Pulmonary blood flow measured by inspiratory inert gas concentration forcing oscillations

The aim of this study was to discover if the forced inspired inert gas sinewave technique could be used to measure pulmonary blood flow, using nitrous oxide as the indicator gas, following inotropic stimulation of the heart by dobutamine, in the presence of a constant alveolar ventilation. Cardiac output (range 1-4.5 L min(-1)) was measured in six dogs by thermodilution and by calculation from the sinusoidal expired partial pressures of argon and nitrous oxide using: (i) analytical equations and a conventional continuous ventilation three-compartment lung model, which did not include recirculation; and (ii) a digital simulation tidal ventilation lung model (Gavaghan and Hahn, 1996. Respir. Physiol. 106, 209-221) which was adapted to include nitrous oxide mixed-venous recirculation from a combined single viscera compartment. The continuous ventilation model calculations always underestimated thermodilution cardiac output, with the bias error increasing to almost -1 L min(-1) at the longest forcing periods, 4-5 min. In contrast, the tidal ventilation model calculations were in close agreement to thermodilution cardiac output, with biases of -0.04 and -0.26 L min(-1) at forcing periods of 2 and 3 min, respectively.

Reporting of 'hypotension' after epidural analgesia during labour. Effect of choice of arm and timing of baseline readings

We studied 20 women in labour to see how reporting 'hypotension' after obstetric epidural analgesia is affected by position of the blood pressure cuff and baseline definition. Blood pressure was recorded from both arms simultaneously while the woman was semirecumbent and then in the left lateral position. Three readings were then taken after epidural bupivacaine, one left lateral and the remainder right lateral. Before the epidural, blood pressure in the dependent arm in the lateral position was similar to blood pressure in either arm in the semirecumbent position and an average of 10 mmHg (systolic) and 14 mmHg (diastolic) higher than blood pressure in the uppermost arm (p < or = 0.00005). This difference persisted in both lateral positions as epidural analgesia became established. Choosing different definitions of hypotension, baselines and arm to measure blood pressure resulted in 'hypotension rates' between 0% and 75%. For blood pressure measurement in the lateral position, the blood pressure cuff should be placed on the dependent arm.

The effects of cardiopulmonary bypass temperature on neuropsychologic outcome after coronary artery operations: a prospective randomized trial

The effect of systemic perfusion temperature on postoperative cognitive function was investigated in 96 adult patients undergoing elective coronary revascularization with cardiopulmonary bypass at 28 degrees C, 32 degrees C, or 37 degrees C. Neuropsychologic performance was assessed 1 day before the operation and 6 weeks after the operation. Five tests were adapted from the Wechsler Adult Intelligence Scale and two from the Wechsler Memory Scale.

RESULTS

No patients had major neurologic complications. Ninety-three patients completed the five Wechsler Adult Intelligence Scale tests, but only 70 went on to complete the Wechsler Memory Scale tests as well. In these, there was an effect of cardiopulmonary bypass temperature on the number of neuropsychologic tests in which there was a preoperative to postoperative deterioration (p = 0.021), the number with bypass at 37 degrees C being significantly greater than the number with bypass at 32 degrees C (p = 0.015). Subsidiary analyses using a multivariate linear model examined the effect of cardiopulmonary bypass temperature on the magnitude of change, with or without allowing for other possible confounding influences. There was an adverse effect of normothermic (37 degrees C) versus moderately hypothermic (32 degrees C) perfusion---more convincingly displayed in the analyses of all seven scores rather than just the Wechsler Adult Intelligence Scale scores. Further cooling to 28 degrees C conferred no additional benefit in terms of cognitive function. The importance of the deterioration is open to question.

Disturbed mother-infant interaction in association with congenital heart disease

OBJECTIVE

To investigate whether difficulties are experienced in the interaction between infants with congenital heart disease and their mothers and to identify infants who show compromised emotional development, in order to offer intervention during the early stages of postoperative compensatory growth.

METHODS

20 infants and their mothers were compared with 20 non-cardiac mother-infant pairs. Infants were filmed in interaction for 30 minutes two days before and six months after corrective surgery. Fifteen minutes of film were analysed in 180 5-s units. The emotional tone (affect) and the interpersonal engagement were classified as positive or negative by prespecified criteria. The percentages of positive scores were analysed. The mental health of the mothers was also assessed.

RESULTS

Cardiac infants showed less positive affect and engagement than the noncardiac group at both sessions. There was no correlation between of positive affect or engagement and the severity of the condition in either group. Cardiac mothers showed less positive affect and engagement than the comparison group, and were psychologically distressed at both sessions. The engagement scores of the mothers of the cardiac infants were also more variable.

CONCLUSIONS

Cardiac infants and their mothers have lower levels of positive affect and engagement than non-cardiac mother-infant pairs. Thus some mothers are unable to adapt to their infant. This leads to disordered interaction which is maintained at six months. This information can be used to offer intervention during the early stages of postoperative compensatory growth.

Patient-controlled epidural diamorphine for post-operative pain: verbal rating and visual analogue assessments of pain

Twenty-two patients were studied while receiving epidural analgesia with diamorphine after major lower abdominal surgery under combined regional and general anaesthesia. Epidural PCA began when the intraoperative epidural block with bupivacaine wore off enough for the patient to request treatment. It was started with 2 mg of diamorphine and continued with a reducible background infusion that was initially set at 0.2 mg h-1 and supplemented by on-demand doses of 0.2 mg, with a lockout time of 15 min. The patients received routine post-operative monitoring and care, with pain at rest being assessed on a four-point verbal rating scale (VRS, none, mild, moderate, severe) at 5, 10, 15, 30, 45, 60, 90 and 120 min from the start of ePCA, then hourly until 24 h and then 2-hourly until 48 h. VRS on coughing and a 10 cm visual analogue score (VAS) at rest and on coughing were recorded at the same times at 4 h, then 4 hourly until 24 h and then at 48 h, at which times, blood samples were also taken to measure morphine concentrations by radioimmunoassay. Analgesia started promptly and reached a maximum at between 30 and 45 min, accompanied by maximum sedation. Thereafter clinically acceptable analgesia was maintained without undue sedation for 48 h, though pain on coughing was less well controlled than pain at rest. After the initial loading dose of diamorphine, the 95% confidence intervals (CI) for further consumption were 3.7 to 17 mg (average 9.7) in the first 24 h and 2.1 to 12.9 mg (average 6.7 mg) in the second 24 h. The plasma morphine concentrations rose to a plateau by about 15 min, with concentrations within 95% CI from 0 to 11 ng mliters-1 (average 5 ng mliters-1. The VRS and VAS pain scores were analysed by a conservative approach that treated them as ordinal data, and by a parametric approach that treated them as interval data. Both approaches conveyed broadly similar information about the post-operative analgesia.

Effects of cardiopulmonary bypass temperature on pulmonary gas exchange after coronary artery operations

BACKGROUND

Pulmonary dysfunction is one aspect of the postoperative morbidity associated with cardiopulmonary bypass. Normothermic systemic perfusion can result in shorter intubation times, which have been attributed to improved pulmonary gas exchange, but the influence of perfusion temperature on pulmonary gas exchange itself is not known.

METHODS

Pulmonary gas exchange was assessed using alveolar-arterial oxygen pressure gradients in 45 patients undergoing routine coronary revascularization who were randomized to undergo cardiopulmonary bypass at 28 degrees C, 32 degrees C, or 37 degrees C. This was part of a more comprehensive study of the effects of temperature on bodily systems. The gradients were estimated preoperatively with the patients breathing air, again over a period between 2 and 4 hours postoperatively during mechanical ventilation with three different oxygen concentrations (30%, 40%, and 60%), and again 1 hour after extubation while breathing the same three oxygen concentrations.

RESULTS

Preoperative alveolar-arterial oxygen pressure gradients on air were 24.4 +/- 8.2 mm Hg (mean +/- standard deviation) (28 degrees C), 24.5 +/- 20.4 mm Hg (32 degrees C), and 20.5 +/- 9.5 mm Hg (37 degrees C). Postoperatively, during ventilation and after rewarming, the gradients increased with the increase in inspired oxygen fraction concentrations (30% to 60%) from 67.1 +/- 12.0 mm Hg to 193.1 +/- 30.5 mm Hg (28 degrees C), from 76.4 +/- 20.6 mm Hg to 246.7 +/- 47.7 mm Hg (32 degrees C), and from 79.0 +/- 18.0 mm Hg to 222.9 +/- 40.5 mm Hg (37 degrees C), respectively. A similar pattern was noted 1 hour after extubation, when the gradients increased from 72.4 +/- 12.5 mm Hg to 256.6 +/- 26.5 mm Hg (28 degrees C), from 75.7 +/- 13.9 mm Hg to 252.7 +/- 38.3 mm Hg (32 degrees C), and from 69.1 +/- 19.3 mm Hg to 253.1 +/- 33.0 mm Hg (37 degrees C). There were no significant differences in alveolar-arterial oxygen pressure gradient between the three groups during ventilation or after extubation.

CONCLUSIONS

Cardiopulmonary bypass perfusion temperature does not influence alveolar-arterial oxygen pressure gradients in the first 12 hours after routine coronary artery bypass grafting in patients with uncompromised pulmonary and left ventricular function.

Epidural diamorphine infusions with and without 0.167% bupivacaine for post-operative analgesia

Forty patients who underwent upper or mid-abdominal surgery were randomly allocated to receive a post-operative epidural infusion of 0.083 mg ml-1 of diamorphine in either 0.167% bupivacaine or 0.9% NaCl solution. The nursing staff, who were unaware of which solution was being infused, managed the patients' pain according to a standardized scheme. They adjusted the epidural infusion rates to 3, 5 or 7 ml h-1 according to the patient's hourly reports of pain on a four point verbal rating scale (none, mild, moderate or severe), aiming to use the lowest allowed infusion rate to prevent or reduce any pain that was more than mild. Additional analgesia was given as diclofenac 75 mg intramuscularly if the patients report moderate pain while on the highest infusion rate. The nurses were instructed to summon anaesthetic help if pain relief was still unsatisfactory after diclofenac, but this was never necessary. Diclofenac was needed by six patients receiving diamorphine in saline and one receiving diamorphine in bupivacaine (P < 0.05). The range of average hourly epidural infusion rates was constrained by design to between 3 and 7 ml h-1 but the median of these values was 5 ml h-1 in the diamorphine-saline group and 3.35 ml h-1 in the diamorphine-bupivacaine group (P < 0.02). In patients receiving diamorphine in saline, a median of 6 (range 0-16) of the 24 h reports were of more than mild pain, whereas in the diamorphine-bupivacaine group, the corresponding figures were 2 (range 0-13) (P < 0.02)).(ABSTRACT TRUNCATED AT 250 WORDS)

Epidural infusions of sufentanil with and without bupivacaine: comparison with diamorphine-bupivacaine

The requirements for supplementary 3 ml epidural injections of bupivacaine 0.5% (top-ups) were used in a randomized double-blind study to compare the effects of five types of thoracic epidural infusions given at 2.5 ml h-1 for the first 24 h after major surgery to the upper abdomen in 99 patients and the lower abdomen in 72. The infusions were: bupivacaine 0.167% alone; diamorphine 0.167 mg ml-1 (0.417 mg h-1) in bupivacaine 0.167%; sufentanil 2 micrograms ml-1 (5 micrograms h-1) in 0.167% bupivacaine; sufentanil 4 micrograms ml-1 (10 micrograms h-1) in 0.167% bupivacaine; and sufentanil 4 micrograms ml-1 (10 micrograms h-1) in normal saline. The patients who had upper abdominal surgery were on average older than those having lower abdominal surgery and a larger proportion of them were female. They received on average fewer top-ups. After both upper and lower abdominal surgery, epidural infusions of bupivacaine alone required the most frequent supplementation (inter-quartile range 6-14 top-ups in 24 h) and the two sufentanil-bupivacaine mixtures required the fewest (interquartile range 0-12 top-ups in 24 h). The infusions of sufentanil without bupivacaine were significantly less effective than the sufentanil-bupivacaine mixtures after upper (but not lower) abdominal surgery. Although the two sufentanil-bupivacaine mixtures were indistinguishable in analgesic effectiveness after either upper or lower abdominal surgery, the lower (5 micrograms h-1) dose rate of sufentanil gave a significantly higher average breathing rate and lower average PaCO2 for the first 24 h after lower (but not upper) abdominal surgery. Blood samples were taken (as an afterthought) from 11 patients receiving sufentanil 10 micrograms h-1, just before the epidural infusion was stopped. The concentrations were mostly above the range for systemic analgesia, but below the values that would have been expected if a steady state had been achieved.

Assessment of cardiorespiratory function using oscillating inert gas forcing signals

A theoretical model (Hahn et al. J. Appl. Physiol. 75: 1863-1876, 1993) predicts that the amplitudes of the argon and nitrous oxide inspired, end-expired, and mixed expired sinusoids at forcing periods in the range of 2-3 min (frequency 0.3-0.5 min-1) can be used directly to measure airway dead space, lung alveolar volume, and pulmonary blood flow. We tested the ability of this procedure to measure these parameters continuously by feeding monosinusoidal argon and nitrous oxide forcing signals (6 +/- 4% vol/vol) into the inspired airstream of nine anesthetized ventilated dogs. Close agreement was found between single-breath and sinusoid airway dead space measurements (mean difference 15 +/- 6%, 95% confidence limit), N2 washout and sinusoid alveolar volume (mean difference 4 +/- 6%, 95% confidence limit), and thermal dilution and sinusoid pulmonary blood flow (mean difference 12 +/- 11%, 95% confidence limit). The application of 1 kPa positive end-expiratory pressure increased airway dead space by 12% and alveolar volume from 0.8 to 1.1 liters but did not alter pulmonary blood flow, as measured by both the sinusoid and comparator techniques. Our findings show that the noninvasive sinusoid technique can be used to measure cardiorespiratory lung function and allows changes in function to be resolved in 2 min.

Measurement of dead-space in a model lung using an oscillating inspired argon signal

In a model lung, airways dead-space can be accurately measured using a forced inspired oscillating argon signal, which varies sinusoidally about a mean concentration of 6% v/v with an amplitude of +/- 4% v/v. With sinusoid forcing periods longer than 120 seconds, and at a breathing rate of 13.4 breaths minute-1, the mean airways dead-space can be measured with a standard error of less than 5%. Sinusoid forcing periods shorter than 120 s provided inaccurate estimates of dead-space and so should not be used with this technique.

Gas exchange in a three-compartment lung model analyzed by forcing sinusoids of N2O

A mathematical gas exchange model, using sinusoidal forcing functions of inert inspired gas (A. Zwart, R. C. Seagrave, and A. Van Dieren. J. Appl. Physiol. 41: 419-424, 1976), has been extended by us to include dead space (VD), a single alveolar compartment (VA) perfused with blood flow (Qp), and a shunt (Qs). In this new work we use N2O as the indicator gas in the mathematical model and in the experimental studies, in low enough concentrations [<6% (vol/vol)] to avoid anesthetic effects. Mathematical relationships between the inspired and expired N2O gas partial pressures, the blood gas N2O partial pressures, and their variation with forcing frequency are derived for a continuous ventilation uptake and a conventional anesthetic gas distribution model. We show that these gas and blood gas N2O relationships give direct derivation of cardiorespiratory parameters such as VA, Qp, the dead space-to-total ventilation ratio (VD/VT), and the shunt-to-total blood flow ratio (Qs/QT) without altering the subject's oxygenation and that they are essentially free from recirculation effects at high forcing frequencies > or = 2 min-1. Theoretical results from the model are presented for a wide range of forcing frequencies between 2 x 10(-2) and 10 min-1 (sinusoid periods 30-0.1 min), and these show that VA, Qp, and VD/VT can all be measured by N2O forcing frequencies > or = 1 min-1. We also present results from five animal studies, with an experimental inspired gas forcing frequency range of 0.125 to 2 min-1, which show qualitative agreement with the predictions of the continuous ventilation model. During these animal studies both mass spectrometric N2O respiratory gas measurements and intravascular polarographic arterial and mixed venous blood N2O partial pressure measurements were made, and examples of these in vivo measurements are presented, together with examples of the calculations derived from them.

A compensation method for membrane-covered (Clark) electrodes

Membrane-covered electrodes (Clark electrodes) are widely used for monitoring blood gases, particularly PO2. A method of compensating for the inherently limited speed of response of Clark electrodes is presented. The theoretical response in the time domain is related to that in the frequency domain, and the latter is deduced from measurement of the former. Although the response functions are both infinite series, both responses are nevertheless completely defined by a single time parameter Te characteristic of the electrode under given measurement conditions. Practical verification was performed using electrodes in the double-pulsed mode, but the theory is applicable equally to direct-current-polarized and simply pulsed electrodes.

A digital system for generating dynamic sinusoidal gas concentration signals

A computer-controlled gas-mixing system is presented. It is capable of mixing four gases, the concentration of three of which will follow a path to be determined by the user. For our purposes the output O2 fraction is maintained constant and the levels of Ar and N2O vary sinusoidally and independently, with periods between 0.25 and 30 min. A fourth gas, N2 is necessary to make the sum of the individual fractions 100%. The system uses banks of between one and four solenoid valves each linked via a sonic choke to a common mixing chamber. A regime of pulse frequency modulation is employed. All calculations and timing of valve switching are performed by a dedicated microcomputer built for the purpose. The device has been used to provide respiratory gas forcing functions for a program of research in respiratory monitoring.

Inter-relations of the volume and timing components of ventilation during carbon dioxide rebreathing in awake and anaesthetized subjects

We studied the relations between the volume and timing components of ventilation during carbon dioxide rebreathing in conscious subjects and patients anaesthetized with propofol. In conscious responses, breath-by-breath minute ventilation (VI) generally correlated better with end-tidal carbon dioxide than did tidal volume (VT), but VT correlated better than VI in the anaesthetized responses. The source of this difference was that, whereas VT and the inspiratory period were both smaller and less variable when subjects were anaesthetized rather than conscious, the expiratory period was no less variable, and this disturbed the usual inverse relation between VT and the duration of the ventilatory cycle. Anaesthesia stabilized the switch from inspiration to expiration, but not that from expiration to inspiration. In some patients, it produced a disturbance pronounced enough to suggest bimodality of the timing of expiration.

Some ventilatory effects of propofol as sole anaesthetic agent

Ventilatory effects of propofol, used as a sole agent for the induction and maintenance of general anaesthesia, were studied in 14 healthy unpremedicated patients. Subarachnoid anaesthesia was established before induction of general anaesthesia. Induction was with propofol 2.5 mg kg-1 given while the patients breathed 100% oxygen. We intended to start an infusion of propofol 100 micrograms kg-1 min-1; maintain it for at least 25 min; make a first set of quasi-steady-state observations; double the infusion; and repeat observations after 25 min. The single induction bolus plus single rate infusion was not totally satisfactory: further boluses were usually needed. At induction there was apnoea in all but three patients, sometimes lasting more than 3 min; hyperventilation before induction, combined with hyperoxia, probably exaggerated this. Established ventilatory rates were generally 30% higher than awake. One patient became bradypnoeic. Tidal volume and minute ventilation, and the Tl:Ttot ratio, were reduced. Doubling the infusion rate had no clear effect on frequency or tidal volume, but it further reduced the Tl:Ttot ratio and caused an increase in PE'CO2 of 1 kPa. The ventilatory response to carbon dioxide was 58% of baseline awake control (95% confidence limits +/- 26%) at the lower infusion rate, with further slight depression when the infusion rate was doubled. Doubling the rate of infusion of propofol did not give twice the effect on ventilation, and probably is not giving twice the "depth" of anaesthesia. We cannot say if this is for pharmacokinetic or pharmacodynamic reasons.