Journal of Clinical Anesthesia (2015) 27, 60–62
Case Report
Successful epidural anesthesia for cesarean delivery in a woman with Fontan repair☆ Edward Mathney MD (Assistant Professor of Anesthesiology), Yaakov Beilin MD (Professor of Anesthesiology and Obstetrics, Gynecology and Reproductive Sciences)⁎ Department of Anesthesiology, Icahn School of Medicine at Mount Sinai, New York, NY Received 28 April 2014; revised 15 August 2014; accepted 22 August 2014
Keywords: Cesarean delivery; Fontan; Parturient
Abstract The cohort of the adult population with congenital heart disease is steadily growing, as surgical and medical management advances; and many women with congenital heart disease are becoming pregnant and presenting to the labor and delivery suite [1]. The anesthesiologist must therefore have an understanding of the physiology of the corrected heart lesion and the impact of the physiologic changes of pregnancy ,[2]. A woman status post-Fontan procedure requiring cesarean delivery is presented. © 2014 Elsevier Inc. All rights reserved.
1. Case presentation A 21-year-old, gravida 1, para 0, presented at 30 weeks of gestation after having tried to conceive unsuccessfully for 1.5 years. The patient was born with pulmonary atresia for which she underwent a modified Blalock-Taussig shunt in infancy, right ventricular (RV) outflow tract reconstruction at 21 months, and atrial septal defect closure with further reconstruction of the RV outflow tract and a side-to-side connection of the superior vena cava to the right pulmonary artery at 6 years. Cardiac physiology was described as classic Fontan. Three years before pregnancy, cardiac evaluation showed systemic venous pressures of 15 mm Hg and left ventricular ejection fraction (LVEF), 33%. Echocardiography at 30 weeks of gestation confirmed the above anatomy as well as Ebstein ☆
Funding: Departmental only. ⁎ Corresponding author at: The Mount Sinai Hospital, Department of Anesthesiology, Box 1010, 1 Gustave L Levy Place, New York, NY 10029-6574. E-mail address: [email protected] (Y. Beilin). http://dx.doi.org/10.1016/j.jclinane.2014.08.006 0952-8180/© 2014 Elsevier Inc. All rights reserved.
anomaly of the tricuspid valve, a markedly dilated right atrium, and a hypoplastic RV. Holter monitoring revealed frequent self-limited episodes of supraventricular tachycardia that was clinically associated with symptoms ranging from fluttering to chest pain without syncope. The patient was started on metoprolol at approximately 3 months of gestation with the addition of amiodarone at approximately 7 months of gestation. Shortness of breath and lower extremity edema were improved with the addition of furosemide. By the 7-month appointment, the patient's New York Heart Association's status had declined from class II to class III, with shortness of breath after walking 1 block. Because of her worsening cardiac status and concern that she would decompensate further during delivery, the decision was made to perform a cesarean delivery in a cardiac operating room with the immediate availability of cardiopulmonary bypass. Multiple interdisciplinary meetings were held to discuss the delivery and anesthetic plan. The patient was admitted to the hospital on the night before scheduled cesarean delivery at 36 weeks of gestation, where an infusion of milrinone was started at 0.5 μg/kg per minute (dose range, 0.375-0.75 μg/kg
Cesarean delivery in woman status post-Fontan procedure per minute). She presented to the cardiac surgical suite, where standard American Society of Anesthesiologists monitors and oxygen via nasal cannula were applied. In addition, central venous access was placed to monitor central venous pressure (CVP), invasive arterial monitoring was started, and an epidural catheter was placed at L3-4 with loss of resistance to air technique. The patient was prehydrated with approximately 1000 mL of crystalloid to increase CVP from 15-18 mm Hg. A surgical level of anesthesia was obtained with a total of 16 mL lidocaine 2% + epinephrine 1:200 k administered over a period of 40 minutes. Milrinone infusion was continued throughout the surgery. Mean arterial pressure decreased from 100-85 mm Hg, and CVP decreased from 18-15 mm Hg, when a surgical level of anesthesia was obtained. The procedure was uncomplicated. Fluid administration totaled 1800 mL of crystalloid with an estimated blood loss of approximately 800 mL. A healthy infant male was born with both 1- and 5-minute Apgar scores of 9, weighing 2360 g. The mother was admitted to the cardiac care unit and was discharged to home 5 days later.
2. Discussion The Fontan procedure was first described in 1971 as a treatment for tricuspid atresia [3] with its use now expanded to all patients with a single functional ventricle, including complete atrioventricular septal defects and hypoplastic left heart syndrome [4]. Currently, encountering a parturient that has undergone a Fontan procedure is an uncommon occurrence. One study using the CONCOR registry of 7486 patients with congenital heart disease to identify women with Fontan palliation of childbearing age found 44 women, 38 of which were included in the study, with only 6 becoming pregnant [5]. As patients survive longer with improved functional status post-Fontan, it should be expected that having to care for pregnant patients with Fontan physiology will be a more common occurrence. Between 1997 and 2009 the 1, 5, and 10-year survival rates have significantly improved from 80%, 78%, and 71% [6] to 97%, 96%, and 94% [7]. Although longevity has been improved, event-free survival is significantly lower at 96%, 87%, and 64% at 1, 5, and 10 years, respectively [7]. Classic Fontan physiology separates pulmonary and systemic circulations, allowing the single functional ventricle to maintain systemic flow, whereas pulmonary blood flow becomes entirely passive [8]. Our patient had a right ventricle, although with the Ebstein anomaly and hypoplasia, the contribution to pulmonary flow was minimal; and her physiology was described as classic Fontan. Further complicating this patient's course is that cardiac output increases approximately 40% during pregnancy, and blood volume increases by 45% [9]. A heart with depressed function due to surgical repair as well as long-standing altered physiology may not be able to increase the cardiac
61 output required during pregnancy and then at delivery. Our patient had an LVEF of 33%, which is depressed as compared with a normal patient but not unexpected for a patient after Fontan procedure. Eicken et al [10] found that the mean LVEF 10 years after Fontan was 49.3%, with a range of 20%-63%. Supraventricular tachyarrhythmias are a common occurrence even without the added blood volume during pregnancy, and the incidence of arrhythmia progressively increases with time [5,11]. These arrhythmias may lead to further functional decline; and, as in our patient, efforts to maintain normal sinus rhythm may prove helpful in preserving cardiac output [11]. The passive pulmonary blood flow in the patient with Fontan presents multiple anesthetic considerations in the parturient such as maintaining higher-than-normal CVPs, minimizing intrathoracic pressure by avoiding positive pressure ventilation, and preventing increased pulmonary vascular resistance (PVR) by avoiding hypercarbia, hypoxia, and acidosis. These goals are can be best accomplished with neuraxial anesthesia [12-15], although some have used general anesthesia successfully [12,16]. Recent modeling suggests that PVR is the major determinant of preload and, thus, cardiac output in Fontan physiology [17]; and given that PVR progressively increases over time when presented with nonpulsatile flow [11], it is likely that phosphodiesterase inhibitors or inhaled nitric oxide may be needed. Milrinone, a phosphodiesterase inhibitor, was started before the procedure to reach a steady state of medication in anticipation of the increase in cardiac output required at delivery. Nitric oxide was reserved in case of decompensation. To promote forward flow through the passive pulmonary circuit, elevated right atrial pressures (and therefore CVP) as high as 20 mm Hg are necessary to maintain a gradient between the right and left atria of approximately 7 mm Hg—enough to surmount an elevated PVR [18]. For this reason, CVP monitoring may be useful, and prehydration with cautious titration of local anesthetic via an epidural catheter would be prudent. A concern in the presented case was the LVEF of 33%, which would lead to elevated left ventricular end diastolic pressure and left atrial pressure, subsequently necessitating even higher right atrial pressures to maintain a gradient. Furthermore, there was concern that the patient would not tolerate the further increase in cardiac output at delivery; and therefore, the case was performed in the cardiac operating room in case there was a need for emergent cardiopulmonary bypass. Taking this into consideration, CVP and mean arterial pressure were both monitored carefully, as a surgical level of anesthesia was gradually achieved. Monitoring cardiac output with a noninvasive cardiac output device can be considered; but due to limitations with current available equipment, especially when trying to monitor rapid changes in cardiac output, the decision was made not to use one [19]. In summary, our patient presented with Fontan physiology to the cardiac operating room at 36 weeks of gestation with several of the adverse sequelae related to the Fontan
62 procedure including decreased New York Heart Association functional status, depressed myocardial function with associated elevations in right-sided pressures, and history of supraventricular arrhythmia requiring multiple medications to maintain sinus rhythm.
E. Mathney, Y. Beilin
[9] [10]
[11]
References [1] van der Velde ET, Vriend JW, Mannens MM, Uiterwaal CS, Brand R, Mulder BJ. CONCOR, an initiative towards a national registry and DNA-bank of patients with congenital heart disease in the Netherlands: rationale, design, and first results. Eur J Epidemiol 2005;20:549-57. [2] Birnbach DJ, Browne IM. Anesthesia for obstetrics. In: Miller RD, Eriksson LI, Fleisher L, Wiener-Kronish JP, Young WL, editors. Miller's Anesthesia. 7th ed. Philadelphia, PA: Lippincott; 2010. p. 2204-7. [3] Fontan F, Baudet E. Surgical repair of tricuspid atresia. Thorax 1971; 26:240-8. [4] AboulHosn JA, Shavelle DM, Castellon Y, Criley YM, Plunkett M, Pelican P, et al. Fontan operation and the single ventricle. Congenit Heart Dis 2007;2:2-11. [5] Drenthen W, Pieper PG, Roos-Hesselink JW, van Lottum WA, Voors AA, Mulder BJ, et al. Pregnancy and delivery in women after Fontan palliation. Heart 2006;92:1290-4. [6] Gentles TL, Mayer JE Jr, Gauvreau K, Newburger JW, Lock JE, Kupferschmid JP, et al. Fontan operation in five hundred consecutive patients: factors influencing early and late outcome. J Thorac Cardiovasc Surg 1997;114:376-91. [7] Tweddell JS, Nersesian M, Mussatto KA, Nugent M, Simpson P, Mitchell ME, et al. Fontan palliation in the modern era: factors impacting mortality and morbidity. Ann Thorac Surg 2009;88:1291-9. [8] Naguib MA, Dob DP, Gatzoulis MA. A functional understanding of moderate to complex congenital heart disease and the impact of
[12]
[13]
[14] [15]
[16]
[17]
[18]
[19]
pregnancy. Part II: tetralogy of Fallot, Eisenmenger's syndrome and the Fontan operation. Int J Obstet Anesth 2010;19:306-12. Capeless EL, Clapp JF. Cardiovascular changes in early phases of pregnancy. Am J Obstet Gynecol 1989;16:1449-53. Eicken A, Fratz S, Gutfried C, Balling G, Schwaiger M, Lange R, et al. Hearts late after Fontan operation have normal mass, normal volume. And reduced systolic function: a magnetic resonance imaging study. J Am Coll Cardiol 2003;42:1061-5. de Leval MR, Deanfield JE. Four decades of Fontan palliation. Nat Rev Cardiol 2010;7:520-7. Eid L, Ginosar Y, Elchalal U, Pollak A, Weiniger CF. Caesarean section following the Fontan procedure: two different deliveries and different anaesthetic choices in the same patient. Anaesthesia 2005;60: 1137-40. Carp H, Yayaram A, Vadhera R, Nichols M, Morton M. Epidural anesthesia for cesarean delivery and vaginal birth after maternal Fontan repair: report of two cases. Anesth Analg 1994;78:1190-2. Cohen AM, Mulvein J. Obstetric anaesthetic management in a patient with the Fontan circulation. Br J Anaesth 1994;73:252-5. Ioscovich A, Briskin A, Fadeev A, Grisaru-Granovsky S, Halpern S. Emergency cesarean section in a patient with Fontan circulation using an indwelling epidural catheter. J Clin Anesth 2006;18:631-4. Nitsche JF, Phillips SD, Rose CH, Brost BC, Watson WJ. Pregnancy and delivery in patients with Fontan circulation: a case report and review of obstetric management. Obstet Gynecol Surv 2009;64: 607-14. Gewillig M, Brown SC, Eyskens B, Heying R, Ganame J, Budts W, et al. The Fontan circulation: who controls cardiac output? Interact Cardiovasc Thorac Surg 2010;10:428-33. Hosking MP, Beynen FM. The modified Fontan procedure: physiology and anesthetic implications. J Cardiothorac Vasc Anesth 1992;6:465-75. Funk DJ, Moretti EW, Gan TJ. Minimally invasive cardiac output monitoring in the perioperative setting. Anesth Analg 2009;108: 889-97.
Case Report
Successful epidural anesthesia for cesarean delivery in a woman with Fontan repair☆ Edward Mathney MD (Assistant Professor of Anesthesiology), Yaakov Beilin MD (Professor of Anesthesiology and Obstetrics, Gynecology and Reproductive Sciences)⁎ Department of Anesthesiology, Icahn School of Medicine at Mount Sinai, New York, NY Received 28 April 2014; revised 15 August 2014; accepted 22 August 2014
Keywords: Cesarean delivery; Fontan; Parturient
Abstract The cohort of the adult population with congenital heart disease is steadily growing, as surgical and medical management advances; and many women with congenital heart disease are becoming pregnant and presenting to the labor and delivery suite [1]. The anesthesiologist must therefore have an understanding of the physiology of the corrected heart lesion and the impact of the physiologic changes of pregnancy ,[2]. A woman status post-Fontan procedure requiring cesarean delivery is presented. © 2014 Elsevier Inc. All rights reserved.
1. Case presentation A 21-year-old, gravida 1, para 0, presented at 30 weeks of gestation after having tried to conceive unsuccessfully for 1.5 years. The patient was born with pulmonary atresia for which she underwent a modified Blalock-Taussig shunt in infancy, right ventricular (RV) outflow tract reconstruction at 21 months, and atrial septal defect closure with further reconstruction of the RV outflow tract and a side-to-side connection of the superior vena cava to the right pulmonary artery at 6 years. Cardiac physiology was described as classic Fontan. Three years before pregnancy, cardiac evaluation showed systemic venous pressures of 15 mm Hg and left ventricular ejection fraction (LVEF), 33%. Echocardiography at 30 weeks of gestation confirmed the above anatomy as well as Ebstein ☆
Funding: Departmental only. ⁎ Corresponding author at: The Mount Sinai Hospital, Department of Anesthesiology, Box 1010, 1 Gustave L Levy Place, New York, NY 10029-6574. E-mail address: [email protected] (Y. Beilin). http://dx.doi.org/10.1016/j.jclinane.2014.08.006 0952-8180/© 2014 Elsevier Inc. All rights reserved.
anomaly of the tricuspid valve, a markedly dilated right atrium, and a hypoplastic RV. Holter monitoring revealed frequent self-limited episodes of supraventricular tachycardia that was clinically associated with symptoms ranging from fluttering to chest pain without syncope. The patient was started on metoprolol at approximately 3 months of gestation with the addition of amiodarone at approximately 7 months of gestation. Shortness of breath and lower extremity edema were improved with the addition of furosemide. By the 7-month appointment, the patient's New York Heart Association's status had declined from class II to class III, with shortness of breath after walking 1 block. Because of her worsening cardiac status and concern that she would decompensate further during delivery, the decision was made to perform a cesarean delivery in a cardiac operating room with the immediate availability of cardiopulmonary bypass. Multiple interdisciplinary meetings were held to discuss the delivery and anesthetic plan. The patient was admitted to the hospital on the night before scheduled cesarean delivery at 36 weeks of gestation, where an infusion of milrinone was started at 0.5 μg/kg per minute (dose range, 0.375-0.75 μg/kg
Cesarean delivery in woman status post-Fontan procedure per minute). She presented to the cardiac surgical suite, where standard American Society of Anesthesiologists monitors and oxygen via nasal cannula were applied. In addition, central venous access was placed to monitor central venous pressure (CVP), invasive arterial monitoring was started, and an epidural catheter was placed at L3-4 with loss of resistance to air technique. The patient was prehydrated with approximately 1000 mL of crystalloid to increase CVP from 15-18 mm Hg. A surgical level of anesthesia was obtained with a total of 16 mL lidocaine 2% + epinephrine 1:200 k administered over a period of 40 minutes. Milrinone infusion was continued throughout the surgery. Mean arterial pressure decreased from 100-85 mm Hg, and CVP decreased from 18-15 mm Hg, when a surgical level of anesthesia was obtained. The procedure was uncomplicated. Fluid administration totaled 1800 mL of crystalloid with an estimated blood loss of approximately 800 mL. A healthy infant male was born with both 1- and 5-minute Apgar scores of 9, weighing 2360 g. The mother was admitted to the cardiac care unit and was discharged to home 5 days later.
2. Discussion The Fontan procedure was first described in 1971 as a treatment for tricuspid atresia [3] with its use now expanded to all patients with a single functional ventricle, including complete atrioventricular septal defects and hypoplastic left heart syndrome [4]. Currently, encountering a parturient that has undergone a Fontan procedure is an uncommon occurrence. One study using the CONCOR registry of 7486 patients with congenital heart disease to identify women with Fontan palliation of childbearing age found 44 women, 38 of which were included in the study, with only 6 becoming pregnant [5]. As patients survive longer with improved functional status post-Fontan, it should be expected that having to care for pregnant patients with Fontan physiology will be a more common occurrence. Between 1997 and 2009 the 1, 5, and 10-year survival rates have significantly improved from 80%, 78%, and 71% [6] to 97%, 96%, and 94% [7]. Although longevity has been improved, event-free survival is significantly lower at 96%, 87%, and 64% at 1, 5, and 10 years, respectively [7]. Classic Fontan physiology separates pulmonary and systemic circulations, allowing the single functional ventricle to maintain systemic flow, whereas pulmonary blood flow becomes entirely passive [8]. Our patient had a right ventricle, although with the Ebstein anomaly and hypoplasia, the contribution to pulmonary flow was minimal; and her physiology was described as classic Fontan. Further complicating this patient's course is that cardiac output increases approximately 40% during pregnancy, and blood volume increases by 45% [9]. A heart with depressed function due to surgical repair as well as long-standing altered physiology may not be able to increase the cardiac
61 output required during pregnancy and then at delivery. Our patient had an LVEF of 33%, which is depressed as compared with a normal patient but not unexpected for a patient after Fontan procedure. Eicken et al [10] found that the mean LVEF 10 years after Fontan was 49.3%, with a range of 20%-63%. Supraventricular tachyarrhythmias are a common occurrence even without the added blood volume during pregnancy, and the incidence of arrhythmia progressively increases with time [5,11]. These arrhythmias may lead to further functional decline; and, as in our patient, efforts to maintain normal sinus rhythm may prove helpful in preserving cardiac output [11]. The passive pulmonary blood flow in the patient with Fontan presents multiple anesthetic considerations in the parturient such as maintaining higher-than-normal CVPs, minimizing intrathoracic pressure by avoiding positive pressure ventilation, and preventing increased pulmonary vascular resistance (PVR) by avoiding hypercarbia, hypoxia, and acidosis. These goals are can be best accomplished with neuraxial anesthesia [12-15], although some have used general anesthesia successfully [12,16]. Recent modeling suggests that PVR is the major determinant of preload and, thus, cardiac output in Fontan physiology [17]; and given that PVR progressively increases over time when presented with nonpulsatile flow [11], it is likely that phosphodiesterase inhibitors or inhaled nitric oxide may be needed. Milrinone, a phosphodiesterase inhibitor, was started before the procedure to reach a steady state of medication in anticipation of the increase in cardiac output required at delivery. Nitric oxide was reserved in case of decompensation. To promote forward flow through the passive pulmonary circuit, elevated right atrial pressures (and therefore CVP) as high as 20 mm Hg are necessary to maintain a gradient between the right and left atria of approximately 7 mm Hg—enough to surmount an elevated PVR [18]. For this reason, CVP monitoring may be useful, and prehydration with cautious titration of local anesthetic via an epidural catheter would be prudent. A concern in the presented case was the LVEF of 33%, which would lead to elevated left ventricular end diastolic pressure and left atrial pressure, subsequently necessitating even higher right atrial pressures to maintain a gradient. Furthermore, there was concern that the patient would not tolerate the further increase in cardiac output at delivery; and therefore, the case was performed in the cardiac operating room in case there was a need for emergent cardiopulmonary bypass. Taking this into consideration, CVP and mean arterial pressure were both monitored carefully, as a surgical level of anesthesia was gradually achieved. Monitoring cardiac output with a noninvasive cardiac output device can be considered; but due to limitations with current available equipment, especially when trying to monitor rapid changes in cardiac output, the decision was made not to use one [19]. In summary, our patient presented with Fontan physiology to the cardiac operating room at 36 weeks of gestation with several of the adverse sequelae related to the Fontan
62 procedure including decreased New York Heart Association functional status, depressed myocardial function with associated elevations in right-sided pressures, and history of supraventricular arrhythmia requiring multiple medications to maintain sinus rhythm.
E. Mathney, Y. Beilin
[9] [10]
[11]
References [1] van der Velde ET, Vriend JW, Mannens MM, Uiterwaal CS, Brand R, Mulder BJ. CONCOR, an initiative towards a national registry and DNA-bank of patients with congenital heart disease in the Netherlands: rationale, design, and first results. Eur J Epidemiol 2005;20:549-57. [2] Birnbach DJ, Browne IM. Anesthesia for obstetrics. In: Miller RD, Eriksson LI, Fleisher L, Wiener-Kronish JP, Young WL, editors. Miller's Anesthesia. 7th ed. Philadelphia, PA: Lippincott; 2010. p. 2204-7. [3] Fontan F, Baudet E. Surgical repair of tricuspid atresia. Thorax 1971; 26:240-8. [4] AboulHosn JA, Shavelle DM, Castellon Y, Criley YM, Plunkett M, Pelican P, et al. Fontan operation and the single ventricle. Congenit Heart Dis 2007;2:2-11. [5] Drenthen W, Pieper PG, Roos-Hesselink JW, van Lottum WA, Voors AA, Mulder BJ, et al. Pregnancy and delivery in women after Fontan palliation. Heart 2006;92:1290-4. [6] Gentles TL, Mayer JE Jr, Gauvreau K, Newburger JW, Lock JE, Kupferschmid JP, et al. Fontan operation in five hundred consecutive patients: factors influencing early and late outcome. J Thorac Cardiovasc Surg 1997;114:376-91. [7] Tweddell JS, Nersesian M, Mussatto KA, Nugent M, Simpson P, Mitchell ME, et al. Fontan palliation in the modern era: factors impacting mortality and morbidity. Ann Thorac Surg 2009;88:1291-9. [8] Naguib MA, Dob DP, Gatzoulis MA. A functional understanding of moderate to complex congenital heart disease and the impact of
[12]
[13]
[14] [15]
[16]
[17]
[18]
[19]
pregnancy. Part II: tetralogy of Fallot, Eisenmenger's syndrome and the Fontan operation. Int J Obstet Anesth 2010;19:306-12. Capeless EL, Clapp JF. Cardiovascular changes in early phases of pregnancy. Am J Obstet Gynecol 1989;16:1449-53. Eicken A, Fratz S, Gutfried C, Balling G, Schwaiger M, Lange R, et al. Hearts late after Fontan operation have normal mass, normal volume. And reduced systolic function: a magnetic resonance imaging study. J Am Coll Cardiol 2003;42:1061-5. de Leval MR, Deanfield JE. Four decades of Fontan palliation. Nat Rev Cardiol 2010;7:520-7. Eid L, Ginosar Y, Elchalal U, Pollak A, Weiniger CF. Caesarean section following the Fontan procedure: two different deliveries and different anaesthetic choices in the same patient. Anaesthesia 2005;60: 1137-40. Carp H, Yayaram A, Vadhera R, Nichols M, Morton M. Epidural anesthesia for cesarean delivery and vaginal birth after maternal Fontan repair: report of two cases. Anesth Analg 1994;78:1190-2. Cohen AM, Mulvein J. Obstetric anaesthetic management in a patient with the Fontan circulation. Br J Anaesth 1994;73:252-5. Ioscovich A, Briskin A, Fadeev A, Grisaru-Granovsky S, Halpern S. Emergency cesarean section in a patient with Fontan circulation using an indwelling epidural catheter. J Clin Anesth 2006;18:631-4. Nitsche JF, Phillips SD, Rose CH, Brost BC, Watson WJ. Pregnancy and delivery in patients with Fontan circulation: a case report and review of obstetric management. Obstet Gynecol Surv 2009;64: 607-14. Gewillig M, Brown SC, Eyskens B, Heying R, Ganame J, Budts W, et al. The Fontan circulation: who controls cardiac output? Interact Cardiovasc Thorac Surg 2010;10:428-33. Hosking MP, Beynen FM. The modified Fontan procedure: physiology and anesthetic implications. J Cardiothorac Vasc Anesth 1992;6:465-75. Funk DJ, Moretti EW, Gan TJ. Minimally invasive cardiac output monitoring in the perioperative setting. Anesth Analg 2009;108: 889-97.
