Department of Anaesthesiology, Section of Pathophysiology and Process Development, University of Ulm, D-89073 Ulm, Germany*Corresponding author
Accepted for publication: May 4, 2001
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Abstract |
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Br J Anaesth 2001; 87: 4903
Keywords: ventilation, mechanical; surgery, cardiovascular
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Introduction |
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Oxygen consumption and cardiac index increase in patients during weaning from mechanical ventilation after cardiac surgery,3 and the additional load on the cardiovascular system could be crucial in some patients. To study the underlying mechanisms we measured V·O2, WOB, cardiovascular effects, and the plasma concentrations of catecholamines and other hormones involved in the stress response during weaning using different ventilatory modes, in patients who had aortocoronary bypass surgery.
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Methods and results |
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Mechanical ventilation was with an EVITA-2 ventilator (Drägerwerk AG, Lübeck, Germany) using the following settings: volume-controlled continuous positive pressure ventilation (CPPV), tidal volume (VT)=810 ml kg1, and respiratory rate (f)=10 breaths min1) until rewarming and recovery from anaesthesia was complete. If cardiovascular measurements were stable and no surgical complications had occurred, a set of control data was obtained as described below. The propofol infusion was then stopped. As soon as the patients were able to breathe spontaneously at a respiratory rate of 1025 breaths min1, the ventilator was set according to a preset random order (which was masked for the physicians involved in patient care) to one of the two study settings, for example, synchronized intermittent mandatory ventilation (SIMV) or biphasic positive airway pressure (BIPAP), both combined with inspiratory pressure support (PS). The mechanical respiratory rate (fSIMV) was reduced to 5 breaths min1, and the pressure support adjusted to give a VT of about 50% of the passive mechanical breaths. During SIMV the VT was set as during CPPV, whereas during BIPAP the inspiratory pressure level (Pinsp) was set to the plateau inspiratory pressure noted during CPPV. At least 120 min after the propofol infusion had been stopped, a set of data was obtained. We then switched to the other test mode and sampled data again after 60 min.
We recorded the following variables:
Flow, airway pressure and oesophageal pressure (measured from a balloon catheter inserted using a laryngoscope immediately after induction of anaesthesia and positioned using the occlusion test4), as continuously digitised values to allow off-line calculation of respiratory rate (f), minute ventilation (MV), pressure time product (PTP), and WOB methods described previously.5
Heart rate (HR), systemic vascular pressure, and cardiac index (CI).
V·O2 (V·O2 measured), carbon dioxide production rate (V·CO2), respiratory quotient (RQ=V·CO2/V·O2 measured) and mean expired PCO2 using the Deltatrac® metabolic monitor (Datex Corp., Helsinki, Finland).
Arterial and mixed venous blood gases for the calculation of venous admixture (QS/QT), dead space ventilation (VD/VT) and V·O2 (V·O2 calculated) according to standard equations and the Fick principle, respectively.
Plasma levels of epinephrine, norepinephrine, ACTH, cortisol, vasopressin and prolactin using methods previously described.6 7
Statistical analysis was performed using a Friedman rank-sign analysis for repeated measures and a subsequent non-parametric Student NewmanKeuls test.
All data are presented as medians and quartile values in Table 1. Although cardiovascular and gas exchange measurements were in the normal range during all study conditions, plasma values of catecholamines and hormones, except for cortisol and prolactin (probably partly suppressed by the low-dose dopamine-infusion administered during weaning from cardiopulmonary bypass), were elevated regardless of the ventilatory mode used during the corresponding test period. The values noted in the three study conditions, however, were not statistically significant. f, WOB, and PTP were greater during both partial ventilation support modes than during control. However, no statistically significant differences were found when comparing SIMV+PS with BIPAP+PS.1
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Comment |
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The catecholamine and stress hormone blood concentrations we found generally were greater than normal and also greater than values previously measured after abdominal surgery.6 The hormone release may have contributed to haemodynamic stability as was demonstrated in that study.6 These catecholamine and hormone concentrations were already present in the control period and did not increase after withdrawal of sedation and switching from passive ventilation to partial spontaneous breathing. Therefore, we believe that after cardiac surgery the hormonal response is not affected by the discomfort of breathing. Even BIPAP, which can make breathing more comfortable during weaning and reduce the need for sedative drugs9 did not affect the plasma catecholamine and stress hormone levels.
The hormonal stress response may provide an objective method to measure patient comfort during mechanical ventilation. It has been used successfully in preterm infants to demonstrate the advantage of mechanically supported spontaneous breathing over controlled mechanical ventilation in terms of reduced stress response.10 Assessment of the attempt to determine the physiologic stress response during weaning by measuring stress hormone blood concentrations in adults have not been attempted before.
We found that during the early stages of weaning from mechanical ventilation after cardiac surgery, switching from passive ventilation to partial spontaneous breathing and interruption of continuous sedation did not alter the postoperative hormonal stress response and V·O2. Obviously, since we studied a small group of subjects after uncomplicated heart surgery, without pre-existing pulmonary diseases and limited myocardial function, the results may not apply to patients with compromised circulatory or respiratory function or if weaning from prolonged mechanical ventilation.
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References |
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