240 SECTION IV Pediatric Critical Care Cardiovascular tract obstruction or another problem or problems that I need to know about? Will Dakota have sufficient Do2 in 10 hours when postbypass myocardial[.]
240 S E C T I O N I V Pediatric Critical Care: Cardiovascular tract obstruction or another problem or problems that I need to know about? Will Dakota have sufficient Do2 in 10 hours when postbypass myocardial depression is at its worst? Might additional therapy secure adequate Do2 at a lower filling pressure, thereby minimizing adverse effects of systemic venous hypertension? As this example illustrates, the QOT concept is useful for three reasons: The QOT is, in part, a function of the patient’s overall condition and can reflect anatomic or physiologic problems that require further exploration Physiologic trajectory is key, especially early in the course of certain illnesses or after cardiopulmonary bypass, when Do2 predictably declines over the first 12 hours.2 Taking into account the QOT relative to tissue oxygenation at any point in time helps one better estimate the likelihood of the need for augmented support (e.g., mechanical support of the circulation) as time passes Some therapies (e.g., fluid infusion to obtain high filling pressure or high airway pressure), while helpful at a point in time, can be pernicious over the longer run: high venous pressure, especially in infants, causes third spacing of fluid, and the effect of ventilator-associated lung injury on lung function can be devastating By using high central venous or airway pressures, the intensivist is, in effect, incurring a debt to secure short-term perfusion that will have to be repaid later Experienced clinicians always take the QOT into account in their work, which influences their level of concern about a patient and guides subsequent timing and choice in adjusting therapy Almost any form of therapy might be included in the QOT concept However, this chapter focuses on medical therapies that have the most important effect on hemodynamics, including Do2 and perfusion pressure These therapies include inotropic/vasoactive agents, volume infusion, and airway pressures used during mechanical ventilation The amount of inotropic and vasoactive drugs administered, assuming that they are used appropriately, seems to be a crude indicator of patient illness.2 Volume infusion to achieve adequate filling pressure is required even for the normal heart, but the QOT concept applies when higher-than-normal filling pressure is needed to maintain adequate CO The consequences of high filling pressures are body edema (especially in infants), pleural and other cavity space effusions, and pulmonary edema If high venous pressure is coupled with systemic hypotension (e.g., with a failing Fontan circulation), there may be critically reduced transtissue perfusion pressure with a potentially negative impact on cerebral and splanchnic perfusion With respect to mechanical ventilation, the need for high mean airway pressure (Paw) is most commonly a reflection of lung disease, but pulmonary edema on a hydrodynamic basis may occasion the use of high Paw for optimal lung recruitment High Paw can reduce venous return to the heart, increase pulmonary vascular resistance (PVR), and contribute to ventilator-associated lung injury Variables That Determine Tissue Oxygenation Tissue oxygenation is directly related to both Do2 and systemic arterial blood pressure (SAP) Do2, the quantity of O2 delivered to the tissues per minute, is the product of systemic blood flow (SBF), which equals CO except in patients with certain cardiac malformations, and arterial O2 content: DO ( mL ) 10 CO ( L ) CaO ( mL 100 mL blood ) where CO is cardiac output or SBF (in liters per minute or liters per minute per square meter) and Cao2 is quantity of O2 bound to hemoglobin plus the quantity of O2 dissolved in the plasma in arterial blood The O2 content of arterial blood (mL O2/dL blood) equals: ( ) CaO SaO Hbg [ g dL ] 1.36 ( PaO 0.003) where Sao2 is arterial O2 saturation, Hgb is hemoglobin concentration (in grams per deciliter), 1.36 (constant) is the amount of O2 bound per gram of hemoglobin (mL) at atm of pressure, Pao2 is arterial partial pressure of O2, and 0.003 (constant) multiplied by the Pao2 equals amount of O2 dissolved in plasma at atm The quantity of dissolved O2 is generally considered to be negligible in the normal range of Pao2 Hypoxia, because of poor gas exchange within the lungs (i.e., intrapulmonary shunt), or in the setting of CHD with right-to-left shunting, is an important determinant of blood O2 content CO is the product of stroke volume (quantity of blood ejected per beat) and heart rate; SAP is determined by CO and systemic vascular resistance (SVR) The four primary determinants of cardiac function are preload (which determines the precontractile lengths of the myofibrils); end-systolic wall stress (function of systemic blood pressure and physical characteristics of the arterial system, ventricular wall thickness, and chamber dimension); myocardial contractility; and heart rate These determinants of ventricular function can be altered by many factors in the intensive care setting Preload, or end-diastolic volume, is affected by ventricular compliance (rate and extent of cardiomyocyte relaxation and cardiac connective tissue), intravascular volume, and intrathoracic pressure Expansion of the heart resulting from transmural filling pressure, rather than the LA pressure per se, determines the force of contraction Therefore, intrathoracic (or intrapericardial) pressure is a key determinant of preload Ventricular hypertrophy, vasodilator and diuretic therapies, and positive pressure mechanical ventilation all adversely affect preload Similarly, cardiac function is inversely related to afterload, or endsystolic wall stress Anatomic obstructions and systemic or pulmonary hypertension may negatively affect ventricular systolic and diastolic function Excessively fast or slow heart rates and inappropriately timed atrial contraction (relative to ventricular systole) may negatively affect ventricular filling or function Finally, myocardial contractility is often negatively affected by the following factors: hypoxemia, acidosis, hypomagnesemia, hypocalcemia, hypoglycemia, hyperkalemia, cardiac surgery, sepsis, and cardiomyopathies Monitoring Tissue Oxygenation CO can be assessed qualitatively by physical examination and other modalities and quantitatively by a variety of techniques using invasive and noninvasive devices, laboratory data, and other clinical indicators Do2 is easily derived if systemic blood flow can be measured, but it is only indirectly inferred if this information is lacking Qualitative Assessment of Cardiac Output Physical Examination The physical examination is often the initial and most common technique used to assess and monitor cardiovascular function Significantly diminished CO may manifest as diminished peripheral pulses, cool or mottled extremities, and delayed capillary CHAPTER 27 Assessment of Cardiovascular Function refill However, certain clinical signs of low CO may be unreliable depending on the particular diagnosis For example, in the context of cardiac lesions associated with a large arterial pulse pressure (e.g., severe aortic insufficiency and aortopulmonary shunts), peripheral pulses may be increased despite low CO and reduced systemic Do2 Patients in septic shock are often peripherally vasodilated and warm despite hypotension and reduced tissue Do2 Central cyanosis from either cardiac or respiratory causes results from arterial O2 desaturation In contrast, peripheral cyanosis results from vasoconstriction or low blood flow at the microcirculatory level In some patients, cyanosis is a relatively subtle physical finding, particularly if the patient is anemic or has a dark complexion Hydration status can be assessed by skin turgor, dryness of mucous membranes, and fullness of the anterior fontanel (in infants), but these manifestations of hydration status relate mostly to interstitial fluid and may poorly reflect intravascular volume, which must be directly measured Cardiac auscultation for abnormal heart sounds—including valve clicks, rubs, gallops, and murmurs—may provide the first indication of a significant functional or structural cardiac abnormality, although these sounds not directly reflect Do2 Unfortunately, the lack of a heart murmur, especially in low CO states, does not necessarily rule out a significant residual cardiac lesion The presence of crackles on pulmonary auscultation, particularly in the older pediatric patient, may signify pulmonary edema However, crackles are nonspecific and may be caused by lung disease or fluid overload in addition to disorders of cardiac structure or function Finally, jugular venous distension and hepatomegaly are indicative of high right-sided filling pressures often associated with RV dysfunction Chest Radiography Although the chest radiograph is of little value in assessing a patient’s hemodynamic profile per se, it may be helpful for assessing certain aspects related to cardiovascular status Provided that the chest radiograph is technically adequate, the clinician can assess heart size, contour and configuration, pulmonary vascularity, pleural effusions, lung parenchyma, and abdominal situs Some of these findings, when abnormal, may help determine the etiology of cardiovascular dysfunction Chronically increased pulmonary arterial pressure may be indicated by enlarged pulmonary arteries in the hila that radiate toward the periphery of the lung Conditions that increase pulmonary blood flow (PBF, or Qp) at least twice normal also increase the size of the pulmonary arteries Increased pulmonary capillary pressure may be inferred by the presence of pulmonary edema The edema may present as a “fluffy” hilum but may have a more diffuse granular appearance in neonates Pleural effusions may accompany pulmonary edema, particularly in conditions associated with poorly compensated congestive heart failure Increased pulmonary venous markings are indicative of elevated pulmonary venous pressures of any cause, although usually from decreased left ventricular compliance or obstruction in the pulmonary veins or mitral stenosis In the early postoperative period, the most important information obtained from the chest radiograph is (1) the positions of the endotracheal tube, chest tubes, and intracardiac lines; (2) the presence of extrapulmonary fluid or air; and (3) the presence of pulmonary edema The cardiothoracic ratio gives a quantitative estimate of cardiac size, which is obtained by dividing the transverse measurement of the cardiac shadow in the posteroanterior view by the width of the thoracic cavity Cardiomegaly is present if this value is greater than 0.5 in adults and 0.6 in infants 241 Although useful for assessing left ventricular (LV) enlargement, the cardiothoracic ratio is not as sensitive to RV enlargement RV enlargement results in lateral and upward displacement of the cardiac apex on the posteroanterior view and filling of the retrosternal space on the lateral view It is perhaps more important to know what the chest radiograph may not reveal; significant cardiac problems, such as constrictive pericarditis, acute fulminant myocarditis, and even acute pericardial tamponade are often associated with a normal-sized heart on a chest radiograph Quantitative Assessment of Cardiac Output Quantitative measures of CO in the ICU can be obtained by a variety of techniques, including the Fick method, thermodilution, dye dilution techniques (now rarely used), and Doppler echocardiography Each of the first three methods applies a similar principle of dilution of an indicator: O2, cold, or indocyanine green dye, respectively The change in concentration of a substance is proportional to the volume of blood in which it is being diluted In general, thermodilution is the method most widely used in the intensive care setting However, for conditions of low CO, the Fick method is more reliable than the thermodilution or dye dilution techniques Conversely, the Fick method is less accurate in conditions of high systemic blood flow because of difficulty in measuring narrow arteriovenous O2 differences in the blood Thermodilution Technique The thermodilution technique requires use of a specialized pulmonary artery (PA) catheter CO is calculated by injecting a known volume of iced water or saline solution into the right atrium (proximal catheter port) and measuring the temperature change at the catheter tip in the PA CO is calculated by the following equation: CO 1.08 Vi ( Tb Ti ) Tb ( t ) dt where Vi is injectate volume (mL), Tb is temperature of blood, Ti is injectate temperature, and Tb(t)dt is area under the curve In general, thermodilution CO measures are performed using a completely automated system, and the calculations are performed by a computer This method (as with any indicator dilution method) requires complete mixing; thus, it is most accurate in situations in which a mixing chamber is located proximal to the thermistor It is generally used only in patients who not have intracardiac or great vessel–level shunts or an insufficient valve between the injection site and sampling site The injection must be made rapidly because a slow injection will give a falsely elevated CO Possible sources of error with this method include inaccurate measurement of the volume of injectate or temperature of the blood or injectate, close approximation of the thermistor to a vessel wall, and inadequate mixing, as is sometimes seen in venous systems with low flow Fick Method According to the Fick principle, CO equals O2 consumption divided by the arteriovenous O2 content difference: CO VO ( CaO CvO ) where Vo2 is O2 consumption (in milliliters per minute) and Cao2 and Cvo2 are arterial and venous O2 content (mL O2/100 mL blood), respectively Care must be taken to select the appropriate sampling site for a true mixed venous blood sample With a normal heart, the best site to obtain a mixed venous sample is within 242 S E C T I O N I V Pediatric Critical Care: Cardiovascular the PA If a left-to-right shunt is present, however, the mixed venous site should be the cardiac chamber proximal to the site of the shunt When a site other than the PA is used for the mixed venous site, the resultant value for arteriovenous O2 difference is a less reliable reflection of the absolute CO, but it can be used for serial observations and for monitoring response to therapy over time Because measuring Vo2 in the intensive care setting requires special equipment and is somewhat cumbersome, the arteriovenous O2 difference is often used as an indirect measure of CO A wide arteriovenous O2 difference generally reflects a low CO and indicates a large O2 extraction by the tissues, whereas a narrow arteriovenous O2 difference usually reflects a high CO (Note that it is the arteriovenous content difference, not O2 saturation difference, that matters; with anemia, the O2 saturation difference may be wider than normal despite normal CO.) Unfortunately, studies suggest that Vo2 is quite variable for any individual patient in an intensive care setting.9 Furthermore, mixed venous O2 saturation (and, hence, arteriovenous O2 difference) may be misleading in patients with decreased tissue O2 extraction, as can be seen with septic shock.10,11 Doppler Echocardiography Doppler techniques can be used to measure CO using the mean velocity of systolic flow, heart rate at the time of measurement, and cross-sectional area of the artery in which measurements are being made (usually the ascending aorta): CO V HR where A is the area of the orifice, V is integrated flow velocity, and HR is heart rate To determine the integrated flow velocity, the area under the Doppler curve must be measured The area of the aortic orifice is commonly obtained by measuring the aortic diameter from the two-dimensional image, where A 0.785 ∞ d2 This technique requires special care for accurate Doppler interrogation of blood flow and is seldom used in the critical care unit Pulse Oximetry Pulse oximetry measures the quantity of hemoglobin saturated with O2 in peripheral arterial blood It depends on two principles: (1) oxygenated and reduced hemoglobin have different absorption spectra; and (2) at constant light intensity and hemoglobin concentration, O2 saturation of hemoglobin is a logarithmic function of the intensity of transmitted light (Beer-Lambert law) Two wavelengths of light that have different absorption spectra for reduced hemoglobin and oxyhemoglobin are transmitted from the light-emitting diodes through the arterial bed Light absorption at the two wavelengths is compared, yielding the ratio of oxyhemoglobin to reduced hemoglobin, or the O2 saturation Pulse oximeters have a high potential for error at saturations below 80%.12 Furthermore, the O2 dissociation curve flattens out at the high range so that at saturations greater than 95%, large changes in Pao2 accompany small changes in saturation This phenomenon should be kept in mind when monitoring premature infants, for whom it is important to avoid hyperoxia Other Measures of Oxygen Delivery Acid-Base Status When tissue hypoxia occurs and affected tissues and organs resort, in part, to anaerobic metabolism, increased production of lactate, carbon dioxide (CO2), and hydrogen ions occurs The anion gap, the difference in unmeasured serum anions and unmeasured serum cations, can yield information regarding the cause of metabolic acidosis If the anion gap is normal (8–16 mEq/L), loss of bicarbonate has occurred, usually via the kidneys or gastrointestinal tract, or rapid dilution of the extracellular fluid has occurred Blood Lactate Blood lactate concentration is a laboratory measure that indirectly reflects perfusion.13,14 Blood lactate measurements are extensively used for monitoring and evaluating response to therapy It has been demonstrated that initial absolute blood lactate levels are less important than the temporal trend in lactate concentrations for predicting morbidity and mortality in postoperative cardiac patients.15,16 Unfortunately, the specificity of blood lactate is imperfect and may lack sensitivity for detecting supply-dependent O2 consumption, particularly if it is only regional In addition, blood lactate depends on hepatic metabolism and the rate of production and clearance It may also be elevated in cases of severe hypoxic ischemic brain injury or in advanced bowel ischemia Serum Biomarkers In recent years, there has been an increased interest in the measurement and use of serum biomarkers as an assessment of cardiac, renal, and neurologic function in critical illness The most commonly used cardiac-specific serum biomarkers in the assessment of cardiovascular health are troponin and B-type natriuretic peptide (BNP).17 Troponin release is specific to myocardial injury therefore, it may be elevated in patients with myocarditis, pericarditis, coronary injury or occlusion, and sepsis After cardiac surgery, troponin levels can be elevated due to myocardial injury as a result of cardiopulmonary bypass and aortic cross-clamping Therefore, its utility in this circumstance is unclear unless there is a specific concern about coronary blood flow BNP is released in response to ventricular wall stress due to volume or pressure overload High levels of circulating BNP have been correlated with congestive heart failure states—a trend over time is likely the most helpful for the clinician Other biomarkers to measure the health of the kidneys and brain also hold promise as potentially important tools to assess these systems noninvasively Gastric Tonometry Gastric tonometry, a technique available for clinical use in adult and some pediatric ICUs, allows indirect assessment of perfusion by measuring gut intramucosal pH or partial pressure of carbon dioxide (Pco2).18,19 It may have an advantage over blood lactate concentration in that it can uncover regional hypoxia and hypoperfusion involving the gut and can be adapted for continuous online measurement.19 Nonetheless, this technique assumes that a critical reduction in O2 transport manifests in the splanchnic circulation before it can be detected systemically (probably a reasonable assumption), and tonometric methods are not entirely noninvasive Urine Output Urine output generally reflects CO, but oliguria may occur in the first 24 hours after open heart surgery, especially in neonates, even in the context of good CO and blood pressure Thus, it is important to consider urine output in the context of other indicators of organ perfusion and not as an isolated variable It should also be noted that the kidneys are quite sensitive to perfusion pressure and that good systemic blood flow coupled with low systemic arterial pressure (due to low SVR) may adversely affect urine output more than other measures of tissue perfusion CHAPTER 27 Assessment of Cardiovascular Function Near-Infrared Spectroscopy Near-infrared spectroscopy (NIRS), a noninvasive technique that has been applied to assess systemic and regional O2 transport in several clinical and laboratory studies,20–28 is now commonly used, particularly in patients with CHD, as a means of trending regional Do2 or as a surrogate for mixed venous O2 saturation or systemic Do2.29 Retrospective studies in patients with CHD have shown associations with lower NIRS measurements and acute kidney injury,30,31 risk of necrotizing enterocolitis,32 and postoperative mortality in infants with single-ventricle physiology.33 Abdominal site NIRS has been shown to correlate with simultaneous intramucosal pH measurements by gastric tonometry in neonates and infants with CHD undergoing catheter-based or surgical intervention.34 In an experimental setting in which Do2 is controlled, NIRS has been used to correlate cytochrome aa3 (the terminal link in the electron transport chain responsible for mitochondrial respiration), Vo2, and lactate flux.25 Thus, NIRS has the potential to identify a critical regional reduction in O2 transport at the cellular level Systemic Arterial Blood Pressure Invasive Blood Pressure Monitoring Intravascular pressure monitoring is often essential in the management of critically ill neonates and infants in the ICU Typically, an end-hole catheter is inserted into a vessel (or cardiac chamber) and connected to a pressure transducer by a coupling system composed of fluid-filled extension tubing, a stopcock for withdrawing blood and balancing the transducer to atmospheric pressure, and a continuous infusion device to flush out blood and air The transducer translates pressure into an electrical signal that can be processed through a preamplifier into a waveform and numerical display on a monitor The pressure transducer must be properly calibrated, dampened, and positioned (at mid-chest level) Inaccurate measurements can occur for a variety of reasons In the pediatric population, blood pressure is age dependent and is a relatively insensitive marker of CO and Do2 Because blood pressure is the product of CO and SVR, hypotension may result from diminished CO and/or decreased SVR.35 Because the treatment options are different, distinguishing low CO from low SVR is important Noninvasive Blood Pressure Monitoring The auscultatory method of blood pressure measurement with a cuff and pressure gauge is difficult if access to the patient is limited, if the patient is small or uncooperative, and when frequent recordings are required Therefore, two techniques, Doppler and oscillometric measurements, have been developed The Doppler technique uses a Doppler ultrasound probe that is applied to the radial or brachial artery A cuff wrapped around the upper arm is inflated until the audible Doppler signal is obliterated and then deflated until the signal first becomes audible again (systolic blood pressure) This method has been validated in low-flow states and in small children.36 The oscillometric method has the advantage of being readily automated The device for indirect noninvasive mean arterial pressure (Dinamap, GE) is based on the principle that blood flow through a vessel produces oscillation of the arterial wall that may be transmitted to an inflatable cuff encircling the extremity As cuff pressure decreases, a characteristic change occurs in the magnitude of oscillation at the levels at which systolic, diastolic, and mean pressures are registered Accuracy of Dinamap blood pressures has been validated in children, and it 243 correlates well with direct intravascular radial artery pressures.37 The accuracy of these two techniques relates to the cuff size If the cuff is too narrow, the pressure recorded may be erroneously high; if the cuff is too wide, the pressure recorded may be underestimated Both techniques are unreliable and inadequate in patients with low CO, hypotension, dysrhythmias, significant edema, or systemic vasoconstriction Central Venous or Intracardiac Pressure Monitoring Pressures can also be measured in the cardiac chambers or in the pulmonary vasculature However, the necessity for intravascular or intracardiac lines should always be carefully considered and should be removed as soon as the clinical condition permits The placement of relatively large catheters in small vessels for prolonged periods carries a risk of thrombosis and systemic thromboembolism Central venous access affords the opportunity to measure central venous pressure (CVP), deliver drugs or high-osmolarity nutritional solutions, and repeatedly sample blood to monitor venous O2 saturations and for other laboratory studies Intraarterial lines offer the opportunity to continuously monitor arterial pressure and for intermittent blood gas analysis Intravascular pressures provide information about ventricular preload and afterload RV preload is assessed by the CVP The CVP is determined by a variety of factors, including patient age, preoperative status (i.e., a patient with RV hypertrophy and increased RA pressure), cardiac performance, intrathoracic pressure, blood volume, vasopressor therapy, and status of the pericardium The CVP a wave reflects atrial contraction; the v wave reflects atrial filling Serial measurements of CVP are frequently used to evaluate the response to fluid administration RV afterload can be assessed using a PA catheter This catheter is particularly important for monitoring PA pressure and therapeutic response to vasodilators in patients with elevated PVR The PA wedge pressure reflects LA pressure (in the absence of pulmonary vein stenosis) In the postoperative cardiac patient, a direct LA line can be placed to directly assess LV preload LV afterload is assessed by measurement of SAP, provided that no LV outflow tract obstruction is present High-Frequency Physiologic Data Capture and Streaming Analytics Given the large amount of data that intensivists are presented with on a moment-to-moment basis, there has been recent interest in the use of software and computer algorithms to collect and display the relevant data in real time In addition, there is ongoing and promising research into the use of streaming analytics and machine learning to help clinicians predict which patients may be at higher risk of certain clinical outcomes, thus enhancing opportunities for timely decisions and therapeutic or preventive interventions.38,39 Assessing Variables That Affect the Quantity of Therapy If it is important to take into account the QOT needed to secure adequate tissue perfusion, it follows that one would like to assess the variables that affect the QOT Table 27.1 summarizes the effect of abnormalities of cardiovascular and pulmonary function on QOT What follows is a brief description of how these cardiovascular variables may be assessed in the critical care unit 244 S E C T I O N I V Pediatric Critical Care: Cardiovascular TABLE Cardiovascular Function and Quantity 27.1 of Therapy (QOT) Cardiac System Variable Impact on QOT Cardiac Function g Ventricular systolic function h Filling pressure, h inotropic/pressor support g Ventricular diastolic function h Filling pressure, h pressor support Abnormal rhythm h Filling pressure, h inotropic/pressor support Intracardiac structural lesions (g efficiency) h Filling pressure, h inotropic/pressor support Single ventricle with A-P shunt (g efficiency) h Filling pressure, h inotropic/pressor support Peripheral Vasculature h SVR h Ventricular work n h filling pressure, h inotropic/pressor support g SVR h Filling pressure, h inotropic/pressor support h PVR ventricles h Ventricular work n h filling pressure, h inotropic/pressor support Aortopulmonary shunt gPBF n g O2 n h filling pressure, h inotropic/pressor support Bidirectional Glenn h SVC pressure Fontan h Filling pressure, h inotropic/pressor support Vascular Function “Leaky” vascular bed n edema n volume infusion n edema h Filling pressure, h inotropic/pressor support Pulmonary Function g Lung compliance, g gas exchange h Airway pressure n g venous return n h systemic venous pressure, barotrauma A-P, Aortopulmonary; O2, oxygen, PBF, pulmonary blood flow; PVR, pulmonary vascular resistance; SVR, systemic vascular resistance Ventricular Systolic Function Precise measurement of ventricular systolic function is difficult,40 especially in the critical care setting A commonly used surrogate is the echocardiographic demonstration of ventricular wall excursion/ shortening LV shortening and ejection fractions can be measured using echocardiography with reasonable accuracy, but these variables are influenced by preload and afterload and by inotropic conditions RV morphology makes echocardiographic measurement of systolic function even more problematic despite a variety of described techniques to assess this variable.41 For either ventricle, most often one resorts to a qualitative assessment of ventricular wall excursion as a crude estimate of systolic function Although cardiac magnetic resonance angiography/magnetic resonance imaging (MRI) can accurately measure LV and RV ejection fraction, it is often of limited practical utility in the critically ill pediatric patient Ventricular Diastolic Function Ventricular diastolic function is also difficult to measure precisely In the critical care unit, diastolic dysfunction is usually manifested as a need for increased filling pressures for a given magnitude of ventricular output Echocardiography sometimes will demonstrate an apparently underfilled ventricle despite adequate or high filling pressures, but most often a lack of compliance is inferred from high filling pressures alone Echocardiographic measures of ventricular compliance exist, but many clinicians have found them to be of limited practical value It is important to emphasize that transmural filling pressure, not atrial pressure per se, is what determines diastolic filling; elevated pericardial or intrathoracic pressure will reduce ventricular filling for any given atrial pressure.42 Rhythm Disturbance A variety of abnormal rhythms can decrease systemic perfusion and therefore lead to increased QOT A surface electrocardiogram may be sufficient to delineate the type and mechanism of an arrhythmia; however, especially with tachycardia, all too often one cannot clearly discriminate P waves from the T and QRS deflections The use of atrial leads in conjunction with limb leads can be exceedingly helpful for both diagnosing and treating arrhythmias in postoperative patients Alternatively, esophageal electrodes sometimes can be helpful, although they are somewhat cumbersome to use (and cannot always affect atrial capture) It is important to frequently and carefully reassess the rhythm because significant changes (e.g., from sinus rhythm to junctional ectopic tachycardia) may escape casual detection Abnormal Systemic Vascular Resistance SVR is determined by the following equation: SVR ( SAP CVP ) CO where SAP is mean systemic arterial pressure (mm Hg), CVP is mean central venous pressure (mm Hg), and CO is cardiac output, usually indexed to surface area (in liters per minute per square meter) Increased SVR can be useful when CO is insufficient for adequate systemic perfusion pressure with normal SVR On the other hand, SVR increased beyond that needed for adequate SAP increases systemic ventricular afterload and therefore may negatively affect CO.43 For reasons discussed in the following section on single-ventricle physiology, increased SVR may also result in excess PBF in patients with an aortopulmonary shunt Finally, increased SAP in a newly postoperative patient may contribute to excessive bleeding In contrast, low SVR can cause systemic hypotension despite adequate or supranormal CO Anecdotal observations and some published information indicate that low SVR may occur after cardiac surgery as well as with other systemic illnesses (e.g., sepsis) As previously noted, because CO is infrequently measured in PICUs, SVR is most commonly inferred from observation of cutaneous perfusion and SAP Indeed, it is important to evaluate systemic hypotension in the context of cutaneous perfusion (brisk capillary refill suggests low SVR) because rational therapy for decreased SVR with adequate CO (vasopressor support) is quite different from that useful for hypotension due to inadequate CO ... shadow in the posteroanterior view by the width of the thoracic cavity Cardiomegaly is present if this value is greater than 0.5 in adults and 0.6 in infants 241 Although useful for assessing left... performed using a completely automated system, and the calculations are performed by a computer This method (as with any indicator dilution method) requires complete mixing; thus, it is most accurate... rapidly because a slow injection will give a falsely elevated CO Possible sources of error with this method include inaccurate measurement of the volume of injectate or temperature of the blood