Mechanical Ventilation Following Cardiac Surgery in Children

Transcription

1 44 Current Respiratory Medicine Reviews, 2012, 8, Mechanical Ventilation Following Cardiac Surgery in Children Alexandre Tellechea Rotta *,1 and Werther Brunow de Carvalho 2 1 Indiana University School of Medicine, Director, Pediatric Cardiac Critical Care, Riley Hospital for Children, Indianapolis, IN, USA Children s Institute, Hospital das Clínicas, University of São Paulo, Chief, Pediatric Intensive Care Unit, Hospital Santa Catarina, Sao Paulo, Brazil Abstract: The application of positive pressure mechanical ventilation can result in complex changes in pulmonary and cardiovascular physiology. These cardiopulmonary interactions are particularly important in pediatric patients undergoing surgery for repair or palliation of congenital cardiac defects. In this article, we review the various effects of mechanical ventilation on right and left ventricular preload, afterload and contractility. We also address specific clinical scenarios, such as mechanical ventilation of the uncomplicated patient following cardiac surgery, ventilation of patients with delayed sternal closure, the Norwood procedure, bidirectional and total cavopulmonary anastomoses and patients with right ventricular diastolic dysfunction. Keywords: Heart-lung interactions, cardiorespiratory interactions, cardiocirculatory function, systemic hemodynamics, cardiovascular monitory, mechanical ventilation, cardiac surgery, cardiopulmonary interactions, congenital cardiac defects. INTRODUCTION The heart and lungs work in tandem to supply oxygen to be consumed by the various tissues. Maintenance of an adequate cardiorespiratory function is essential to the care of critically ill patients and can be accomplished with the use of medications, fluid management, as well as invasive and noninvasive respiratory support. Paradoxically, interventions designed to improve the function of one system may, at times, lead to unwanted effects on another. Positive pressure mechanical ventilation is one such intervention, as it can result in complex cardiovascular changes with decrease in cardiac output and reduced tissue oxygen delivery, despite an apparent increase in the arterial oxygen content [1, 2]. This article reviews the impact of spontaneous breathing and mechanical ventilation on the circulatory system (cardiorespiratory interactions) and discusses ventilation strategies for the management of children following surgery for repair or palliation of selected congenital cardiac defects. THE INFLUENCE OF VENTILATION ON CARDIAC FUNCTION Mechanical ventilation and spontaneous breathing cause changes in intrapleural or intrathoracic pressure and can independently influence key determinants of cardiocirculatory performance: atrial filling or preload, impedance to left ventricular ejection or afterload, contractility and heart rate. Changes in intrathoracic pressure are transmitted to all intrathoracic structures, including the heart, pericardium, great arteries and veins. Spontaneous inspiration causes a negative pleural pressure and the reduction in intrathoracic pressure significantly influences venous return to the right *Address correspondence to this author at the Indiana University School of Medicine, Director, Pediatric Cardiac Critical Care, Riley Hospital for Children, Indianapolis, IN, USA; Tel: (317) ; Fax: (317) ; arotta@iupui.edu atrium. In contrast, positive pressure mechanical ventilation causes an inspiratory increase in the intrapleural pressure, and, if positive end-expiratory pressure (PEEP) is applied during exhalation, intrapleural pressure will remain above atmospheric pressure for the entire respiratory cycle [3]. Considering spontaneous inspirations can occur among mandatory positive pressure breaths, the hemodynamic effects of spontaneous breathing can also frequently be observed in patients undergoing mechanical ventilation. BREATHING AND THE AUTONOMIC TONE The autonomic response to changes in pulmonary volume during spontaneous breathing results in a slight variation in cardiac rhythm, commonly referred to as sinus arrhythmia. Sinus arrhythmia is characterized by an increase in heart rate during spontaneous inspiration related to a decrease in vagal tone, while the opposite change occurs during exhalation. An increase in lung volume results in bradycardia and reflex arteriolar dilatation, which are discreet when usual tidal volumes are employed but can become clinically significant with the delivery of excessive tidal volumes [4]. This vasodilatory response also appears to be mediated by afferent vagal fibers [3, 4]. The faster respiratory rates and higher resting sympathetic tone of infants make this group particularly sensitive to vagal overstimulation upon initiation of positive pressure ventilation [3]. MECHANICAL HEART-LUNG INTERACTIONS The greatest determinants of hemodynamic responses to an increased lung volume are mechanical in nature. Since lung volume increases with the application of positive airway pressure, the diaphragm is pushed down and the heart is compressed between the two expanded lungs, thus increasing the pressure surrounding the heart. Unlike the chest wall and diaphragm that can move beyond the expanded lungs, the heart is in essence trapped between those structures. As such, the juxta-cardiac intrathoracic 1-8 /12 $ Bentham Science Publishers

2 Mechanical Ventilation Following Cardiac Surgery in Children Current Respiratory Medicine Reviews, 2012, Vol. 8, No pressure is greater than the intrathoracic pressure at the lateral or diaphragmatic chest walls. This increase in intrathoracic pressure is disadvantageous to bi-ventricular filling [1]. Extreme pulmonary volumes can impair ventricular filling to the point of creating a tamponade physiology. This adverse effect can normally be avoided with the application of more conservative tidal volumes, particularly in patients with already hyperinflated lungs. EFFECTS OF MECHANICAL VENTILATION ON RIGHT VENTRICULAR FUNCTION Right Ventricular Pre-Load Blood flows passively from the low pressure systemic venous reservoir into the right atrium. The pressure in the venous reservoir is a function of the circulating blood volume, peripheral vasomotor tone and blood distribution within the vasculature [1]. Venous return is the main determinant of cardiac output and is dependent on the pressure gradient between the extrathoracic veins and the right atrium. Spontaneous inspiration increases this gradient, thus increasing venous return. As such, right ventricular preload and stroke volume increase during spontaneous breathing (or during negative pressure mechanical ventilation) [5]. The opposite occurs during positive pressure ventilation: the increased right atrial pressure during inspiration with positive pressure decreases the pressure gradient for systemic venous return, thus decreasing right atrial and right ventricular filling, right ventricular stroke volume and potentially reducing cardiac output [3, 5]. The Valsalva effect, a physiologic response to a sustained increase in airway pressure against a closed glottis, is characterized by an early increase in arterial pressure and a decrease in cardiac output, which is secondary to a decrease in venous return. The Valsalva effect mimics positive pressure ventilation and clearly demonstrates how an elevation in intrathoracic pressure influences the right heart [3] (Fig. 1). Right Ventricular Afterload The right ventricular afterload can be defined as the systolic wall stress of the right ventricle. The pulmonary vascular resistance (PVR) is the main determinant of right ventricular afterload and can be affected by arteriolar vasomotor tone and by changes in lung volume. The total pulmonary vascular resistance depends on the balance of the vascular tone and of its two components: the alveolar and extra-alveolar blood vessels. The alveolar vessels are small arterioles, venules and capillaries located within the alveolar septum and subjected to the alveolar pressure and surrounding pressure [1]. The PVR can become elevated in the two extremes of lung volume (Fig. 2). In states of low lung volumes, extra-alveolar vessels become exceedingly tortuous and tend to collapse. There is also spontaneous collapse of alveoli due the loss of interstitial traction, leading to alveolar hypoxia. With a decrease in regional alveolar po 2, there is an increase in regional vasomotor tone that results in pulmonary hypoxic P pl SVC P atmos IVC EGV Head Lung RA LA Systemic Circulation Diaphragm Periphery Thorax Abdomen Periphery Fig. (1). Schematic representation of a three-compartment model of circulation. SVC, superior vena cava; IVC, inferior vena cava; P pl, pleural pressure; RA, right atrium;, right ventricle; LA, left atrium;, left ventricle; EGV, extrathoracic great veins; P atmos, atmospheric pressure. Modified from ref. [3]. Pulmonary Vascular Resistance FRC Lung Volume TLC Total Alveolar vessels Parenchymal vessels Fig. (2). Schematic representation of the lung volume and pulmonary vascular resistance (PVR) relationship, showing the contribution of alveolar and parenchymal vessels on the total PVR. Both extremes of pulmonary hyperinflation and hypoinflation adversely affect PVR, while a lung volume that approximates functional residual capacity (FRC) has the least impact on PVR., residual volume; TLC, total lung capacity. vasoconstriction and reduced blood flow [3]. As the lung volume increases, larger vessels become more linear and have increased capacitance, which in turn reduces hypoxia

3 46 Current Respiratory Medicine Reviews, 2012, Vol. 8, No. 1 Rotta and de Carvalho and lowers the PVR. Further increases in lung volume result in higher transpulmonary pressures (alveolar pressure intrathoracic pressure) with alveolar overdistension and capillary compression, reducing vascular cross section diameter and increasing PVR. As such, conditions associated with pulmonary hyperinflation also lead to increases in PVR. Positive pressure ventilation and positive end-expiratory pressure (PEEP) can reduce right ventricular afterload in patients with low lung volumes by re-expanding collapsed pulmonary units, improving oxygenation and alveolar gas exchange, reducing hypoxic pulmonary vasoconstriction, lowering PVR and improving right ventricular stroke volume. However, positive pressure ventilation and PEEP more often result in increased right ventricular afterload due to excessive alveolar expansion and capillary compression [6, 7] (Fig. 3). The application of PEEP prevents the intrathoracic pressure from returning to atmospheric pressure during exhalation and, if exceedingly high PEEP levels are used, there could be a decrease in cardiac output during the respiratory cycle. Right Ventricular Contractility Myocardial contractility is an important modulator of stroke volume and myocardial oxygen delivery is an important determinant of ventricular contractility. Myocardial blood flow is determined by the myocardial perfusion pressure, and is dependent on intrathoracic pressure, aortic pressure and right ventricular systolic pressure. In the non-hypertensive right ventricle, coronary blood flow occurs mainly during systole and is dependent on the pressure differential between the aorta and the right ventricle. Considering that positive pressure ventilation results in an increased right ventricular pressure, all else being equal, coronary perfusion pressure decreases and coronary blood flow is reduced during mechanical inspiration. As a result, right ventricular contractility, cardiac output and oxygen delivery are reduced. EFFECTS OF MECHANICAL VENTILATION ON LEFT VENTRICULAR FUNCTION Left Ventricular Preload Three physiologic principles have been proposed to explain the decrease in left ventricular preload during positive pressure ventilation: 1) the left ventricle can only eject the volume of blood it receives from the right ventricle. Since the right ventricular output is decreased during positive pressure ventilation, the left ventricle receives a smaller amount of blood and its preload is reduced; 2) Right ventricular systolic pressure and afterload increase during positive pressure ventilation. The elevated right ventricular pressure results in a conformational change of the interventricular septum leading to a decrease in left ventricular compliance and preload; 3) Direct compression of the left ventricle due to elevated intrathoracic pressures, reducing even further the left ventricular preload [3, 7, 8]. Changes in lung volume can alter left ventricular preload by changing instantaneous blood flow inside the left ventricle or by altering the diastolic compliance of the left ventricle. With an increase in lung volume, the relative capacitance of alveolar and extra-alveolar blood vessels also changes. Should lung volumes increase beyond the functional residual capacity (FRC), the capacitance of alveolar vessels decreases as these vessels are compressed. The effect of increasing lung volume on pulmonary venous flow depends on the relative filling state of the pulmonary circulation. In a hypovolemic state, where alveolar vessels are relatively empty, an increase in lung volume will pool blood in the extra-alveolar vessels, thus reducing venous return to the left ventricle. In a fluid overloaded state, alveolar and extra-alveolar Pleural pressure Preload Afterload Ejection Preload Ejection Transpulmonary pressure Afterload Preload Ejection Systolic pressure, Pulse pressure, Aortic blood velocity maximum at the end of inspiration Systolic pressure, Pulse pressure, Aortic blood velocity minimum at the end of expiration Fig. (3). Hemodynamic changes during a mechanical ventilator breath. Left ventricular stroke volume is maximum at the end of inspiration and minimum after 2 to 3 systoles during the expiratory period. Adapted from ref. [8].

4 Mechanical Ventilation Following Cardiac Surgery in Children Current Respiratory Medicine Reviews, 2012, Vol. 8, No vessels are distended and an increase in lung volume would shift blood to the extra-alveolar vessels, thus increasing pulmonary venous flow to the left ventricle. Left Ventricular Contractility Left ventricular contractility is generally not directly affected by changes in pulmonary ventilation. When contractility appears decreased during mechanical ventilation, it is usually secondary to high airway pressures that reduce preload, cardiac output and myocardial oxygen delivery. Clinical signs that suggest these important cardiorespiratory interactions include wide fluctuations in systolic arterial pressure during inspiration. The variability of the systolic arterial pressure (difference between the maximal and minimal systolic arterial pressures during a mechanical ventilation cycle) has been shown to be a sensitive indicator of hypovolemia [9, 10] (Fig. 4). Fig. (4). Changes in systolic blood pressure induced by the respiratory cycle. Adapted from ref. [10]. Left Ventricular Afterload The left ventricular afterload is dependent on the left ventricular transmural pressure. The transmural pressure of an intrathoracic structure such as the left ventricle is a function of the measured pressure within that structure and the pressure surrounding it, which in this case is the pleural (intrathoracic) pressure: Transmural Pressure = Systolic Pressure Intrathoracic Pressure The application of positive intrathoracic pressure during mechanical ventilation decreases the left ventricular transmural pressure and reduces the left ventricular afterload [11]. Conversely, the negative intrathoracic pressures generated during spontaneous breathing or negative pressure ventilation increase the left ventricular transmural pressure and the left ventricular afterload. The effect of positive intrapleural pressure in reducing left ventricular afterload is more evident in infants and small children as compared to adults. Infants generate relatively low systolic ventricular pressures (systolic aortic pressure) compared to adults, but may be subjected to similar positive pressures while receiving mechanical ventilation. As such, the application of the same positive intrathoracic pressure would disproportionately affect the afterload of an infant compared to an adult. The effect of relaxed spontaneous breathing on left ventricular afterload of healthy individuals with normal myocardial function is not clinically significant due to the minor swings in intrathoracic pressures and the fact that the effects on the right heart predominate. However, in patients with a severe asthma attack or acute airway obstruction, the inspiratory pleural pressure is already significantly negative and the left ventricular afterload elevated. In these situations, a further acute negative variation in intrathoracic pressure can precipitate a sudden increase in left ventricular afterload and result in acute pulmonary edema, even in previously healthy hearts. Positive pressure ventilation with the application of PEEP can attenuate or overcome these negative inspiratory variations in intrathoracic pressure and reduce left ventricular afterload. MECHANICAL VENTILATION OF THE UNCOMPLI- CATED POSTOPERATIVE PATIENT The surgical repair of congenital cardiac defects requires the use of mechanical ventilation to ensure adequate support throughout the procedure. Many of the repairs performed today also involve the institution of cardiopulmonary bypass to support circulation and gas exchange during the main portions of the surgical repair. Upon completion of the surgical procedure, the heart is rewarmed and mechanical ventilation resumes as the patient is prepared to transition off cardiopulmonary bypass physiology. Blood gases are followed frequently as the metabolic needs during this complex phase of the procedure are dynamic and require frequent adjustments. Once stable and separated from cardiopulmonary bypass, mechanical ventilation of the uncomplicated patient following surgical repair of a congenital heart defect is not much different than that employed in surgical patients with normal lungs. Most patients undergoing cardiac surgery do not have significant preexisting lung disease and are generally in a healthy lung state prior to surgery. Pressure or volume limited time-cycled modes can be employed with similar results, provided the tidal volumes are maintained at approximately 8 ml/kg and that the driving pressures (plateau pressures PEEP) are kept in the 15 to 25 cm H 2 O range. PEEP is applied to prevent de-recruitment during exhalation and is generally started between 4 and 6 cm H 2 O. Age appropriate inspiratory times and lesion appropriate FiO 2 are also used. In the early decades of congenital cardiac surgery under cardiopulmonary bypass, patients exhibited significant lung alterations simply from the exposure to the noxious artificial surfaces from the circuit, which led to rapid activation of a proinflammatory cascade, capillary leak and pulmonary dysfunction [11]. With recent improvements in cardiopulmonary bypass technology through the use of more biocompatible components, intravascular volume optimization and removal of pro-inflammatory mediators by modified ultrafiltration, pulmonary function immediately following the procedure can, in many cases, be compatible with spontaneous breathing [12, 13]. Provided the patient is stable after the completion of the operation and there is consensus regarding a good surgical

5 48 Current Respiratory Medicine Reviews, 2012, Vol. 8, No. 1 Rotta and de Carvalho result, many patients nowadays are rapidly weaned and successfully separated from mechanical ventilation shortly after the procedure. In our institution, a high volume tertiary referral center for complex cardiac defects, 54% of patients are separated from mechanical ventilation in the operating room or within 6 hours of the completion of the surgical procedure. Some institutions have reported rates as high as 79% [14] for patients successfully extubated in the operating room following cardiac surgery, undoubtedly with lower complexity. Duration of mechanical ventilation is inversely proportional to the patient s age and directly related to the duration of cardiopulmonary bypass and complexity of the surgical procedure [14, 15]. MECHANICAL VENTILATION OF THE PATIENT WITH AN OPEN STERNUM Although most patients that undergo cardiac surgical repairs have their sternotomy wound closed at the completion of the procedure, selected patients might require delayed sternal closure for reasons such as bleeding, edema or concerns about low cardiac output syndrome. Some of these patients have an open sternum with closed skin, while others have an open sternum and open skin with the mediastinal structures covered by a polyester fiber patch, under sterile conditions. Great attention should be given to the proper choice of ventilator settings in the patients with an open sternum, as the chest wall and lung have a different compliance profiles compared to those of patients with intact chests. The fact that the lungs are no longer constrained by opposing forces from the intact chest wall can lead to significant hyperinflation and volutrauma. Those lungs might be able to accept very high and injurious tidal volumes while generating only modest peak inspiratory pressures, which can give the clinician not preoccupied with the effective inspiratory volume a false sense of security (Fig. 5). Patients with open sternotomy wounds require careful monitoring of the effective tidal volume, which is generally set between 8 and 10 ml/kg of dry body weight. Further reduction of tidal volume in these patients might not be beneficial and can lead to de-recruitment and atelectasis. The PEEP should be set slightly higher than usual in the patient with an open sternum, usually in the range of 5 to 7 cm H 2 O. This might be necessary to prevent de-recruitment of the lung during exhalation when the lung recoil force is no longer opposed in its entirety by the anteriorly open chest wall an could, otherwise, lead to a functional residual capacity at or below the closing capacity. MECHANICAL VENTILATION FOLLOWING STAGE I PALLIATION (NORWOOD PROCEDURE) FOR HYPOPLASTIC LEFT HEART SYNDROME The hypoplastic left heart syndrome (HLHS) is, in fact, a spectrum of congenital heart lesions ranging from the milder mitral and/or aortic stenosis with a small non apex-forming left ventricle to the most severe mitral and aortic atresia, severely hypoplastic arch and diminutive left ventricle. Although some patients in the mildest end of the spectrum might be candidates for a bi-ventricular repair [16, 17], most patients in the more severe end of the spectrum require staged palliation or a heart transplant in order to survive. The goals of stage I palliation for HLHS are to: 1) create unobstructed systemic flow; 2) ensure unobstructed pulmonary and systemic venous return and 3) provide a controlled source of pulmonary blood flow. To address the first goal, a neo-aorta is fashioned with the hypoplastic ascending aorta, the adjacent pulmonary artery trunk and a pulmonary allograft patch. The second goal is addressed by performing an atrial septectomy so as to permit adequate mixing of the pulmonary venous blood. The third goal is accomplished by placement of a systemic to pulmonary artery shunt that is generally a modified Blalock-Taussig shunt (right subclavian artery to pulmonary artery) or a Sano shunt (right ventricle to pulmonary artery) [18-20] (Fig. 6). Volume Above FRC (ml/kg) Open sternum Closed sternum Mean Airway Pressure (cm H 2 O) Fig. (5). Volume/pressure relationship (compliance) of the entire respiratory system of a subject with a closed sternum (circles) and an open sternum (triangles). The hysteresis between the inspiratory limb (black symbols) and expiratory limb (open symbols) can be seen in both curves. The leftward and upward deviation of the Open sternum curve indicates that very high tidal volumes might be inadvertently applied, despite a relatively low inspiratory pressure, unless effective tidal volumes are closely monitored. The cardiac output of patients following stage I palliation is partitioned into pulmonary (Qp) and systemic (Qs) components, and the actual proportion of each is dependent on the amount of anatomical restriction to flow and the vascular resistances of the respective components. Postoperative management of these patients centers on balancing Qp and Qs, which might be challenging due to the nature of parallel circuits. The Qp:Qs relationship in single ventricle physiology can be estimated by a simplification of the Fick equation: Qp:Qs = (SaO 2 SmvO 2 ) / (SpvO 2 SaO 2 ) where SaO 2 is the oxygen saturation of arterial blood, SmvO 2 is the oxygen saturation of mixed venous blood, and SpvO 2 is the oxygen saturation of pulmonary venous blood.

6 Mechanical Ventilation Following Cardiac Surgery in Children Current Respiratory Medicine Reviews, 2012, Vol. 8, No A B Repaired Neo-Aorta Pulmonary Artery Base Modified Blalock-Taussig Shunt Fig. (6). Postoperative anatomy following Stage I palliation (Norwood procedure) for the Hypoplastic Left Heart Syndrome. A) The pulmonary artery root has been used to fashion the proximal part of the neo-aorta. The base of the pulmonary artery is closed by a patch and a modified Blalock-Taussig shunt connecting the right subclavian artery and the right pulmonary artery is the source of pulmonary blood flow. B) Pulmonary venous return to the left atrium crosses to the right atrium where it mixes with the systemic venous return prior to ejection through the right ventricular outflow tract into the neo-aorta. Once ejected into the neo-aorta, part of the cardiac output will perfuse the systemic circulation, while part will enter the pulmonary circulation through the modified Blalock-Taussig shunt. After stage I palliation, one should attempt to keep the pulmonary-to-systemic blood flow ratio (Qp:Qs) at a balanced state (close to 1). The Qp:Qs is influenced by the size (diameter) and length of the shunt, systemic vascular resistance and pulmonary vascular resistance. Therapies that increase systemic vascular resistance, such as the administration of dopamine or epinephrine, can lead to a rise in blood pressure, increased Qp:Qs and high arterial oxygen saturation with the highly undesirable side effect of decreased systemic perfusion and acidosis. Hyperventilation is associated with a decrease in pulmonary vascular resistance and should be avoided for the same reasons. Conversely, maneuvers that increase pulmonary vascular resistance, such as hypercapnic acidosis (through hypoventilation or administration of exogenous inhaled CO 2 ), sub-atmospheric FiO 2 ( ) or elevated airway pressures can decrease the Qp:Qs and facilitate systemic perfusion [21-23]. Mechanical ventilation exerts various changes to the cardiorespiratory physiology of patients following stage I palliation (Table 1). The ventilator should be adjusted to achieve a ph of 7.4, PaCO 2 of 40 torr and PaO 2 of 40 torr. This translates to an arterial oxygen saturation of approximately 75% and a Qp:Qs near 1. Ventilation and oxygenation should be tightly controlled for the first 24 hours following surgery and any changes should be carefully monitored by frequent arterial blood gas analysis. Patients are usually kept well sedated for the first 24 hours, with our without neuromuscular blockade. Once stability is achieved, separation from mechanical ventilation follows a gradual and closely monitored wean. Table 1. Effects of Respiratory Interventions on Pulmonary and Systemic Circulations Intervention PVR SVR Qp:Qs Increase MAP Increase No effect Decrease Increase PEEP Increase No effect Decrease Hyperventilation Decrease Increase Increase Increase PaCO 2 Increase Decrease Decrease Increase FiO 2 Decrease Increase Increase Sub-atmospheric FiO 2 Increase Decrease Decrease PVR, pulmonary vascular resistance; SVR, systemic vascular resistance; Qp:Qs, ratio of pulmonary to systemic blood flow; MAP, mean airway pressure; PEEP, positive end-expiratory pressure; PaCO 2, partial pressure of arterial carbon dioxide; FiO 2, fraction of inspired oxygen. MECHANICAL VENTILATION FOLLOWING BIDI- RECTIONAL CAVOPULMONARY ANASTOMOSIS (STAGE II PALLIATION) Cavopulmonary connections (bidirectional Glenn shunt and Hemi-Fontan) involve the connection of the superior vena cava to the pulmonary artery so that venous return from the upper part of the body is passively directed to the pulmonary circulation and is the sole source of pulmonary blood flow (Fig. 7). This creates a very unique physiology in that pulmonary blood flow is dependent on the resistance of two distinct vascular beds: the cerebral and pulmonary circulations. Mechanical ventilation can significantly influence pulmonary blood flow after cavopulmonary anastomosis due

7 50 Current Respiratory Medicine Reviews, 2012, Vol. 8, No. 1 Rotta and de Carvalho decrease the transpulmonary gradient, increase pulmonary blood flow and improve oxygen saturation [26]. RPA SVC RA LPA MECHANICAL VENTILATION FOLLOWING TOTAL CAVOPULMONARY ANASTOMOSIS (FONTAN PRO- CEDURE, STAGE III PALLIATION) The Fontan procedure is the last step in the staged repair for palliation of single ventricle physiology. This operation has undergone several technical transformations since its original description [27]. In its contemporary form, it involves the redirection of inferior vena cava blood to the pulmonary arteries in patients who have already undergone a bidirectional cavopulmonary anastomosis. In the Fontan operation, inferior vena cava blood is directed to the pulmonary arteries by means of an extracardiac conduit or through the creation of a trans-atrial lateral tunnel fashioned with a semi-circular Gore-Tex patch and the lateral wall of the right atrium. The Fontan conduit (lateral tunnel or extracardiac) is generally fenestrated so as to allow for a pop-off (right to left shunt) and maintenance of cardiac output should there be a significant increase in pulmonary vascular resistance [28, 29] (Fig. 8). Neo-Aorta RPA SVC LPA Fig. (7). Postoperative anatomy following Stage II palliation (Bidirectional Glenn shunt). The superior vena cava (SVC) is anastomosed to the right pulmonary artery (RPA) and provides passive blood flow in a bidirectional fashion. Blood returns from the inferior vena cava (IVF) into the right atrium (RA) prior to entering the single ventricle for ejection into the systemic circulation. LPA, left pulmonary artery. to its effects on pulmonary vascular resistance and cerebral blood flow. Hyperventilation and alkalosis are well known to cause a decrease in pulmonary vascular resistance. However, hyperventilation also leads to cerebral vasoconstriction, decreased cerebral blood flow and decreased venous return from the superior vena cava into the pulmonary vascular bed. As such, hyperventilation following cavopulmonary anastomosis usually leads to a net decrease in pulmonary blood flow and a consequent decreased systemic oxygen saturation [24]. On the other hand, a mild degree of hypoventilation and hypercapnia can lead to cerebral vasodilatation and increased cerebral blood flow, which in turn might increase venous return from the superior vena cava and pulmonary blood flow, thus increasing systemic oxygenation [25]. In cases where the pulmonary vascular resistance might be adversely affected by hypercapnic acidosis or by primary lung pathology, the administration of inhaled nitric oxide has been used to selectively relax the pulmonary vascular bed, IVC LT RA Fig. (8). Postoperative anatomy following Stage III palliation (fenestrated lateral tunnel Fontan procedure). Blood from the inferior vena cava (IVC) is directed to the pulmonary arteries through a fenestrated (arrow) lateral tunnel (LT) that is visible through the exposed right atrium (RA). Superior vena cava (SVC) also drains passively into the pulmonary artery. Pulmonary venous return to the left atrium is directed to the RA where in mixes with any right-to-left shunt across the fenestration prior to ejection from the single ventricle into the neo-aorta. LPA, left pulmonary artery; RPA, right pulmonary artery.

8 Mechanical Ventilation Following Cardiac Surgery in Children Current Respiratory Medicine Reviews, 2012, Vol. 8, No Following the Fontan procedure, pulmonary blood flow is very dependent on the central venous pressure. Intravascular volume status should be carefully monitored by following the left atrial pressure, as patients often need volume expansion in the early postoperative period. Provided the intravascular volume is optimized, common causes for persistent hypoxemia in the patient with a fenestrated Fontan operation include elevated pulmonary vascular resistance and obstruction at the site of the venopulmonary anastomosis. An obstruction at the site of the veno-pulmonary anastomosis is mechanical in nature and generally requires surgical revision or dilatation (stenting) in the interventional cardiac catheterization laboratory. On the other hand, persistent hypoxemia due to pulmonary hypertension is often managed clinically. Considering that pulmonary blood flow in patients with Fontan physiology is largely dependent on venous return that is redirected to the pulmonary arteries, spontaneous breathing is the most advantageous form of ventilation in patients after the Fontan procedure. Following a technically successful operation without the occurrence of a low cardiac output state or bleeding, patients can generally be extubated in the operating room or shortly after arrival in the pediatric cardiac intensive care unit. Those who require continued ventilation should have lung inflation optimized by the application adequate PEEP to minimize atelectasis and target the lowest effective mean airway pressure to avoid hyperinflation. In the absence of lung disease or effusions, the PEEP is generally set at 3 cmh 2 O and effective tidal volumes of approximately 8 ml/kg are employed. These settings should be optimized to achieve lung volumes close to the functional residual capacity. Ventilation at low long volumes leads to atelectasis and a subsequent increase in pulmonary vascular resistance. Ventilation at high lung volumes (and high intrapleural pressures) leads to hyperinflation and adversely affects pulmonary blood flow and oxygenation (Fig. 2). Elevated pulmonary artery pressures (> 15 mmhg) have been associated with poor outcomes in patients with Fontan physiology [30]. Elevated pulmonary artery pressures lead to venous congestion and third spacing of fluid, pleural effusions, ascites, anasarca and multi-organ dysfunction. Pleural effusions and significant ascites should be readily drained and intra-abdominal pressure minimized. Selected patients with significant ventilation-perfusion mismatch or increased pulmonary vascular resistance might benefit from inhaled nitric oxide therapy. Nitric oxide is usually started at 20 ppm, which should allow for a reduction in the FiO 2 to 0.6 or lower. Cuirass negative pressure ventilation has been shown to augment cardiac output and pulmonary blood flow in patients with Fontan physiology and can be considered in patients refractory to management by more standard ventilation strategies [31]. MECHANICAL VENTILATION IN PATIENTS WITH RIGHT VENTRICULAR DIASTOLIC DYSFUNCTION With the various advances in myocardial protection, surgical techniques and postoperative care over the past decade, complete correction of Tetralogy of Fallot (TOF) has become a procedure that is now associated with a low operative morbidity and mortality. However, in a small but important sub-group of patients, the early postoperative course can be complicated by a severe low cardiac output state that is often refractory to conventional therapeutic measures [32]. A common theme that unifies this group of patients is a history of ventriculotomy or muscle resection to relieve outflow obstruction of an already hypertrophied right ventricle. These patients present with markedly elevated right atrial pressures and a low cardiac output state, partly because of systolic ventricular dysfunction but mostly due to diastolic dysfunction of the poorly compliant ventricle [32]. Right ventricular diastolic dysfunction is usually managed with fluids to optimize right ventricular preload and inotropic agents, such as milrinone, to improve contractility and decrease the afterload [33]. Mechanical ventilation can play a significant role in the clinical course of these patients, as cardiopulmonary interactions can be exuberant in patients with restrictive right ventricular physiology. These patients tend to develop pleural effusions and increased intraabdominal pressure from ascites, which further interfere with respiratory performance. As such, rapid ventilator wean and early extubation are usually not a viable option for these patients who tend to have a slower postoperative recovery and longer ICU length of stay than their counterparts without diastolic dysfunction. Mechanical ventilation of these patients should center on optimizing lung volumes after drainage of pleural effusions and ascites, while maintaining low intrathoracic pressures so as not to further impair right ventricular function. Manipulation of cardiopulmonary interactions by means of negative pressure (cuirass) ventilation has been successfully performed in selected patients following TOF repair and might provide an alternative to increase cardiac output in patients refractory to more conventional treatment strategies. REFERENCES [1] Pinsky MR. The effects of mechanical ventilation on the cardiovascular system. Crit Care Clin 1990; 6: [2] Cheifetz IM, Craig DM, Quick G, et al. 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