We analysed the signals for 5 min epochs over a 15 h recording period. RESULTS For matched breath analysis, the instantaneous respiratory rates matched within 2 bpm on 86% of occasions. A similar match was found between epoch averages of the respiratory rate. The mean absolute difference between the respiratory rate measured by nasal cannula and Orient speck was 0.6 bpm. The Orient speck generated reliable measures of respiratory rate every 5 min in 95.4% of epochs. CONCLUSIONS The Orient speck provides a reliable measure of respiratory rate at frequent intervals in subjects receiving patient-controlled morphine analgesia after surgery.

Spinal anesthesia and lumbar lordosis

Hyperbaric bupivacaine 0.5% (3.0 ml) was injected intr-thecally in two groups of 20 patients. Both groups of patients lay in the lateral position with their hips flexed at 90°. In groupF, the hip flexion was maintained for 5 minutes after turning supine. In group S, the hips were straightened before the patients were turned to the supine position. The technique of hip flexion to reduce the lumbar lord sis did not significantly limit the height of anesthetic blockade. The distribution of height of anesthetic blockade showed marked bimodality (P < 0.05) in both groups, in group F at T4 and T9 and in group S at T3 and T9. Cardiovascular side effects were minimal and equal in both groups.

Characterization of breathing patterns during patient-controlled opioid analgesia

BACKGROUND Respiratory rate is an important measurement in patient care, but accurate measurement is often difficult. We have developed a simple non-invasive device to measure respiratory movements in clinical circumstances, with minimal interference with the patient. We investigated respiratory patterns in patients receiving postoperative morphine analgesia to assess the capacity of the device to detect abnormalities. METHODS We studied subjects during self-administered opioid analgesia after major gynaecological surgery, and related the derived signals with a signal from a nasal cannula. Respiratory movement signals were transmitted wirelessly to a recorder from two encapsulated tri-axial accelerometer (RESpeck) sensors. We analysed the signals using two different sensor placements, each for 30 min. The nasal cannula signal was used to classify breathing patterns as obstructive or non-obstructed. RESULTS We studied 20 patients for a mean duration of 49 min each. Breathing patterns were very variable, between and within patients. The median breathing rates ranged from 6.4 to 19.5 bpm. Breathing was partly obstructed in 10 patients, and six patients had repeated cycles of obstruction and transient recovery. In these patients, we found a consistent and statistically significant pattern of changes in chest wall movement, with increased abdominal and decreased rib cage movement during obstruction. In patients with slow respiratory rates, breath-to-breath times were highly variable. CONCLUSIONS In undisturbed subjects receiving patient-controlled morphine analgesia after surgery, abnormal breathing patterns are extremely common. Cyclical airway obstruction is frequent and associated with a typical pattern of changes in chest wall movement.

Does sleep enhance the effect of subanesthetic isoflurane on hypoxic ventilation

After surgery, patients may receive little audiovisual stimulation and may sleep.Lack of audiovisual stimulation enhances the suppression of the hypoxic ventilatory response (HVR) by 0.1 minimum alveolar anesthetic concentration (MAC) isoflurane. Sleep also reduces the HVR and may thus increase the risk of hypoxia in patients at this time. We therefore measured the ventilatory response in volunteers to a sustained step hypoxic stimulus (mean arterial oxygen saturation [SaO2] 80% [SEM 0.3] for 20 min) in the presence of 0.1 MAC isoflurane, with subjects in the awake and asleep states. The behavioral states were studied in random order in nine male subjects. The combination of isoflurane and sleep significantly reduced (P < 0.05) normoxic ventilation (6.71 [0.39] vs 8.24 [0.29] L/min) and increased end-tidal PCO2 (PETCO2) (43.1 [0.5] vs 40.4 [0.8] mm Hg) compared with the awake state. However, ventilation was similar in the asleep and awake states during early (15.10 [1.35] vs 15.50 [1.61] L/min) and late (10.45 [0.97] vs 11.03 [0.99] L/min) hypoxia in the presence of isoflurane. Thus sleep did not reduce ventilation during hypoxia in the presence of isoflurane sedation. The increase in PETCO2 during sleep may have offset suppression of the HVR. (Anesth Analg 1995;81:751-6)

Direct measurement of nitrous oxide kinetics.

BACKGROUND Using conscious subjects, measurement of the effects of low concentrations of anaesthetic agents can allow the dynamics of onset and offset of the agent to be measured and kinetic values estimated. However, the tests have to be rapid and preferably assess cerebral function. METHODS We used a short version of the digit symbol substitution test (DSST) that allowed frequent measurement of the impairment caused by nitrous oxide. We compared 10 min of onset and offset of breathing 5% and 30% nitrous oxide in 30% oxygen, compared with 30% oxygen only. End-tidal nitrous oxide concentrations were used to predict the concentration in a central compartment, according to a range of T(1/2) values chosen to be consistent with possible cerebral blood flow values. RESULTS We studied 19 volunteers and estimated a mean response. Only 30% nitrous oxide decreased the DSST. When DSST scores were related to the values in the predicted central compartment, the best dose-effect relationship was found when the T(1/2) was 37 s, consistent with a regional blood flow of about 120 ml 100 g(-1) min(-1). CONCLUSIONS The onset of nitrous oxide effect on DSST is rapid, consistent with the perfusion of metabolically active cerebral cortical tissues. The rate of onset is greater than previous measures based on a motor test which involved the function of subcortical structures in the central nervous system.

Systemic recirculation assessed in apnoeic anaesthetized patients using carbon dioxide concentration measurements during stepwise expiration

BACKGROUND Mixed venous partial pressure of carbon dioxide has often been estimated in anaesthesia, usually by rebreathing techniques. This assumes equilibrium between respired gas and mixed venous blood before significant recirculation can occur, and requires vigorous rebreathing and precise identification of equilibrium. Modern clinical sidestream capnometers do not measure gas composition as rapidly as the devices used in previous studies, and cannot measure end-tidal values adequately during vigorous rebreathing. In contrast, a single-staged exhalation provides a stable sample for more accurate measurement of gas composition, and clinical measurements then allow examination of the evidence that equilibration occurs before recirculation. Theoretically, this assumption is optimistic, although it forms the basis of a method for non-invasive measurement of cardiac output. We used measurements of staged exhalation to study the evidence for equilibrium more closely. METHODS We studied 19 patients during general anaesthesia. The lungs were inflated with mixed exhaled gas. Stepwise expiration allowed exhaled gas to be analysed over approximately 1 min. The rate of increase in exhaled carbon dioxide fraction was related to the duration of expiration. RESULTS Carbon dioxide concentration continued to change throughout the study period. The lack of equilibrium of carbon dioxide concentration over this time supports simulation studies which predict recirculation from well-perfused body compartments within 20 s, and that the subsequent increase of carbon dioxide reflects the wash in of multiple body compartments. CONCLUSIONS This method allows adequate time for full response of sidestream analysers. Recirculation is an important early feature affecting breath-hold estimates of mixed venous carbon dioxide. Carbon dioxide accumulation during apnoea or rebreathing will have prompt effects on arterial carbon dioxide values and a constant mixed venous composition cannot be assumed when methods based on partial rebreathing are used over this time period.

Kinetics of anesthetic onset measured with a direct index of neural activity.

BACKGROUND: Previous modeling of the kinetics of uptake and elimination of anesthetic drugs from the site of action has used measures derived from the electroencephalogram. Such measures lag the current brain activity because of the time needed to acquire a signal sample and derive the measure. With a direct measure of anesthetic activity, we could model brain uptake more exactly. METHODS: In volunteers, using a double-blind single-session design, we made repeated measurements using a well-known psychomotor test, the 2 target tapping test, during the washin and washout of 30% nitrous oxide. We also assessed maximal drug effect with a test of cognitive function, the digit symbol substitution test. Concentration at the site of action was modeled from end-tidal measurements, using a simple exponential washin and washout function, with half-times between 0.5 and 3 minutes. Comparisons were made within subjects, using 0 and 5% nitrous oxide. RESULTS: We studied 20 subjects. Nitrous oxide, at 30%, consistently reduced performance of the digit symbol substitution test. Tapping frequency was also reduced, but the effect was less consistent, and only 9 of 20 subjects showed a significant individual reduction in tapping frequency. In these subjects, the relationship between the modeled brain concentration and drug effect was better with a half-time set at 2 minutes, compared with 1.5 or 3 minutes. CONCLUSIONS: Given in subanesthetic concentrations, nitrous oxide has rapid onset and offset, consistent with a half-time of 2 minutes. This value is less than the values expected from studies during anesthesia using processed electroencephalogram, but consistent with measures of blood flow to active cerebral tissue in conscious subjects. Studies of performance in conscious subjects may aid further studies of anesthetic kinetics.