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Managing Hemodynamics During High Frequency Oscillatory Ventilation

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Managing Hemodynamics During High Frequency Oscillatory Ventilation Managing hemodynamics during high frequency oscillatory ventilation Introduction There is an inextricable link between hemodynamic characteristics and response to ventilation settings. Therefore, a careful evaluation of cardiovascular function in critically ill patients with pulmonary disease is an important aspect of their ventilatory management. It is also then important to understand and utilize those tools and measurements that will enhance our understanding of these inter-relationships. Matching ventilation/perfusion (V/Q) Clinical cardiovascular monitoring heart rate The goal of breathing is to match alveolar ventilation Monitoring the heart rate is an essential element for with pulmonary perfusion to obtain an ideal V/Q evaluating the cardiovascular status. The pediatric heart balance as it is the major determinant of PaO2 and rate is influenced by age and disease process. PaCO2. While our ventilation management determines the “V” component, there are three major determinants of pulmonary “Q”. They are myocardial function, the pulmonary blood volume, and pulmonary vascular 0 resistance (PVR). 2 cmH There are many factors which play a role in PVR. The ( lung is not a passive participant in PVR but plays an active role affecting PVR by lung volume (Figure 1). The lung parenchyma not only tethers the airways, but tethers the extra alveolar vessels. At low lung Vascular resistance Vascular cc/min) volume, there is a reduction in radial traction reducing the cross-sectional area of the extra alveolar vascular bed, increasing PVR in these vessels. As lung volume increases, Lung volume (cc) PVR falls to its minimal level at an optimum lung volume. However, if mean airway pressure (Paw) continues to Figure 1. increase lung volume resulting in over distention of the Neonates have the added influence of gestational alveoli, this results in compression of the intra-alveolar age and weight. Additional influences on heart rate vessels which lack perivascular support resulting in a rise include central nervous system dysfunction, autonomic of PVR. Because of this direct action by the ventilator on dysfunction, stress, drugs, sepsis, anemia, hyperthermia, a major component of hemodynamic performance, it is and endocrine dysfunction (e.g., thyroid). Since cardiac therefore important to track hemodynamic responses while changing Paw. output equals stroke volume times heart rate, monitoring Central venous pressure changes in heart rate can reflect an acute change in the Central venous pressure (CVP) is the pressure obtained patient’s condition or trend impending problems. from a catheter inserted into the superior vena cava or Patients in respiratory failure frequently have an increase in the right atrium. It is crucial when using this pressure to heart rate to improve cardiac output and to compensate be certain of the catheter’s placement (Figure 2). In the for decreased oxygenation. Although, increasing heart neonate, slight displacement of the catheter from the rate is an effective mechanism for increasing the cardiac right atrium can result in fictitious measurements. output, there is a point when increasing the heart rate These errors can result from placing the catheter across will cause the cardiac output to decrease as the decreased the foramen ovale into the left atrium, or by placing it time for ventricular filling and coronary blood flow effect too far into the right atrium with partial obstruction from cardiac efficiency. the intra-atrial septum. If the catheter is not inserted Bradycardia can be associated with episodes of apnea far enough, the pressure may reflect hepatic pressures. and hypoxia. Other physiologic dysfunctions associated Both an AP and lateral chest x-ray for placement is very with a decrease in heart rate include cardiac ischemia, helpful. In our nursery, we document UVC placement mechanical defects, hypovolemia, and increased vascular with ultrasonography. resistance. On rare occasions, it has been seen secondary About 70 percent of the time, the correct placement to a reflex response to high PEEP or Paw. is approximately T9 on AP chest x-ray. With normal cardiac status, CVP reflects right atrium pressure, Blood pressure which correlates with right ventricular pressure. CVP Arterial blood pressure is the dynamic measurement of is used to evaluate pre-load to the right ventricle. It the force of blood against the wall of the artery reflected allows us to evaluate hemodynamic states such as during systole (tension during LV contraction), and intravascular volume, intra-thoracic pressure, and diastole (relaxation pressure during ventricular diastole). response to volume replacement. Pulse pressure, the difference between the systolic and The normal range for CVP in the neonate and pediatric diastolic, reflects the ventricular ejection amplitude. patient varies, ranging from 0 to 8 mmHg. However, in Normal values vary in the pediatric patient by age, sex, our experience, we cannot usually use high frequency and method of monitoring. Neonates have the additional oscillatory ventilation (HFOV) effectively if the CVPs in effects of gestational age, birth weight, and postnatal the 0 to 3 mmHg range, but may require CVPs of 5 to 8 age to consider. mmHg to allow increasing Paw above the critical opening Hypotension may result from true low blood volume, pressure of the lung. low effective blood volume secondary to peripheral vasodilatation, or left ventricular dysfunction. Hypotension induced reduction in oxygenation delivery may result in tissue hypoxia and elevation of lactic acid levels. Hypertension, or an elevated blood pressure, can be the result of increased peripheral vascular resistance or fluid overload. Pulse pressure can be increased with the left-to-right shunt from a patent ductus arteriosus (PDA) or decreased by shock, hypovolemia, or heart failure. In the pediatric population, considerable information can be obtained from the arterial pressure tracing. The shape of the tracing, pulse pressure, and location of the dicrotic notch, all Figure 2. give pertinent hemodynamic information about the patient. CVPs decrease in hypovolemia or with any mechanism and therefore, the arterial oxygen content (CaO2) or the total causing delayed venous return to the right heart. amount of oxygen carried is dramatically affected by changes Increasing levels of Paw without blood volume expansion in hemoglobin. can also cause decreases in venous return. Increases in Normal CaO2 is approximately 20 mL O2/dl blood (vol%). CVPs are seen in right heart failure, hypervolemia, or To emphasize the difference between PaO2 and CaO2 myocardial dysfunction. (CaO2 = O2 carried by Hb + dissolved O2), consider the aspects While the validity of CVP in the critically ill or mechanically of varying Hb concentration on the following patients: ventilated patient has been questioned, in our experience, if the CVP is placed in the appropriate position (entrance Patient 1 of right atrium), it is very valuable in evaluating right The CaO for a child with a Hb of 15 gm, PaO of 100 torr ventricular preload. 2 2 and O2 Sat of 97%: = (15 x 1.34 x 0.97) + (0.0031 x 100) Pulmonary artery occlusion or “wedge” pressure = 19.50 ml O2/dl + 0.31 ml O2/dl = 19.81 ml O /dl Pulmonary artery occlusion or “wedge” pressure is 2 used for measuring the preload for the left ventricle. The Swan-Ganz catheter introduction by right heart Patient 2 catheterization into the pulmonary artery (PA) is the The CaO for a child with a Hb of 8 gm, PaO of 100 torr and standard method in pediatrics and adults for evaluating 2 2 O2 Sat of 97%: pulmonary vascular hemodynamics. The indications for CaO2 = O2 carried by Hb + dissolved O2 a PA catheter include discriminating cardiac and = (8 x 1.34 x 0.97) + (0.0031 x 100) = 10.40 ml O /dl + 0.31 ml O /dl noncardiac causes of pulmonary infiltrates, titration of 2 2 = 10.71 ml O /dl fluid therapy, evaluation of pharmacologic interventions 2 in pulmonary hypertension, and determining the etiology of hypotension. These two examples demonstrate the dramatic fall in blood To obtain a pulmonary capillary wedge pressure (PCWP), oxygen content that occurs with the fall in Hb. Although the distal lumen of the catheter is used. Balloon inflation both patients have the same PaO2 and O2 Sat, the second isolates the distal tip of the catheter from upstream patient must almost double cardiac output to maintain the pulmonary arterial pressure creating a static, nonflowing same oxygen delivery as the first patient. column of blood distal to the balloon. Without flow Oxygen extraction there is no pressure drop along the column of fluid. The pressure at the catheter tip measures pressure in The Arterial-Venous difference (A-VDO2) is the amount of the pulmonary vein. Because there is normally minimal O2 extracted by the tissues and is commonly used in the resistance in the pulmonary veins, the pressure measured ICU as an indirect measurement of cardiac output. The approximates the pressure in the left atrium. A-VDO2 is derived from oxygen content calculations from arterial and mixed venous blood drawn simultaneously. Oxygen delivery The normal CaO2 is approximately 20 vol% while the Oxygen tension (PaO ) is the partial pressure of oxygen 2 CvO2 is 15 vol%. Therefore only 5 mL O2 in each 100 mL is in the arterial blood. PaO2 is affected by FiO2, alveolar extracted for utilization by the tissues. An increase in the ventilation, diffusion defects, V/Q mismatch, and shunt A-VDO2 indicates that either tissue extraction has increased (intrapulmonary or intracardiac). Oxygen is carried by or cardiac output has decreased. Conversely, a decrease in the blood in two ways. Although we frequently focus the A-VDO2 usually indicates an increase in cardiac output. on PaO2, this dissolved oxygen (0.0031 mL/torr) is only Ideally, a true venous sample should be drawn from the a small fraction (< 5 percent) of O2 carried in the blood. pulmonary artery. If drawn from the right atrium, there is The remaining and most significant portion is carried an increased probability for poor venous mixing and loss bound with the hemoglobin (Hb) as determined by the of data reliability.
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