The Intensive Care Manual - part 2 ppt

left ventricle and, therefore, implies contractility information, and the rate of downstroke in the arterial waveform allows inferences regarding systemic vascular resistance. The blood pressure measured by an intra-arterial cannula depends to some extent on the properties of the vessel cannulated. The arterial pressure waveform is susceptible to artifacts, such as catheter whip and damping, which influence the validity of the pressure data. Catheter whip, or systolic amplification, occurs when arterial pressure waves are reflected back to the catheter tip from points of constriction, branching, or noncompliant arterial walls. Reflection of pressure waves off arterial walls can distort pressure waveforms, causing overreading of systolic pressure. Peripheral catheters are more susceptible to systolic amplification because the velocity of blood flow increases gradually as the blood pulse moves peripherally, since the walls of the large arteries are more compliant and absorb energy. The systolic pressure increases and the systolic wave narrows pro- gressively as the arterial pressure wave is measured more peripherally, and systolic amplification of the waveform increases as the compliance of the arteries decreases peripherally. Spontaneous oscillation is a characteristic of fluid-filled transducer systems. The resonant frequency of a transducer system is the inherent oscillation frequency produced by a pressure signal introduced into the system. Mechanical transducers absorb some of the energy of the systems they monitor and release of some of this energy. This causes a vibration to occur at the natural resonance frequency specific to the system. Damping is the tendency for the vibration, or oscillation, to stop and is a function of compliance, air, tube length, tube coiling, connections in the tubing, and stopcocks. Air in the form of bubbles in the flush solution is very compressible and absorbs a great deal of energy, resulting in significant damping. Excessive damping results in an underestimation of the systolic blood pressure and an overestimation of the diastolic blood pressure, whereas the opposite is true for underdamped systems. Mean pressure is only minimally affected by damping. The resonant frequency can be quantitatively determined using the “flush formula,” 6 in which the frequency (in hertz) equals the paper speed (in millimeters per second) divided by the distance (in millimeters) between oscillation waves. The more closely matched a pressure signal is to the resonant frequency of the system, the greater the likeli- hood of signal amplification, which defines the underdamped system. An underin- flated pressure bag causes an artifactual drop in the blood pressure reading. A transducer that has fallen to the floor causes the displayed blood pressure to be greatly elevated. Pulse Oximetry The adjunctive use of pulse oximetry in CCUs has added an additional level of monitoring, which allows the saturation of arterial blood to be measured directly using the law of Beer-Lambert and the principle of reflectance spectrophotometry. 2 / Intravascular Access and Hemodynamic Monitoring 27 ch02.qxd 11/7/01 4:08 PM Page 27 The mandated use of pulse oximetry during anesthesia has greatly improved anesthesia safety; ideally therefore, pulse oximeters should be used on all critically ill patients. However, pulse oximetry alone is not considered an appropriate early warning of apnea, because significant desaturation may not occur for 15 minutes or more in patients with a normal functional residual capacity who are breathing pure oxygen. Furthermore, the pulse oximeter does not indicate the adequacy of ventilation. Clinically detectable cyanosis does not occur until the oxygen saturation of arterial blood reaches 80% or less. Oxyhemoglobin reflects more red light than does reduced hemoglobin, whereas both hemoglobins reflect infrared light identically. Adult blood usually contains four types of hemoglobin: oxyhemoglobin (HbO 2 ), methemoglobin (MetHb), reduced hemoglobin (Hb), and carboxyhemoglobin (HbCO 2 ). How- ever, except in pathologic conditions, methemoglobin and carboxyhemoglobin occur only in very low concentrations. Pulse oximeters emit light only at only two wavelengths, 660 nm (red light) and 940 nm (near-infrared light). Reduced hemoglobin absorbs approximately 10 times more light, at a wavelength of 660 nm, than does oxyhemoglobin; at a wavelength of 940 nm, the absorption coefficient of oxyhemoglobin is greater than that of reduced hemoglobin. Signal processing based on a calibration curve determines the saturation of the arterial blood as it pulses past the probe. The pulse oximeter has substantially affected the use of ABG analysis for the determi- nation of oxygenation saturation alone. The Sa O 2 displayed by the pulse oximeter is correctly referred to as the Sp O 2 , to differentiate it from the Sa O 2 obtained by ABG analysis and is represented by the following equation: Where Sp O 2 is Saturation of Hb with 0 2 measured by pulse oximetry HbO 2 is oxyhemoglobin concentration in blood HbO 2 + Hb is total hemoglobin concentratin in blood Using the oxyhemoglobin dissociation curve, an Sp O 2 of 90% corresponds to a PaO 2 of approximately 60 mm Hg and an SpO 2 of 75%, to a PaO 2 of 40 mm Hg. The SpO 2 measured by pulse oximetry can be expected to be within 2% of the value for hemoglobin saturation of blood measured by a co-oximeter. Anemia does not interfere with the accuracy of the SpO 2 as long as the hematocrit remains above 15%. The heart rate on the oximeter must correlate with the true heart rate for the SpO 2 to be considered accurate. The SpO 2 is falsely elevated in the presence of carboxyhemoglobinemia and the SpO 2 falsely reads 85% when significant methemoglobinemia is present. Sp Hb Hb Hb O O O 2 2 2 100= + × 28 The Intensive Care Manual ch02.qxd 11/7/01 4:08 PM Page 28 Methemoglobinemia may occur more frequently in septic critically ill patients than previously recognized, since methemoglobin is generated in the presence of nitrites, which are a by-product of the nitric oxide pathway. In addition, since the pulse oximeter requires pulsatile flow, placement of the probe on the index finger or thumb of a patient with a radial arterial cannula serves as an early warning of ischemia in the radial artery distribution. The accuracy of the pulse oximeter is greatly reduced when the arterial oxygen saturation falls below 75%. CENTRAL VENOUS CATHETERIZATION The central veins are the major veins that drain directly into the right heart. Indi- cations for central venous cannulation include a need for both access and monitoring (Table 2–2). The approaches to the central circulation can be classified on the basis of whether the inferior or superior vena cava is used. Venous air embolism is a possibility whenever the venous system is opened to atmospheric pressure above the level of the right atrium, or phlebostatic axis. Inadvertent en- trainment of air through a 14-gauge catheter can occur at a rate of 90 mL/sec and produce a fatal air embolism in less than 1 second. Air embolism is most likely to occur during hypovolemia and spontaneous respiration when the hydrostatic pressure in the right side of the heart falls significantly below atmospheric pressure during early inspiration. The probability of air embolism is diminished, but not eliminated, by placing the patient in Trendelenburg’s (head down) position for superior vena cava (SVC) cannulation and reverse Trendelenburg’s (head up) 2 / Intravascular Access and Hemodynamic Monitoring 29 TABLE 2–2 Indications for Central Venous Cannulation Access for rapid infusion of fluid Long-term access required Monitoring of cardiac function • Preload • Cardiac output • Mixed venous saturation Drug administration • Vasoactive medications • Highly osmotic or irritant drugs • Hyperalimentation • Chemotherapy • Long-term antibiotics Long-term inotropic medications (outpatient inotropic therapy) Dialysis access Temporary transvenous pacing wire placement Aspiration of air emboli Jugular venous bulb monitoring ch02.qxd 11/7/01 4:08 PM Page 29 position for femoral inferior vena cava (IVC) cannulation. Venous air embolism is best treated by aspiration of the air from the heart, but immediate temporizing measures include placing the patient in the left lateral decubitus Trendelenburg’s position, increasing preload cautiously, and using aggressive inotropic support. Embolization of catheter fragments or the guide wire most often indicate serious deviation from proper technique. Difficulty in obtaining successful venipuncture is most often the result of poor anatomic landmarks, previous phlebitis or thrombosis, or distortion of anatomy by surgery or trauma. The complications of central venous cannulation are many, including those based in the patient’s anatomic variability, inadvertent complications despite maintaining the standard of care, a breach in technique, and operator inexperience. Complications of central vein cannulation are listed in Table 2–3. Approaches to the Central Venous Circulation INFERIOR VENA CAVAL ACCESS The IVC is accessed via the femoral vein, which lies medial to the palpable femoral artery and below the inguinal ligament in the femoral triangle (Figure 2–1). Radiographic confirmation of subsequent catheter placement is not necessary. The primary advantage to the femoral venous access site is the relatively low rate of insertion-related complications, mak- ing it a good choice for emergent high-volume infusion. However, higher rates of catheter infection have been reported at this site, especially when the catheter is being used for total parenteral nutrition, and higher rates of deep venous thrombosis (DVT), especially in trauma patients, may outweigh the potential advantages. During cardiopulmonary resuscitation, thoracic compressions may increase inferior vena caval pressure, prolonging the circulation time of drugs to the heart. The femoral vein should not be cannulated for volume infusion in trauma patients if abdominal or pelvic venous injury or hepatic trauma is suspected or if surgical clamping of the IVC is anticipated. In the presence of known or suspected DVT, the femoral approach should be used with great caution, since instrumentation of the vein may dislodge thrombi proximally. The occurrence of DVT after prolonged instrumentation of the femoral venous system, especially in patients who are immobile as a result of trauma or who are in hypercoagulable states, is another consideration before planning femoral vein access. SUPERIOR VENA CAVAL ACCESS The SVC is accessed directly via the subclavian, internal jugular, or external jugular veins (Figure 2–2) and indirectly via the antecubital veins. The proximity of these veins to major arteries in the neck and thorax and the possibility of pneumothorax reflect the more common complications of these approaches. Since the catheters and guide wires are of sufficient length to reach the right atrium and ventricle, arrhythmias caused by mechanical stimulation of the heart are common. Transient ectopy is very common and need not be treated. How- ever, the ability to immediately recognize and treat ventricular tachyarrhythmias 30 The Intensive Care Manual ch02.qxd 11/7/01 4:08 PM Page 30 2 / Intravascular Access and Hemodynamic Monitoring 31 TABLE 2–3 Complications of Central Venous Cannulation Hematoma Microshock Pneumothorax Hemothorax Chylothorax: Left internal jugular (LIJ) approach Arterial puncture from cannulation Subcutaneous infiltration: proximal port Data misinterpretation Perforation • Superior vena cava • Right atrium • Right heart (tamponade) Arrhythmias • Bundle branch block • Ectopy • Ventricular tachycardia Nerve injury • Brachial plexus • Stellate ganglion • Phrenic nerve • Recurrent laryngeal Emboli • Air • Clot • Catheter fragment • Guide wire • Systemic embolization 1. patent foramen ovale 2. arterial cannulation Thrombosis • Vein • Aseptic thrombotic endocarditis • Catheter-related infections 1. Puncture site 2. Catheter: colonization or infection 3. Suppurative thrombophlebitis 4. Endocarditis Pulmonary artery catheter • Pulmonary artery rupture • Catheter knotting • Valvular injury of the external jugular valve or tricuspid valve • Pulmonary infarction • Chordae tendineae rupture ch02.qxd 11/7/01 4:08 PM Page 31 is necessary; therefore, continuous monitoring of the electrocardiogram (ECG) during central venous access is highly recommended. The tip of central venous catheters should lie in the SVC and not in the right atrium, where the catheter tip can perforate or erode into the pericardium, or in the right ventricle, where stimulation of conduction pathways can lead to paroxysmal arrhythmias and conduction block. Perforation of the SVC and right atrium have resulted in mortality rates that approach 70% and 100%, respectively. SUBCLAVIAN VEIN CANNULATION The subclavian vein is the preferred site for central venous cannulation (Figure 2–2), since it is a large vein with relatively 32 The Intensive Care Manual FIGURE 2–1 Anatomy of the femoral triangle. The femoral vein is the most medial neurovas- cular structure within the femoral triangle. The palpable landmark is the femoral artery. The base of the inverted triangle is the inguinal ligament; the vastus intermedius (laterally) and the adductor longus (medially) are the muscular boundaries. The femoral nerve lies laterally and must be avoided. The arrow represents the direction of flow in the femoral vein. ch02.qxd 11/7/01 4:08 PM Page 32 constant anatomy and is the vein most likely to be patent, even during profound hypovolemia since the vein is tethered to the surrounding dense connective tissue. The subclavian vein crosses under the clavicle, medial to the midclavicular line. The vein is most often entered at the junction of the outer one-third and medial two-thirds of the clavicle, with the needle parallel to the clavicle and di- rected at the sternal notch. The subclavian vein is the direct continuation of the axillary vein as it passes over the first rib and under the clavicle. The veins run anterior to the anterior scalene muscle, which separates the vein from the subclavian artery and pleura. The subclavian vein and internal jugular veins join at the thoracic inlet to form the brachiocephalic vein, which drains directly into the SVC. The left side is somewhat preferable for right heart catheterization because the angulation from the right subclavian vein into the right side of the heart is more acute. In experienced hands, the incidence of pneumothorax is no greater with the subclavian approach than it is with the internal jugular approach. INTERNAL JUGULAR VEIN CANNULATION The internal jugular vein passes under the clavicular (lateral) head of the sternocleidomastoid muscle as the most lateral structure in the carotid sheath. Since the internal jugular vein lies posterior to the muscle belly, it can be accessed from either a medial (anterior) approach or a lateral (posterior) approach (Figure 2–2). The use of portable ultrasonography to guide internal jugular vein cannulation is becoming increas- 2 / Intravascular Access and Hemodynamic Monitoring 33 FIGURE 2–2 Anatomical landmarks for superior vena cava access. The internal jugular vein lies under the lateral head (clavicular) of the sternocleidomastoid (SCM) muscle and can be approached anteriorly (a) or posteriorly (b). The vulnerable structures include the carotid artery, brachial plexus cords, the dome of the pleura, and on the left side, the thoracic duct. A superior approach to the subclavian artery is possible in the base of the anterior cervical triangle (c). The subclavian vein passes under the medial aspect of the clavicle and can be accessed there (d); see text. ch02.qxd 11/7/01 4:08 PM Page 33 ingly common and has obvious benefits in those patients in whom the palpation of anatomic landmarks is not possible. The risk of inadvertent carotid artery puncture is always present and is slightly higher with the anterior approach and during periods of hypotension. Carotid puncture with a small-gauge needle carries a low risk of morbidity; hematoma and plaque embolization are relatively rare. Cannulation of the internal carotid artery with a large-bore catheter may provoke serious hemorrhage and may re- quire emergent vascular surgery consultation. A foreign body in the carotid artery carries a high risk of embolic (e.g., air, clot) cerebrovascular complication: definitive therapy must not be delayed. The left internal jugular approach carries a risk of injury to the thoracic duct and resulting chylothorax. The indirect disad- vantages of jugular venous cannulation include limited neck mobility and patient discomfort, proximity to oral and tracheostomal secretions, and overgrowth of the insertion site by facial hair in males, predisposing these catheters to contami- nation and infection. EXTERNAL JUGULAR VEIN CANNULATION The external jugular vein is an alternative jugular approach to the central venous system. The advantages of external jugular cannulation are a low risk of pneumothorax, minimal risk of carotid artery puncture, and easy control of bleeding. However, these advantages are outweighed by the difficulty in accessing the highly mobile and collapsible vein, in anchoring catheters, in passing the guide wire and catheter through a venous valve (which may be made incompetent after catheterization), and the risk of venous injury at the acutely angled junction of the internal and external jugular veins. The external jugular approach is not recommended for routine critical care central venous access. PERIPHERALLY INSERTED CENTRAL CATHETERS A reasonable alternative to direct access to major veins is the use of a peripherally inserted central catheter (PICC) or long-arm central catheter; however, these are more applicable for patients who need long-term care than patients in the CCU. These catheters are inserted into the brachial or cephalic veins in the antecubital area and then threaded into the SVC, where the proper position is confirmed either radio- graphically or electrocardiographically. Anesthesiologists routinely place special long-arm central catheters, which have multiple aspiration ports, in patients for neurosurgical procedures to facilitate aspiration of air embolism, to monitor central venous pressure (CVP), and to administer some medications. The PICC catheter is used mainly for long-term antibiotic or chemotherapy administration. Central Venous Pressure Monitoring CVP can be transduced at any point in the central venous system, including the IVC; however, the reliability and validity of the IVC is affected by intra- abdominal pathology. The phlebostatic axis is at the level of the tricuspid valve 34 The Intensive Care Manual ch02.qxd 11/7/01 4:08 PM Page 34 or right atrium in a supine patient; this is where intravascular pressure reaches zero and is independent of body habitus. Although changes in posture can be expected to affect the reference pressure at the phlebostatic axis by less than 1 mm Hg, the CVP is a less accurate indicator of filling pressures when it is measured in the lateral or upright position, because of venous pooling. The CVP is most often used as an approximation of preload and reflects a balance between venous return and right-sided cardiac output. Under normal conditions, the right side of the heart is composed of a thin wall of myocardium and is more compliant than the more muscular left side of the heart. Since CVP measures intravascular pressure and not transmural pressure, which is the actual determinant of ventricular preload, its validity as an index of preload is influenced by pulmonary variables, such as intrathoracic pressure, and by cardiac variables, such as cardiac compliance. Central filling pressures, such as the CVP, pulmonary artery wedge pressure (PAWP), and pulmonary capillary wedge pressure (PCWP), are measured at end-expiration, when the relative intrathoracic pressure is zero (i.e., it equals atmospheric pressure), and therefore intravascular pressure equals transmural pressure. High levels of positive pressure ventilation, which affect the CVP, should never be discontinued to determine a “more accurate” CVP. In instances where the CVP is thought to be falsely elevated by intrathoracic pressure, an alternative form of preload assessment should be considered or esophageal manometry should be used to estimate transthoracic pressure. The transthoracic pressure can then be subtracted from the CVP to provide a better estimate of preload. Graphic depiction of the CVP (also the PCWP and left atrial pressure) waveforms consists of three positive wave deflections (a, c, and v) and two descents (x and y) (Figure 2–3). The a wave is the increase in venous pressure that is generated by atrial contraction. The c wave occurs when the atrioventricular valve (tricuspid or mitral) is displaced into the atrium during isovolumetric ventricular contraction. The v wave reflects the increase in atrial pressure that occurs as venous return begins to fill the atrium during isovolumetric relaxation, while the atrioventricular valves are still closed. The x descent corresponds to ventricular ejection, as the emptying ventricle draws down on the floor of the atrium and decreases the CVP. The y descent occurs as the atrioventricular valve opens and blood enters the ventricle during ventricular diastole. The importance of these waveforms lies in their ability to reflect on patho- physiologic processes. Absence of the a wave occurs in atrial fibrillation, in which case the x descent may also be absent. Amplified, or “cannon,” a waves occur in the presence of stenosis of the atrioventricular (mitral) wave. Both the x and y descents are exaggerated in the presence of constrictive pericarditis, whereas cardiac tamponade magnifies the x descent and abolishes the y descent. In the presence of atrioventricular valve incompetency, free transduction of ventricular pressure during ventricular contraction generates large “cannon” V waves that are pathognomonic for regurgitant flow, especially mitral regurgita- 2 / Intravascular Access and Hemodynamic Monitoring 35 ch02.qxd 11/7/01 4:08 PM Page 35 tion. In the case of the CVP, pulmonary hypertension increases right ventricular afterload, decreases right ventricular compliance, and accentuates the v waveform depicted on the monitor. PULMONARY ARTERY CATHETER The pulmonary artery catheter (PAC), or Swan-Ganz catheter, provides a more accurate measure of left ventricular preload; however, it also is subject to operator bias and misinterpretation. 7,8 The pulmonary artery catheter was originally introduced in 1970 by Swan and Ganz, whose names are still attached to the catheter today. The importance of basing clinical interventions on judicious in- terpretation of the data obtained from the PAC cannot be overemphasized. 9,10 Although the discipline of critical care medicine is to a large extent rooted in use of the PAC, recent suggestions that the use of the PAC in clinical medicine is as- sociated with increased morbidity and mortality may reflect, in large part, deci- sions made on the basis of inaccurate data alone, without sufficient consideration of the underlying physiologic principles. 36 The Intensive Care Manual FIGURE 2–3 The CVP wave form as it relates to the electrocardiogram (see text). ch02.qxd 11/7/01 4:08 PM Page 36 [...]... modulation of the oxygen extraction ratio (O2ER) The O2ER is the ratio of VO2 ˙ O2 and represents the fraction of delivered oxygen that is taken up into the to D tissues, usually in the range of 20 % to 30% O2 ER = ˙ VO2 ˙ DO2 × 100 Maximal oxygen extraction occurs when the O2ER approximates 50% to 60% At this point, known as the point of critical oxygen delivery, the VO2 becomes dependent on DO2 (supply-dependent... pulmonary artery balloon in the wedge position should be accompanied by a return to the PA waveform, as blood flow resumes past the catheter tip ch 02. qxd 11/7/01 38 4:08 PM Page 38 The Intensive Care Manual reaches the distal pulmonary artery, the diastolic pressure characteristically rises Further advancement of the catheter causes the waveform to flatten and signifies that the “wedge” position has... by a rapid response thermistor capable of determining end–diastolic volume (EDV) and subsequent volume changes (ESV) as the right ventricle empties The ejection fraction (EF) is the EDV-ESV, which is the stroke volume SV, divided by EDV CO = ˙ VO2 CaO2 − CVO2 Where CO is cardiac output VO2 is whole body O2 consumption CaO2 is content of O2 in arterial blood CvO2 is content of O2 in venous blood ˙ This... the care of critical care patients MUCOSAL TONOMETRY The mucosal tonometer is a potentially useful method for the assessment of tissue-specific perfusion Tonometer technology has been applied to the esophageal and gastric mucosa, the mucosa of the rectum, and the sublingual oral mucosa Although the monitoring site varies, the technology and the physiologic principles are the same The central role of the. .. Kumar P, et al The fast flush test measures the dynamic response of the entire blood pressure monitoring system Anesthesiology 19 92; 77: 121 5 7 Iberti TJ, Fischer EP, Leibowitz AB, et al A multicenter study of physicians’ knowledge of the pulmonary artery catheter JAMA 1990 ;26 4 :29 28 8 Connors AF, Speroff T, Dawson NV, et al The effectiveness of right heart catheterization in the initial care of critically... PHARMACOLOGIC INTERVENTION The PAC is the gold standard for the clinical assessment of the physiologic response of the ch 02. qxd Variable CI (L /min/m 2 ) = Normal Range CO (L /min) 2. 8–4 .2 L/min/m2 2 BSA (m ) Stroke index (SI) SV (mL per beat) = CO (L /min) × 1000 (mL / L) 60–90 mL per beat HR (beats per min) SI (mL per beat per m 2 ) = SV (mL per beat) 30–65 mL per beat per m2 BSA (m 2 ) Systemic vascular... toward the apex of the lung In order to best reflect cardiac function, the tip of the PAC should lie in zone 3 or 4 when the catheter tip is in zone 3 or 4, since only in these zones is there a continual column of uninterrupted blood between the catheter tip and the left atrium High mean airway pressure and hypovolemia are the most common causes of relatively decreased zones 3 and 4 of the lung The risks... pyruvate dehydro- ˙ FIGURE 2 8 The theoretical relationship between oxygen delivery (D O2) and oxygen uptake ˙ O2): Gradual decrease in D O2 has little or no detectable effect on V O2 since compensation ˙ ˙ (V ˙ occurs by icnreased peripheral extraction Further decrease in D O2 to the inflection point ˙ causes the V O2 to become pathologically flow dependent and a state of dysoxia occurs in which there is... cardiac variations, the effect of respirations can be eliminated Stroke volume (SV) is determined mathematically, based on the specific resistivity of blood, thoracic length, basal thoracic impedance, ventricular ejection time, and the maximum rate of impedance change dur- ch 02. qxd 11/7/01 52 4:08 PM Page 52 The Intensive Care Manual ing systolic upstroke The SV correlates with the impedance change... in the presence of clinically determined pulmonary edema is a major criterion for the diagnosis of ARDS CARDIAC OUTPUT MEASURED BY THERMODILUTION In addition to intracardiac pressure measurements, the PAC also enables the measurement of cardiac output by thermodilution. 12 A thermistor at the tip of the PAC continually measures the temperature of the blood in the pulmonary artery as it flows past the . (Figure 2 2) , since it is a large vein with relatively 32 The Intensive Care Manual FIGURE 2 1 Anatomy of the femoral triangle. The femoral vein is the most medial neurovas- cular structure within the. predates the PAC but nonetheless requires right heart catheterization. The Fick equation is: 42 The Intensive Care Manual ch 02. qxd 11/7/01 4:08 PM Page 42 Where CO is cardiac output VO 2 is whole. of the outer one-third and medial two-thirds of the clavicle, with the needle parallel to the clavicle and di- rected at the sternal notch. The subclavian vein is the direct continuation of the axillary

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