Stress response during weaning after cardiac surgery

E. Calzia*, M. Koch, W. Stahl, P. Radermacher and A. Brinkmann

Department of Anaesthesiology, Section of Pathophysiology and Process Development, University of Ulm, D-89073 Ulm, Germany*Corresponding author

Accepted for publication: May 4, 2001


    Abstract
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
We compared the effects of weaning using synchronized intermittent mandatory ventilation (SIMV) with the use of biphasic positive airway pressure (BIPAP) on the stress response, oxygen uptake (V·O2) and work of breathing (WOB) in 10 patients after aortocoronary bypass surgery. All three ventilatory settings were investigated in each patient, for example, volume-controlled mechanical ventilation immediately before weaning was followed, in randomized order, by both SIMV and BIPAP. In addition to routine monitoring of continuous and respiratory state, we measured V·O2, WOB, and pressure–time product (PTP) as well as the plasma concentrations of epinephrine, norepinephrine, ACTH, cortisol, vasopressin, and prolactin. Although respiratory rate (f), WOB and PTP were greater with both SIMV and BIPAP when compared with control, other variables did not change with the ventilatory mode. In conclusion, weaning from mechanical ventilation using partial support modes does not affect the postoperative stress response in patients who have had uncomplicated cardiac surgery.

Br J Anaesth 2001; 87: 490–3

Keywords: ventilation, mechanical; surgery, cardiovascular


    Introduction
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
The effect of weaning on the postoperative stress response in surgical patients is uncertain. In these circumstances, oxygen consumption (V·O2) and plasma catecholamine levels are considered to be determined by and related to the work of breathing (WOB).1 2 The effects of sedation and the ventilator mode need to be clarified.

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.


    Methods and results
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
After approval by the local ethical committee we studied 10 patients who gave written informed consent. They were admitted to the intensive care unit (ICU) after aortocoronary bypass surgery. Inclusion criteria were as defined in a previous study.4 Anaesthesia was obtained with i.v. propofol (2 mg kg–1 bolus followed by a continuous infusion of 25–75 µg kg–1 min–1), fentanyl (40–80 µg kg–1) and muscle relaxation obtained with pancuronium bromide (0.2–0.3 mg kg–1). After operation, a propofol infusion (25–50 µg kg–1 min–1) was used for sedation.

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)=8–10 ml kg–1, and respiratory rate (f)=10 breaths min–1) 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 10–25 breaths min–1, 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 min–1, 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 test’4), 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 Newman–Keuls 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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Table 1 All values are medians, with quartile values. *P<0.05 control vs SIMV+PS and BIPAP+PS, respectively (Friedman rank sign analysis for repeated measures, followed by non-parametric Student Newman–Keuls test). Abbreviations are: HR=heart rate; MAP=mean arterial pressure; CI=cardiac index; AVDO2=arterial-venous oxygen content difference; PaO2=arterial oxygen partial pressure; PaCO2=arterial carbon dioxide partial pressure; QS/QS=venous admixture; VD/VT=dead space ventilation; V·O2 calculated=oxygen consumption calculated according to Fick’s principle; V·O2 measured=oxygen consumption measured by means of indirect calorimetry; V·CO2=carbon dioxide production; RQ=respiratory quotient which equals the ratio V·CO2/V·O2 measured, f=respiratory rate; MV=minute ventilation; WOB=work of breathing; PTP=pressure-time product. Normal plasma concentrations of hormones are: epinephrine 12–110 pg ml–1; eorepinephrine 120–520 pg ml–1; ACTH 9–25 pg ml–1; cortisol 5–25 µg 100 ml–1; vasopressin <6.7 pg ml–1; prolactin 5–15 ng ml–1 (male) and 5–25 ng–1 ml (female)
 


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Fig 1 The box plots show the work of breathing (WOB, lower panel) and oxygen consumption as measured by the Deltatrac® device (V·O2 measured, upper panel) during the different study conditions. The boxes represent medians (horizontal lines in each box) and quartiles. Five and 95% percentiles are indicated by the error bars. Single values outside of the percentile-ranges are drawn as dots. The circled ‘X’ sign indicates the statistically significant difference between the WOB measured during volume-controlled mechanical ventilation (control condition) and either of the two test-modes corresponding to SIMV+PS and BIPAP+PS.

 

    Comment
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Wet set out to measure V·O2 and the stress hormone response at the same time during weaning after cardiac surgery. Compared with previous measurements in intubated, spontaneously breathing patients,4 WOB and PTP were less with both modes SIMV and BIPAP because of the greater degree of mechanical support applied. Nevertheless, we showed that both ventilatory modes required the patients to sustain a significant portion of their minute ventilation. The patients were sufficiently awake to completely control their own breathing pattern, as seen from their respiratory rates. Interestingly, V·O2 did not increase when WOB was increased after switching to either SIMV or BIPAP, indicating that spontaneous breathing was performed efficiently, with a minimal influence on the cardiovascular system.1 This finding was independent of the method used to determine V·O2. The difference between V·O2 measured and V·O2 calculated was probably related to the pulmonary oxygen uptake, which V·O2 calculated does not measure. Therefore, using the Fick method to measure total body V·O2 after cardiopulmonary bypass may underestimate the true value by up to 20–30%.8

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.


    References
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 Abstract
 Introduction
 Methods and results
 Comment
 References
 
1 Pinsky MR. Breathing as exercise: the cardiovascular response to weaning from mechanical ventilation. Intensive Care Med 2000; 26: 1164–6[ISI][Medline]

2 Oh TE, Bhatt S, Lin ES, Hutchinson RC, Low JM. Plasma catecholamines and oxygen consumption during weaning from mechanical ventilation. Intensive Care Med 1991; 17: 199–203[ISI][Medline]

3 DeBacker D, El Haddad P, Preiser JC, Vincent JL. Hemodynamic responses to successful weaning from mechanical ventilation after cardiovascular surgery. Intensive Care Med 2000; 26: 1201–6[ISI][Medline]

4 Baydur A, Behrakis PK, Zin WA, Jaeger MJ, Milic-Emili J. A simple method for assessing the validity of the esophageal balloon technique. Am Rev Respir Dis 1982; 126: 788–91[ISI][Medline]

5 Calzia E, Lindner KH, Stahl W, Martin A, Radermacher P, Georgieff M. Work of breathing, inspiratory flow response, and expiratory resistance during continuous positive airway pressure with the ventilators EVITA-2, EVITA-4 and SV 300. Intensive Care Med 1998; 24: 931–8.[ISI][Medline]

6 Brinkmann A, Seeling W, Wolf CF, et al. Vasopressor hormone response following mesenteric traction during major abdominal surgery. Acta Anaesthesiol Scand 1998; 42: 948–56

7 Strohmenger HU, Lindner KH, Keller A, Lindner IM, Bothner U, Georgieff M. Release of endogenous vasopressors during and after cardiopulmonary resuscitation. Crit Care Med 1995; 23: 1347–55[ISI][Medline]

8 Oudemans-van Straaten HM, Scheffer GJ, Eysman L, Wildevuur ChRH. Oxygen consumption after cardiopulmonary bypass – implications of different measuring methods. Intensive Care Med 1993; 19: 105–10[ISI][Medline]

9 Rathgeber J, Schorn B, Falk V, Kazmaier S, Spiegel T, Burchardi H. The influence of controlled mandatory ventilation (CMV), intermittent mandatory ventilation (IMV) and biphasic interpositive airway pressure (BIPAP) on duration of intubation and consumption of analgesics and sedatives. A prospective analysis in 596 patients following adult cardiac surgery. Eur J Anaesthesiol 1997; 14: 576–82[ISI][Medline]

10 Quinn MW, de Boer RC, Ansari N, Baumer JH. Stress response and mode of ventilation in preterm infants. Arch Dis Child Fetal Neonatal Ed 1998; 78: F195–8[ISI]