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In the cardiac patient, especially during the first postoperative night, rapid hemodynamic changes may occur. The use of invasive continuous monitoring devices helps to detect complications early. Everyone working bedside has to keep in mind that repeated careful clinical observation by the experienced nurse and medical doctor is fundamental in all bedside monitoring. If monitoring devices show ambiguous results, a careful clinical examination usually resolves the problem. The best “monitor” is the intelligent, experienced, and committed caregiver.
For children with congenital heart disease it is com- mon to have saturations between 70 and 88%, there- fore, the value of the pulse oximeter is always to be compared with blood gas analyzes of PaO2 and should be regarded more as a useful indicator of trends.
At saturations above 90–95%, the oxygen dissocia-
•
tion curve flattens out so that large changes in PaO2
causes small changes in pulse oximetric saturation.
Hyperoxia is not detectable with pulse oximetry.
This fact is important in the avoidance of hyperoxia in the premature infant.
10.1.3 Transcutaneous Oxygenation (PaO
2) and Carbon Dioxide (PaCO
2) Monitoring
Transcutaneous monitoring of PaO2 and PaCO2 are widely used in neonatal and pediatric intensive care. A sensor is fixed to the skin over the anterior chest or the abdomen on an occlusive contact medium and held in place by an adhesive ring. Reported correlations for PaO2 monitoring are 0.90–0.95 and for PaCO2 moni- toring 0.90–0.93 [3]. As transcutaneous derived gas tensions result from complex interactions between hemodynamic, respiratory, and local factors, which can hardly be defined in hemodynamically instable patients;
these methods are of inferior value in the care for the early postoperative cardiac patients. However, in patients with prolonged ventilation the use of PaCO2 monitoring to control mechanical ventilation is useful and reduces the need of blood gas analyzes. As in pulse oximetry, the value of the transcutaneous PaCO2 monitoring is always to be compared with blood gas analyzes of PaCO2 and should be regarded as a useful indicator of trends.
10.1.4 Endtidal CO
2(ETCO
2) Monitoring
In patients with normal pulmonary function and matching of ventilation–perfusion, ETCO2 monitoring provides an accurate estimation of arterial CO2. Capnometers use infrared spectroscopy in the exhaled gas to ana- lyze the CO2 content. In pediatric intensive care, main- stream monitors are commonly used because they can be incorporated at the proximal end of an endotracheal tube. Correlation of arterial PaCO2 and ETCO2 reveals important informations in the postoperative cardiac patient. An increase in the ETCO2 may occur with increase in cardiac output, injection of bicarbonate solution, and hypoventilation. A decrease in the ETCO2 indicates hyperventilation, decrease in cardiac output, mismatching of ventilation–perfusion, or obstruction of the endotracheal tube.
Fig. 10.1 Atrial electrocardiography (ECG) showing normal sinus rhythm. Each p-wave (P) follows a QRS-complex (QRS)
Fig. 10.2 Atrial ECG showing junctional ectopic tachycardia, where the p-wave (P) follows the QRS-complex (QRS) indicating retrograde stimulation of the atria
10.1.5 Blood Pressure Monitoring
10.1.5.1 NonInvasive Blood Pressure Monitoring
Oscillometry is the most commonly used means of indirect blood pressure measurement in automated devices. It is based on the principle that pulsatile blood flow through an artery creates oscillations of the arterial wall. Oscillometric devices like Dinamap® (acronym for Device for Indirect Non Invasive Mean Arterial Pressure) utilize a blood pressure cuff to sense these oscillations that appear as tiny pulsations in cuff pressure. By measuring and analyzing at various cuff pressures, the amplitude and frequency of these pulsations. Oscillometric devices can noninvasively determine blood pressure and pulse rate. Accuracy of Dinamap® blood pressures has been validated in children, and it correlates well with direct intravascular radial artery pressures [4]. Accuracy of this technique is related to the correct size of the cuff. If the cuff is too narrow the pressure recorded will be erroneously high and if too wide may be too low. The width of the inflat- able bladder should be 40% of the midcircumference of the limb and the length should be twice the width.
Noninvasive blood pressure monitoring is inadequate in patients with low cardiac output, hypotension, arrhyth- mias with beat-to-beat changes in blood pressure, vasoconstriction, and significant edema. Due to these limitations in patients after cardiopulmonary bypass, intravascular arterial blood pressure measurement is mandatory.
10.1.5.2 Invasive Blood Pressure Monitoring Intra-arterial access is used to provide continuous moni- toring of systemic arterial blood pressure and offers the opportunity for intermittent arterial blood gas analysis.
Commonly, the radial, femoral, dorsalis pedis, and posterior tibial arteries are used. Less often, the brachial and in neonates the umbilical artery can be used. Usually, percutaneous entry is possible but occasionally a cut- down may be required to get vascular access. Local insertion site complications, such as hematoma, hemor- rhage, thrombosis, and infection can occur. Although the complication rates are low [5], the potential for devastat- ing injury exists and deserves the greatest respect when- ever placement of an arterial catheter is considered.
To avoid infection a closed continuous flush system with a disposable transducer monitoring system is used; all components should be kept sterile and the number of manipulations and entries into the pressure monitoring system should be minimized.
Invasive arterial blood pressure measurement reveals systolic, diastolic, and mean pressure values, and the shape of the pressure curve provides important additional informations (Fig. 10.3). The expected age- dependent arterial pressures are shown in Table 10.1.
10.1.6 Central Venous Access
Central venous lines offer the opportunity to measure central venous pressure, deliver potent drugs or high osmolarity nutritional solutions, and monitor venous oxygen saturation. The cardiac anatomy and physiology affects the information derived from pressure transduc- tion and blood sampling and dictates the decision, where the line is placed. In the patient with single ventricle physiology who has undergone Glenn operation, a line inserted via the upper limb or neck veins measures pulmonary artery pressure. As thrombosis or occlusion of the caval vein restricts pulmonary blood flow in these patients so venous lines should be placed else- where. With the catheters inserted from below, an accurate assessment of central venous pressure is only possible, if the tip of the catheter reaches the inferior caval-atrial junction.
Commonly the internal or external jugular, subcla- vian, or femoral veins are used. In patients with complete arterial or ventricular mixing, care must be taken with line flushing or bolus since, administration of drugs infusion of air or clot has the potential to result in systemic embolism. The most common com- plications caused by central venous lines are thrombo- sis and infection. To avoid catheter related bloodstream infections, central venous catheters should be removed after a maximum of 7–10 days [6]. When signs of infection are observed catheter removal should be con- sidered independent of the duration of placement.
10.1.7 Left Atrial Pressure Monitoring
An intraoperatively surgically-placed intracardiac line for continuous measurement of the left atrial pressure is
helpful, especially during the early postoperative period in hemodynamically instable patients, and in elucidating atrial dysrhthmias and assessing atrioventricular
synchrony during pacing. Intracardiac lines should be removed, as soon as the condition of the patient improves.
10.1.8 Pulmonary Artery Pressure Monitoring
Pulmonary artery pressure monitoring is possible via an intraoperatively placed intracardiac line or using a bedside inserted pulmonary artery Swan–Ganz catheter.
Modern pulmonary artery catheters allow direct, simul- taneous measurement of right atrial, right ventricular, pulmonary arterial, and pulmonary capillary wedge pressure, which reflects left atrial pressure. If the cathe- ter is equipped with a thermistor, cardiac output can be estimated by thermodilution technique. Pulmonary artery catheter insertion may be indicated in patients with myocarditis, cardiomyopathy, pulmonary hypertension, or respiratory distress syndrome (RDS) complicating congenital heart disease. Percutaneous insertion of such catheters is best from the left subclavian or right internal jugular veins, but can also be performed from the femoral vein. The most common complications of pulmonary artery catheterization are ventricular tachycardia, pul- monary artery rupture, infection, endocarditis, venous thrombosis, and pulmonary infarction.
10.1.9 Cardiac Output Monitoring
Adequate tissue oxygen delivery (DO2) has to be ensured in any intensive care patient. The components of DO2 include cardiac output, blood hemoglobin concentration, and the degree of oxygen saturation of the hemoglobin molecule.
DO2 = cardiac output x 1.34 x haemoglobin concentra- tion x oxygen saturation
Fig. 10.3 Typical arterial wave forms: (a), normal arterial wave form, (b) low endsystolic pressure, (c) high endsystolic pressure, (d) flattened (red line) curve due to air in the system, thrombotic formations on the catheter or arterial spasm, (e) normal curve (black line) and systolic overshoot (red line), which leads to over-estimation of arterial blood pressure
Table 10.1 Age dependent normal values of mean systemic arterial blood pressure
Age Mean arterial pressure (mmHg)
Neonate 35–45
Infant 40–45
Small child 45–55
School age 50–65
Adolescents 60–75
Cardiac output measurements never should be interpreted or treated isolated, but used in conjunction with qualitative indicators of adequacy of flow (like blood lactate, mixed venous saturation, urine output, capillary refill).
10.1.9.1 Invasive Methods for Cardiac Output Monitoring
Dilution Techniques
Invasive cardiac output determination is usually done by an indicator technique, where the principle is that blood flow can be calculated after central venous injection of an indicator by measuring the change in indicator concentration over time at a point down- stream of the injection site. Dye (e.g., Evans blue, brilliant red, indocyanine green) or cold saline may be used as indicator. Pulmonary artery thermodilution is the most common clinically used method. Automated systems reveal accurate and reliable measurements in adults [7], but have to be evaluated in children.
In transpulmonary thermodilution techniques the thermistor is percutaneously placed in a large artery (femoral or brachial artery). This technique is validated in children [8,9], and systems are commercially available for patients as small as 3 kg.
All dilution techniques are of limited value in the cardiac patient, because they are not accurate in the presence of intracardiac shunts or significant valvular regurgitation.
Fick’s Principle
Fick’s principle states that the rate of diffusion is proportional to the difference in concentration. Similarly, the volume of oxygen consumed per unit time is propor- tional to the difference in oxygen content between arterial and venous blood. The degree of proportionality depends on the volume of blood pumped per unit time, or cardiac output. Therefore, cardiac output can be calculated from the equation:
Cardiac Output = Systemic oxygen consumption / (systemic arterial O2 saturation - systemic venous O2 saturation)
This technique is limited by the difficulty of measuring oxygen consumption in the intubated and ventilated patient.
The advantage of the Fick’s principle is that it can be used when an intracardiac shunt is present.
Indirect assessment of cardiac output can be accom- plished by following mixed venous saturation, using a fiberoptic catheter for continuous measurement of venous oxygen saturation placed either in the pulmo- nary artery or right atrium. In the presence of intracardiac shunts, the saturation in the superior caval vein can be monitored to estimate cardiac output.
10.1.9.2 NonInvasive Cardiac Output Monitoring
Transesophageal Doppler Methods
This technique is minimally invasive and uses the Doppler principle and pulse contour analyzing for the estimation of blood flow in the descending aorta via an esophageal probe. Pediatric Doppler probes, which allow the use of this technique in patients of at least 3 kg of bodyweight , and a pediatric nomogram allows derivation of stroke volume and cardiac output [10,11].
Bioimpedance
In this technique voltage sensing and current transmitting electrodes are placed on the chest, which may be regarded as a conductor whose impedance is altered by changes in blood volume and velocity with each heartbeat.
In the postoperative cardiac patient this technique is limited, because it is less accurate in the presence of skin edema.
10.1.10 Near Infrared Spectroscopy (NIRS)
Probes placed on the skin of the forehead provide a continuous, noninvasive method to measure regional changes in cerebral tissue oxygenation. The regional cerebral oxygenation has been shown to correlate with SvO2 in children with cyanotic and noncyanotic heart defects during cardiac catheterization [12], cardiac surgery [13] and in the postoperative cardiac patient [14], as well as in critically ill neonates without con- genital heart defect [15]. This technique is increasingly
used in postoperative cardiac intensive care, where in special conditions it is extremely helpful like in estimating cerebral blood flow in patients after bidirectional Glenn–Shunt or in patients with extra- corporeal life support.
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The recent advances in pediatric cardiac surgery have focused on early primary repair and shown a marked improvement in outcome. Currently, the average mortality is 3.5% in children older than 1 year of age undergoing open-heart surgery and between 10 and 40% for repair undertaken in the neonatal period.
Intra-operative advances include a better understanding of the inflammatory process caused by cardiopulmo- nary bypass (CPBP) and its management. Astonishing changes have arisen since Gibbon’s first CPBP in 1953.
However, postoperative problems induced by CPBP, which include vital organ damage, and neurologic dys- function are a current challenge in the management of pediatric patients undergoing cardiac surgery.
The deleterious effects of CPBP are related to the activation of cellular and humoral inflammatory path- ways in response to the exposure of the circulating blood volume to the cardiopulmonary bypass circuit.