Intensive Care Medicine

, Volume 31, Issue 1, pp 79–85

Differential effects of human atrial natriuretic peptide and furosemide on glomerular filtration rate and renal oxygen consumption in humans

Authors

  • Kristina Swärd
    • Department of Cardiothoracic Anesthesia and Intensive CareSahlgrenska University Hospital
  • Felix Valsson
    • Department of Cardiothoracic Anesthesia and Intensive CareSahlgrenska University Hospital
  • Johan Sellgren
    • Department of Cardiothoracic Anesthesia and Intensive CareSahlgrenska University Hospital
    • Department of Cardiothoracic Anesthesia and Intensive CareSahlgrenska University Hospital
Original

DOI: 10.1007/s00134-004-2490-3

Cite this article as:
Swärd, K., Valsson, F., Sellgren, J. et al. Intensive Care Med (2005) 31: 79. doi:10.1007/s00134-004-2490-3
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Abstract

Objective

Imbalance in the renal medullary oxygen supply/demand relationship can cause hypoxic medullary damage and ischemic acute renal failure. Human atrial natriuretic peptide (h-ANP) increases glomerular filtration rate in clinical acute renal failure. This would increase renal oxygen consumption due to increased tubular load of sodium. Loop diuretics are commonly used in acute renal failure. Data on the effects of loop diuretics on glomerular filtration rate and renal oxygen consumption in humans are, however, controversial. We evaluated the effects of h-ANP and furosemide on renal oxygen consumption, glomerular filtration rate, and renal hemodynamics in humans.

Design and setting

Prospective two-agent interventional study in a university hospital cardiothoracic ICU

Patients

Nineteen uncomplicated, mechanically ventilated postcardiac surgery patients with normal renal function.

Interventions

h-ANP (25 and 50 ng/kg per minute, n=10) or furosemide (0.5 mg/kg per hour, n=9)

Measurements and results

Renal plasma flow and glomerular filtration rate were measured using the infusion clearance technique for 51Cr-labeled EDTA and paraaminohippurate, corrected for by renal extraction of PAH. h-ANP increased glomerular filtration rate, renal filtration fraction, fractional excretion of sodium, and urine flow. This was accompanied by an increase in tubular sodium reabsorption (9%) and renal oxygen consumption (26%). Furosemide infusion caused a 10- and 15-fold increase in urine flow and fractional excretion of sodium, respectively, accompanied by a decrease in tubular sodium reabsorption (–28%), renal oxygen consumption (–23%), glomerular filtration rate and filtration fraction (–12% and −7%, respectively).

Conclusions

The filtered load of sodium is an important determinant of renal oxygen consumption. h-ANP improves glomerular filtration rate but does not have energy-conserving tubular effects. In contrast, furosemide decreases tubular sodium reabsorption and renal oxygen consumption and thus has the potential to improve the oxygen supply/demand relationship in clinical ischemic acute renal failure.

Keywords

Human atrial natriuretic peptideFurosemideGlomerular filtration rateRenal oxygen consumptionSodium reabsorption

Introduction

Acute renal failure (ARF) following cardiac surgery is associated with a significant morbidity and mortality [1, 2, 3]. The pathogenesis of postoperative ARF is believed to be predominantly a consequence of renal hypoperfusion and ischemia, particularly of the renal medulla [4, 5]. The renal medullary concentrating mechanism, requiring large amount of oxygen, in combination with the relatively low medullary blood flow, renders the renal medulla hypoxic, with low tissue pO2 levels, even under normal conditions [4]. The renal medulla, particularly the outer portion, is therefore sensitive to acute renal ischemia. A logical approach in the management of clinical ischemic ARF would therefore be to improve the renal oxygen supply/demand relationship by augmenting renal blood flow and/or to reduce oxygen consumption of the renal medulla.

Loop diuretics have been shown to decrease the metabolic demand of the renal tubular cell, reducing its oxygen requirement and thereby maintaining medullary oxygenation [6]. A greater urine flow may also reduce tubular obstruction and tubular back-diffusion of filtered urine [7]. Indeed, there are several reports demonstrating that furosemide exerts a renoprotective effect in experimental ischemic ARF [8, 9, 10, 11, 12]. However, Lassnigg et al. [13] recently studied the potential preventive effect of furosemide on renal function after cardiac surgery and found that furosemide lowered creatinine clearance compared with placebo. Furthermore, in a recent cohort study on patients with ARF treated in the ICU, diuretic use was associated with a significant increase in the risk of death or nonrecovery of renal function [14].

Human atrial natriuretic peptide (h-ANP) is a potent endogenous diuretic and natriuretic substance [15] which may improve renal function and perfusion in animal models [16, 17, 18] of and in clinical ischemic ARF [19, 20, 21, 22]. h-ANP promotes a preglomerular vasodilatation and increases glomerular filtration rate (GFR) by 30–40% in patients with ischemic ARF after cardiac surgery [20]. Increased filtered load to the tubules might, however, increase the medullary oxygen demand and h-ANP therefore jeopardize renal O2 supply/demand relationship in ischemic ARF. Brezis et al. [23] have shown that intravenous h-ANP decreases both cortical and medullary tissue pO2 because of increased reabsorptive work, in animals with normal renal function. On the other hand, experimental data suggest that h-ANP inhibits tubular sodium reabsorption in the medullary collecting duct, which would decrease RVO2 [24].

The aim of this study was to evaluate the effects of h-ANP and furosemide on GFR, renal blood flow (RBF), renal oxygen consumption (RVO2), and central hemodynamics in humans using the infusion clearance technique. This method is a recently validated and reliable bedside technique for estimation of total RBF, GFR, and RVO2 without the need for urine collection (K. Swärd, F. Valsson, J. Sellgren, S.-E. Ricksten).

Methods

Study population

Twenty sedated and mechanically ventilated patients with preoperative serum creatinine level of 150 µmol/l or lower were studied after elective, uncomplicated cardiac surgery with cardiopulmonary bypass. One patient was excluded because of development of heart failure requiring inotropic support during the experimental procedure. Baseline demographic data for the patients are presented in Table 1. Written informed consent was obtained from the patients at the preoperative evaluation the day before surgery. The Human Ethics Committee of the University of Gothenburg approved the study protocol.
Table 1

Patients’ demographic and clinical characteristics (CABG coronary artery bypass surgery, CPB cardiopulmonary by pass)

h-ANP group (n=10)

Furosemide group (n=9)

Gender: MF (%)

70/30

89/31

Age (years)

64.70±10.04

65.22±7.99

Body surface area (m2)

1.88±0.19

1.96±0.23

Left ventricular ejection fraction (%)

0.622±0.151

0.602±0.117

Diabetes type 1 (%)

0

22

Hypertension (%)

30

11

Preoperative serum creatinine (µmol/l)

99.50±10.34

107.78±13.11

CABG

8

7

Valve surgery

2

0

CABG + valve surgery

0

2

CPB time (min)

81.0±15.3

84.3±35.2

Aortic cross-clamp time (min)

48.8±11.6

52.7±37.8

A standardized anesthetic procedure was used. The patients were premedicated with intramuscular morphine (5 mg), scopolamine (0.2 mg), and oral flunitrazepam (1 mg). Anesthesia was induced by 3–5mg/kg thiopentone and 5–7 µg/kg fentanyl followed by 0.1 mg/kg pancuronium and maintained by 10–20 µg/kg fentanyl and enflurane. During cardiopulmonary bypass the anesthesia was maintained by propofol infusion. In the intensive care unit the patients were sedated with propofol and mechanically ventilated to normocapnia. Postoperative hypovolemia was treated according to routine clinical practice with hydroxethylstarch (Haes-steril or Voluven, Fresenius Kabi, Uppsala, Sweden) and crystalloid fluids.

Systemic hemodynamics

A pulmonary artery thermodilution catheter (Baxter Healthcare, Irvine, Calif., USA) was inserted through the left subclavian vein, guided into the pulmonary artery. The arterial blood pressure, pulmonary arterial pressure and central venous pressure (CVP) were continuously measured and stored into a data acquisition software (AcqKnowlegde Biopac, Calif., USA). Measurements of thermodilution cardiac output (CO) were performed in triplicate. Systemic vascular resistance (SVR) was calculated as (MAP−CVP)/CO.

Measurements of renal variables

An 8-F catheter (Webster Laboratories, Baldwin Park, Calif., USA) was introduced into the left renal vein via the right jugular vein under fluoroscopic guidance. The catheter was placed in the central portion of the renal vein and its position was verified by venography [25]. After blood and urine blanks were taken an intravenous priming dose of paraaminohippurate (PAH, 8 mg/kg body weight) and chromium ethylenediaminetetraacetic acid (51Cr-EDTA; 0.6 MBq/m2 body surface area) were given followed by an infusion at a constant rate individualized to body weight and serum creatinine. Serum concentrations of PAH and serum 51Cr-EDTA activity from arterial and renal vein blood were measured by a spectrophotometer (Beckman DU 530, Life Science UV/Vis, Fullerton, Calif., USA) and a well counter (Wizard 3”, 1480, Automatic Gamma Counter, Perkin Elmer, Turku, Finland), respectively. Urine was collected for 30-min periods to measure urine flow and sodium excretion. An indwelling Foley catheter drained the urine bladder. To improve retrieving of urine a solution of sterile water (100 ml) was used for irrigation of the bladder together with gentle suprapubic compression. PAH and 51Cr-EDTA levels were obtained from arterial and renal vein blood at the end of each urine collection period.

Experimental procedure

Measurements started when the patients had a stable body temperature higher than 36.5°C approx. 4–6 h after the end of cardiopulmonary bypass. The patients were mechanically ventilated and sedated with propofol (80±38 µg/kg per minute) during the experimental procedure. After an equilibration period of at least 60 min two 30-min urine collection control periods (periods C1 and C2) were started followed by the administration of h-ANP or furosemide. In the h-ANP group the patients received a continuous infusion of recombinant h-ANP (amino acid residue 99–126; Clinalfa, Zurich, Switzerland) at infusion rates of 25 and 50 ng/kg per minute. Each dose was administered for 60 min, and urine was collected at the second half of each hour of infusion (h-ANP 25, h-ANP 50). In the furosemide group the patients received an intravenous bolus dose of 0.5 mg/kg furosemide (FurosemidRecip, Sweden) followed by a continuous infusion at 0.5 mg/kg per hour for three 30-min periods. Urine was collected during the second and third 30-min periods. During the experimental procedure an isotonic crystalloid solution was continuously infused to substitute for fluid losses to the diuretic response to h-ANP and furosemide. Cardiac output was obtained midway through each collection period.

Data calculation

Infusion clearance for PAH was used to estimate renal plasma flow (RPF) according to the formula: PAH infusion rate/(arterial-renal vein) PAH concentrations (Table 1) and renal blood flow as (RBF=RPF/1−hematocrit). Obtained RBF values were multiplied by 0.85 to compensate for extrarenal clearance [25]. Infusion clearance for 51Cr-EDTA (51Cr-EDTA infusion rate/arterial 51Cr-EDTA concentration) was obtained as a measure of GFR. Filtration fraction (FF) was defined as GFR/RPF. Renal vascular resistance (RVR) were calculated from the formula RVR=(MAP−CVP/RBF) All renal data were normalized to a body surface area of 1.73 m2.

Renal oxygen consumption (RVO2) and oxygen extraction were derived from the formulas RVO2=RBF ×(CaO2−CvO2) and O2EX=(CaO2−CvO2/CaO2) ×100, where CaO2 and CvO2 are the arterial and renal vein oxygen contents.

Fractional urinary excretion of sodium (FENa) was defined as: (UNa ×V)/(GFR ×PNA), where (UNa ×V) is renal sodium excretion and (GFR ×PNA) the filtered load of sodium. The tubular reabsorption of sodium (TRNa) was defined as the difference between the filtered load of sodium and the renal sodium excretion, i.e.: TRNa=(GFR ×PNA)−(UNa ×V)

Statistical analysis

The data are presented as mean ±standard deviation (mean ±SD). Analyses of variance for repeated measurements were used followed by post-hoc, single-degree of comparisons (contrast analyses) to compare the effects of each of the two interventional drugs to the two weighted control periods (C1, C2). Within-subject correlation analyses were performed to determine the relationship of RVO2 and tubular sodium reabsorption and that of tubular sodium reabsorption and RVO2 to GFR [26]. A probability of randomless level (P value) less than 0.05 was considered to indicate statistical significance.

Results

Data obtained during the two control periods C1 and C2 did not differ significantly in any of the measured variables.

Effects of h-ANP

Table 2 summarizes the effects of h-ANP on hemodynamic and renal variables. h-ANP induced a decrease in MAP and SVR. At an infusion rate of 25 ng/kg per minute MAP and SVR decreased by 7% (P<0.05) and 10% (P<0.05), respectively. At an infusion rate of 50 ng/kg per minute MAP and SVR decreased by 13% (P<0.001) and 22% (P<0.001), respectively. This was accompanied by an 11% (P<0.05) increase in CO at the highest infusion rate. Heart rate and cardiac filling pressures were not significantly affected by h-ANP. h-ANP caused a 9–13% decrease in RBF (P<0.05) with no change in RVR. GFR and FF increased by 12% (P<0.001) and 15–24% (P<0.001), respectively. This was accompanied by an 89% increase in FENa (P<0.05), a 48–56% increase in urine flow (P<0.05), and a 9% increase in TRNa (P<0.01). RVO2 increased by 21–31% (P<0.001) and renal oxygen extraction by 35–45% (P<0.001). RVO2 was positively correlated to tubular sodium reabsorption (R2=0.44, P<0.01). Tubular sodium reabsorption and RVO2 were both positively correlated to GFR (R2=0.79, P<0.001; R2=0.61, P<0.001, respectively).
Table 2

Effects of h-ANP on hemodynamic and renal variables (C1 first control period before h-ANP, C2 second control period before h-ANP, h-ANP 25 h-ANP infusion at a rate of 25 ng/kg per minute, h-ANP 50 h-ANP infusion at a rate of 50 ng/kg per minute, CO cardiac output, MAP mean arterial pressure, PCWP pulmonary capillary wedge pressure, CVP central venous pressure, SVR systemic vascular resistance, HR heart rate, RBF renal blood flow, GFR glomerular filtration rate, FF filtration fraction, RVR renal vascular resistance, reab reabsorption, FE Na fractional excretion of sodium, RVO2 renal vascular oxygen consumption, O2 extractionrenal renal oxygen extraction)

C1

C2

h-ANP 25

h-ANP 50

CO (l min−1)

4.5±1.1

4.5±1.0

4.7±1.1

5.0±1.2*

MAP (mmHg)

85.1±6.9

86.0±8.3

79.3±11.0*

74.3±9.1***

PCWP (mmHg)

13.0±4.3

12.8±4.3

12.9±4.0

13.0±3.7

CVP (mmHg)

10.5±2.8

10.2±3.6

9.9±2.6

10.1±2.2

SVR (dyne s−1 cm−5)

1399.9±331.1

1384.8±270.4

1246.8±352.7*

1085.4±336.2***

HR (beats min−1)

73.6±13.3

74.2±13.5

74.4±12.7

75.1±11.5

RBF (ml min−1)

739.4±240.3

694.5±202.1

623.2±165.9**

655.8±180.9*

GFR (ml min−1)

85.2±27.6

88.4±24.3

90.1±24.3

97.3±23.5***

FF (%)

17.9±3.3

19.6±3.3

22.5±3.7***

23.3±5.0***

RVR (mmHg ml−1 min−1)

0.11±0.05

0.12±0.04

0.12±0.05

0.11±0.05

Sodium reab. (mmol min−1)

12.2±4.0

12.4±3.3

12.4±3.1

13.3±3.2**

FE Na (%)

3.0±1.5

2.7±1.4

4.7±2.6

5.4±4.1*

Urine flow (ml min−1)

3.3±1.4

2.8±1.4

4.4±1.8*

4.7±2.5*

RVO2 (ml min−1)

9.8±2.9

10.1±2.8

12.0±2.9***

13.0±2.9***

O2 extractionrenal (%)

8.8±2.3

9.6±2.2

12.6±2.7***

13.3±2.8***

PAH extraction

0.88±0.05

0.87±0.07

0.88±0.07

0.88±0.07

*P<0.05, **P<0.01, ***P<0.001

Effects of furosemide

Table 3 summarizes the effects of furosemide on hemodynamic and renal variables. Furosemide induced a 10–12% decrease in CO (P<0.001), an 11% decrease in pulmonary capillary wedge pressure (P<0.01), and an 18% decrease in CVP (P<0.001). MAP and heart rate did not change significantly during furosemide infusion, while SVR increased by 12–15% (P<0.01). Neither RBF nor RVR was significantly affected by furosemide. GFR decreased by 12%. TRNa decreased by 26–31% (P<0.001), accompanied by 10- and 15-fold increases in urine flow and FENa, respectively (P<0.001). RVO2 decreased by 18–29% (P<0.001) and renal oxygen extraction by 20%. RVO2 was positively correlated to tubular sodium reabsorption (R2=0.64, P<0.001). Tubular sodium reabsorption and RVO2 were both positively correlated to GFR (R2=0.90, P<0.001; R2=0.62, P<0.001, respectively).
Table 3

Effects of furosemide on hemodynamic and renal variables (C1 first control period before furosemide, C2 second control period before furosemide, Furo 1 first measurement period of furosemide infusion 0.5 mg/kg per hour, Furo 2 second measurement period of furosemide infusion 0.5 mg/kg per hour, CO cardiac output, MAP mean arterial pressure, PCWP pulmonary capillary wedge pressure, CVP central venous pressure, SVR systemic vascular resistance, HR heart rate, RBF renal blood flow, GFR glomerular filtration rate, FF filtration fraction, RVR renal vascular resistance, reab reabsorption, FE Na fractional excretion of sodium, RVO2 renal vascular oxygen consumption, O2 extractionrenal renal oxygen extraction)

 

C1

C2

Furo 1

Furo 2

CO (l min−1)

5.6±1.8

5.7±1.7

5.0±1.6***

5.1±1.5***

MAP (mmHg)

80.2±6.6

79.2±6.9

76.3±8.8

80.6±6.6

PCWP (mmHg)

16.0±4.1

16.3±4.1

14.2±3.9***

14.4±3.2**

CVP (mmHg)

11.5±3.9

11.6±3.7

9.5±2.9***

9.5±3.1***

SVR (dyne s cm−5)

1046.3±260.0

1021.1±313.7

1161.4±387.0*

1187.1±325.9**

HR (beats min−1)

76.2±10.7

76.3±10.0

77.2±9.6

76±9.3

RBF (ml min-1)

802.4±338.0

807.1±376.6

767.4±294.3

779.1±325.0

GFR (ml min-1)

89.1±25.5

90.5±25.8

79.5±20.4***

78.5±19.2***

FF (%)

17.3±3.1

17.5±3.7

16.4±3.6

16.1±4.0*

RVR (mmHg ml−1 min−1)

0.096±0.031

0.096±0.033

0.100±0.043

0.105±0.041

Sodium reab. (mmol min−1)

12.0±3.5

12.3±3.5

8.4±2.7***

7.7±2.2***

FE Na (%)

1.8±1.4

1.5±0.9

24.9±6.0***

29.4±5.4***

Urine flow (ml min−1)

2.4±1.2

2.1±0.8

20.3±5.0***

23.3±5.5***

RVO2 (ml min−1)

11.1±4.0

10.6±3.0

8.9±3.1**

7.9±3.4***

O2 extractionrenal (%)

10.5±4.3

10.8±4.1

8.6±2.7*

8.5±5.2*

PAH extraction

0.83±0.08

0.83±0.07

0.85±0.04

0.84±0.07

*P<0.05, **P<0.01, ***P<0.001

Discussion

It is well-known that tubular sodium reabsorption is a major determinant of RVO2 [27, 28]. Positive correlations between GFR and tubular sodium reabsorption and between tubular sodium reabsorption and RVO2 were also found in the present study. It has been shown that h-ANP inhibits tubular sodium reabsorption in the medullary collecting duct both directly [24] and indirectly, by inhibition of aldosterone secretion [29] and by lowering the plasma renin concentration [30, 31], which would decrease RVO2. In patients with cirrhosis and refractory ascites h-ANP induces an increase in GFR which is accompanied by a natriuresis and a decrease in RVO2, which is attributed to an inhibitory effect of h-ANP on the elevated levels of antinatriuretic substances in this condition [32]. In the present study this potential energy-conserving, tubular effect of h-ANP was offset by the increase in GFR and tubular sodium reabsorption.

The h-ANP-induced increase in GFR was accompanied by an increase in filtration fraction with no change in renal vascular resistance, which indicates that h-ANP in patients with normal renal function causes a preglomerular renal vasodilatation and a postglomerular vasoconstriction, as previously reported in both animals and humans [15, 21]. The increase in GFR and RVO2 was not matched by a proportional increase in RBF, indicating that h-ANP in patients with normal renal function induces a renal oxygen supply/demand mismatch. This was also supported by the h-ANP-induced increase in renal oxygen extraction. Our findings are in accordance with the results from Brezis et al. [6, 23] showing that h-ANP decreases medullary interstitial pO2 caused by increased GFR in animals with normal renal function. However, the effects of h-ANP on hemodynamics and excretory function of the normal kidney cannot immediately be transposed to the ischemically injured kidney. In patients with early ischemic ARF it has been shown that h-ANP causes an increase in both GFR and RBF with no change in filtration fraction, indicating a preglomerular vasodilatation as the major intrarenal hemodynamic effect [20, 21, 22]. To our knowledge, there are no data on the effects of h-ANP on RVO2 or the renal oxygen supply/demand relationship in early ischemic ARF.

Furosemide caused a 25–30% reduction in renal sodium reabsorption accompanied by a proportional decrease in RVO2. Although it has repeatedly been shown that furosemide decreases RVO2 in animals by inhibiting active transport and oxygen consumption in the medullary thick ascending limb (mTAL) [27, 33], data on the effect of furosemide on RVO2 in humans are sparse. Ofstad et al. [34] observed no effect of furosemide at the low dose of 10 mg on absolute RVO2 in humans. However, Epstein and Prasad [35] showed by the noninvasive technique of blood oxygen level dependent magnetic resonance imaging that furosemide (20 mg) increased medullary oxygen levels due to inhibited mTAL pump activity, particularly in younger humans, data which are supported by the findings of the present study.

In patients with normal renal function and in healthy volunteers variable effects of furosemide on GFR and RPF have been described. Several authors have shown that furosemide causes a prostaglandin-mediated increase in both GFR and RPF in healthy volunteers [36, 37, 38], while others have observed no such furosemide-induced change in GFR [34, 39]. On the other hand, Epstein and Prasad [35] demonstrated that the furosemide-induced increase in medullary oxygen levels is associated with a decrease in creatinine clearance, findings that are supported by the results of the present study. In ICU patients suffering from acute pancreatitis, sepsis, or massive trauma furosemide infusion caused no changes in GFR or RBF [40]. In the present study furosemide infusion decreased GFR and filtration fraction modestly by approx. 12% and 7%, respectively. One could speculate that this was due to afferent arteriolar vasoconstriction as a part of the tubuloglomerular feedback mechanism, in turn activated by the increased solute delivery to the macula densa [41]. This decrease in GFR and the filtered sodium load to the proximal tubules would further decrease RVO2. In other words, the furosemide-induced decrease in RVO2 might be caused by a combination of a direct inhibition of the mTAL pump activity and an indirect decrease in pump activity of proximal tubules, in turn caused by a furosemide-induced decrease in filtered load of sodium.

h-ANP at an infusion rate of 25–50 ng/kg per minute induced a peripheral vasodilatation with a 7–13% fall in MAP, which explained the decrease in RBF as renal vascular resistance was unaffected by h-ANP. In spite of the fall in MAP, GFR increased with h-ANP. At a higher infusion rate of h-ANP an associated hypotension would probably have blunted this increase in GFR. Furosemide caused an 11% decrease in cardiac output due to a fall in cardiac filling pressures, with no changes in MAP or RBF. The furosemide-induced decrease in GFR and filtration fraction with no change in renal vascular resistance could therefore be explained by an increase in the pre-/post-glomerular resistance ratio, in contrast to the decrease in pre-/post-glomerular resistance ratio seen with h-ANP.

In conclusion we have shown in postoperative patients with normal renal function that h-ANP-induced increase in GFR is accompanied by an increase in tubular sodium reabsorption and RVO2. Thus h-ANP does not have energy-conserving, tubular effects. In contrast, furosemide caused a decrease in tubular sodium reabsorption and RVO2 and thus has the potential to improve the oxygen supply/demand relationship in clinical ischemic ARF. Finally, a close correlation between GFR and RVO2 irrespective of diuretic agent was shown, suggesting that the filtered load of sodium is an important determinant of renal oxygen requirements.

Acknowledgements

The technical assistance of Marita Ahlqvist and the assistance of the nursing staff of the Cardiothoracic Intensive Care Unit and Surgical Theatre of the Sahlgrenska University Hospital are gratefully acknowledged.

Copyright information

© Springer-Verlag 2004