Refer to: Lewis FR: Current concepts in cardiovascular monitoring (Trauma Rounds). West J Med 122:339-344, Apr
Trauma Rounds Chief Discussant FRANK R. LEWIS, MD Editors DONALD D. TRUNKEY, MD F. WILLIAM BLAISDELL, MD
1975
Current Concepts in Cardiovascular Monitoring
This is one of a series of Conferences on Trauma at San Francisco General Hospital
CAROL RAVIOLA, MD: * The patient we would like to discuss today is a 63-year-old woman who was admitted 12 hours ago, having been injured when her runaway automobile crushed her against the door of her garage. She was admitted within thirty minutes of injury and was in shock with poor peripheral skin perfusion, and a blood pressure of 80/60 mm of mercury. The injuries appeared to be limited to the lower abdomen and pelvis. Her abdomen was diffusely tender and the pelvis was grossly unstable. Findings on x-ray studies included a fracture of the right iliac crest and superior and inferior pubic rami and a right sacroilic separation. On laparotomy, one hour after admission, a large retroperitoneal hematoma was noted, which filled the pelvis and extended upward on the right nearly to the diaphragm. This was not entered. After thorough exploration of the abdominal cavity showed no additional injuries, the abdomen was closed. The patient received 5 units of blood during the induction of anesthesia and the surgical procedure. Immediately after operation, peripheral perfusion was poor. The patient's toes appeared blue and mottled, the blood pressure was 140/90 mm of mercury and the pulse rate was 100 beats per minute. Central venous pressure was 10 cm of *Chief Resident, Surgery Department, San Francisco General Hospital. Sponsored in part by NIH Grant GM18470 and the Northern California Trauma Committee, American College of Surgeons. Reprint requests to: D. D. Trunkey, MD, Department of Surgery, San Francisco General Hospital, San Francisco, CA 94110.
water. One unit of blood was given over the first two hours after operation and the central venous pressure rose to 25 cm of water. Urine output which was 30 ml the first hour, fell to 20 ml the second hour. A Swan-Ganz catheter was threaded into the pulmonary artery after 11/2 hours of manipulation. The wedge pressure was 7 to 8 cm of water and additional volume was given with an increase in urine output and stability of vital signs. The patient's condition is still critical at this time and we would like to ask Dr. Lewis if he would discuss the indications for pulmonary artery catheterization and discuss its use in monitoring the critically injured patient. FRANK LEWIS, MD: t In patients admitted with serious injuries, the problem of monitoring can be divided into cardiovascular and respiratory components. Today, as requested by Dr. Raviola, I will limit my discussion to the newer sophisticated cardiovascular monitoring techniques used in the Intensive Care Unit at San Francisco General
Hospital. Two questions arise when patients are in clinical shock. One is, what is the patient's volume status relative to the capacity of the vascular tree? A second is, how well is the heart pumping the load of fluid which is being delivered to it? All of the forms of shock can be defined in terms of alterations in volume status or cardiac action: for tAssistant Professor of Surgery, University of California, School of Medicine, San Francisco General Hospital.
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CARDIOVASCULAR MONITORING
example, hemorrhagic shock is due to the lack of adequate circulating volume with a relatively normal intravascular capacity. Septic shock results from an expanded intravascular capacity with a blood volume that is inadequate to fill it. Cardiogenic shock results from ineffective pumping action even though the vascular capacity and volume are matched. The problem of assessment of cardiovascular function can be divided into two different periods. The first consists of the initial assessment of cardiovascular function in the emergency rooms and during surgical procedures, and the second consists of the assessment of complicated problems such as the patient today presented in the postoperative period. The initial period of assessment and resuscitation of the critically injured patient has been covered in a previous "Trauma Rounds" and I will confine my remarks to the venous pressure and more sophisticated measurements. In the initial evaluation in the emergency room, the status of the neck veins will give a gross indication of venous pressure. This is often all that is needed in making immediate decisions about management (for example, a patient who is hypotensive because he "bled out" will have flat neck veins while the one who is hypotensive from a cardiac problem such as tamponade will have bulging veins). Quantitative assessment of venouts pressure is obtained using central venous pressure (cvp). A central venous pressure line is inserted in the emergency room through an antecubital vein. A long percutaneously placed commercial catheter can be used or, if veins are difficult to find, a cutdown on an antecubital vein can be carried out. A catheter is threaded upward into the great veins of the chest. If a cutdown is done, a long largebore catheter such as a pediatric feeding tube should be used. This will permit rapid transfusion as well as the measurement of central venous pressure. Once the cvp line has been placed, it is connected to a three way stopcock and a saline manometer. Readings should be made with the patient absolutely supine, and with the zero reference point chosen at the anterior axillary line. In order to prevent error from one observer to the other, it is convenient to mark the zero reference level on the patient so that discrepancies in readings do not arise from variations in selecting the base line zero. The errors in cvp readings result from
340
APRIL 1975 * 122 * 4
three major sources: (1) the patient's position is flat, (2) the zero reference point is not maintained the same from one reading to the next and (3) the position of the tip of the catheter is incorrectly located-either being outside the chest or else too far central and in the right ventricle. The former of these two difficulties can be avoided by watching the manometer level and noting the presence of good respiratory fluctuations. The second one can be avoided by noting the presence of ventricular pulsations when the cvp is being measured or by noting the location of the catheter tip on an x-ray film of the chest. A pressure below 3 to 5 cm of water is taken generally to indicate hypovolemia and the normal range is considered to be approximately 5 to 10 cm of water. Above the range of 12 to 15 cm of water, the pressure is clearly elevated. With tamponade, pressures of 30 to 40 cm of water are seen. The central venous pressure has been used since the early 1960's as an indicator of volume status. It replaced nuclear medicine techniques for blood volume assessment which were relatively cumbersome and inaccurate. It has come under considerable criticism in recent years as to its accuracy. In many conditions, studies have shown that left atrial pressure or left ventricular end diastolic pressure do not always move in parallel with the central venous pressure. Particularly in instances where there is myocardial dysfunction (for example, mycardial infarction), the filling pressures on the two sides of the heart may differ. Individual reports have emphasized the variance which exists in many conditions-including sepsis, pneumonia and cirrhosis-but the main problem arises when there is myocardial or cardiac valvular disease or severe respiratory failure such as "shock lung." In a study that compared central venous pressure (cvp) and left atrial pressure (LAP) in a group of normal people, there was a strong correlation between these two values. This is true in a patient with trauma in most instances as well. The central venous pressure is a valuable measurement, and its value should be not overlooked in the discussions which have gone on about subtle differences between the cvp and LAP. It is not as precise as the left atrial pressure in determining volume status, but in the average patient, and when it is carefully done, it is entirely adequate as a volume indicator in most clinical situations. In the emergency room and in the operating room it has not yet proved practical or necessary not
CARDIOVASCULAR MONITORING TABLE 1.-Comparison of Swan-Ganz Catheters Showing Their Cost, Uses and Features. Type of Lumen
Sizes
2 ...
5,7F
Cost $18
7F
35
7F
65
3 4
...
Uses and Features
Pulmonary artery and wedge pressures. Aspiration of mixed venous blood Same as above plus side hole in right atrium for central venous pressure measurement All of features of three-lumen type plus thermodilution cardiac outputs
to consider more sophisticated monitoring. The central venous pressure therefore is the mainstay of volume monitoring in these two situations. Once the patient is out of the operating room and in the intensive care unit, it may become necessary to have a more precise measurement of volume status in order to have better "fine tuning" of his fluid replacement. To obtain this, measurement of left atrial pressure is necessary. The most feasible means of obtaining this is to use the pulmonary capillary wedge pressure. In the last few years, the Swan-Ganz catheter has become available from Edwards Laboratories. It allows measurement of pulmonary artery and wedge pressures without the need to use x-ray or fluoroscopic control. Instead, the Swan-Ganz catheter can be placed while the patient is in the intensive care unit, using only a pressure monitor to determine the position of the tip. A PHYSICIAN: Would you describe the catheter?
DR. LEWIS: The Swan-Ganz catheter comes in two sizes, SF and 7F. The small catheter has a diameter of 1.6 mm and the larger, 2 mm. They may be introduced either through a cutdown or through a large bore needle. The catheters come with two, three or four lumens depending upon monitoring requirements. Table 1 gives a comparison of the various types of catheters, their cost and the features available in each. The basic two lumen type is available in two sizes, and may be passed to the wedge position in the pulmonary artery so that pulmonary artery pressures and pulmonary wedge pressures can be measured with the end hole depending whether or not the balloon is inflated. In addition, mixed venous blood can be aspirated from the tip of the catheter to determine arteriovenous (Av) oxygen content differences. The three lumen catheter has
essentially the same features but in addition a side hole is added approximately 20 cm proximal to the tip, which places it in the right atrium when the catheter is in the wedge position. This allows simultaneous measurement of the cvp, as well as the pulmonary artery pressure. The four lumen version incorporates the same features as the three but in addition has a small temperature sensing device (thermistor) at the tip of the catheter. With the use of this device the cardiac output can be determined by the thermodilution method. This technique requires the injection of cold glucose solution through the right atrial side port and then measuring the temperature change between the site of injection and the thermistor at the catheter tip. A time temperature curve so obtained can be easily analyzed to give direct reading of cardiac output. When the cardiac output as well as the arteriovenous oxygen difference is known, the patient's oxygen consumption can be ascertained. DONALD TRUNKEY, MD:* Could you describe the procedure for inserting a pulmonary artery line? DR. LEWIS: The method of insertion of a SwanGanz catheter is the following. Through a cutdown or introducer the catheter is advanced into the superior vena cava. Markings on the catheter at 10 cm intervals from the tip facilitate the placement. When the site of insertion is an antecubital vein, passage to the 35 cm mark will place the catheter in the superior vena cava. The balloon should then be inflated with the required amount of air, which in the case of the 5F catheter is 0.8 ml and in the 7F catheter is 1.2 ml. The catheter should also be connected to a strain gauge so that pressure at the tip can be continuously monitored. With the balloon inflated, the catheter is advanced while watching the pressure tracing. As the catheter tip reaches the right atrium, the flow of blood tends to drag the inflated balloon through the atrium and the tricuspid valve into the right ventricle. These locations can be assessed by noting the irregular phasic tracing with respiratory fluctuations when the catheter is in the vena cava, the accentuated and very regular atrial waves when the catheter enters the heart and then the high spiking ventricular pressure waves when the catheter passes through the tricuspid valve. Further advancement results in the catheter balloon being carried out the ventricular outflow *Assistant Professor of Surgery, University of of Medicine, San Francisco General Hospital.
California, School
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CARDIOVASCULAR MONITORING
track into the pulmonary artery where the high spiking systolic waves are accompanied by a measurable diastolic pressure. From the main pulmonary artery the catheter tip is advanced into progressively smaller branches of the artery until it reaches a wedge position. At this point the catheter position is adjusted so that wedge readings reflecting the left atrial pressure can be taken with the balloon inflated while pulmonary artery pressures can be obtained with the balloon deflated. The normal procedure is to advance the catheter with the balloon expanded to the wedge position, deflate the balloon and look for pulmonary artery waves. The catheter is pulled back if necessary until the waves are seen on the monitor. The balloon is then reinflated to determine if a wedge pressure reading results. If not, the catheter is advanced slightly and the process repeated. Ultimately a position can usually be found where the wedge and pulmonary artery pressures are both obtainable. Should this not be possible, the catheter should be left in the free pulmonary artery position and not in the wedge position, since it will occlude the vessel when wedged and may produce thrombosis in the pulmonary artery distal to the catheter. It is sometimes difficult to tell when the catheter passes from the right ventricle into the pulmonary artery, because the systolic pressure is the same in both and the upsweep of the pressure wave form is the same. However, the down slope of the pressure wave in the ventricle falls rapidly to the range of the central venous pressure. In contrast the downslope in the pulmonary artery is slower and there is usually a valve closing artifact visible along the downslope. In addition, the pressure does not fall to central venous levels but rather to pulmonary artery diastolic levels which is usually higher than the central venous pressure. A PHYSICIAN: Is it possible to place the catheter in all patients? DR. LEWIS: Our success in passing the catheter has been similar to that reported in the literature. We have approximately a 95 percent success rate in passing the catheter into the pulmonary artery and can pass it into the wedge position in about three quarters of patients. In approximately 20 percent of patients, there will be premature ventricular contractions. The primary problem is the time required to pass the catheter and place it properly in the pulmonary artery. With experience
342
APRIL 1975 * 122 * 4
this can often be accomplished in five to ten minutes but there are often times when it may take 30 minutes to an hour to place the catheter. In general it is easier to place the 5F catheter because it is smaller and more flexible and seems to negotiate the curve in the ventricle more easily than the 7F. In certain instances failure to pass the SF has been followed by success with the 7F and there is no definite way of predicting which will drop in most easily. We usually use the two lumen Swan-Ganz catheter. This allows the drawing of mixed venous blood as well as the sampling of pulmonary artery pressures, the most essential data that we require. In certain circumstances it may also be desirable to obtain cardiac output and in this situation the four lumen Swan-Ganz with the thermistor at the tip is used. Once the catheter is in place, wedge pressure measurements can be obtained as often as needed to monitor the patient's volume status. This reading is a direct reflection of the filling pressure of the left atrium, therefore is the most accurate single number we have for determining the patient's relative volume status and myocardial function. The same pressure ranges as noted for the cvp also apply to the wedge except that they average 2 to 3 cm of water higher. Once the catheter is in place it can be left in for several days and is managed the same as any other indwelling venous line. In order to keep the lumen free of thrombus, a continuous slow infusion of heparinized saline or dextrose solution should be maintained. The other advantage of using a Swan-Ganz catheter is that it allows mixed venous blood to be ob`ained. A cautionary note is in order, in that this blood must be withdrawn slowly with the 700
100% 02 HGB-14.5gms
600
500 c0o
fi 400 j-
A- V
02
DIFFERENCE
300
200
0.4 0.5 0.6 SHUNT FRACTION
.3
Chart 1.-Relationship between arterial oxygen pressure (pO2) shunt fraction and cardiac outputs.
CARDIOVASCULAR MONITORING
balloon deflated in order to prevent arterialized blood from being pulled back across the pulmonary capillaries, making the mixed venous sample appear more oxygenated than it actually is. The mixed venous blood allows two physiologic measurements to be obtained; the arteriovenous oxygen content difference and the pulmonary shunt fraction. By measuring the oxygen saturation in arterial and mixed venous blood simultaneously, the content difference may be easily established. In a normal person the difference is 5 ml of oxygen per 100 ml blood. When the heart is pumping ineffectively, the fractional oxygen extraction from arterial blood increases and the mixed venous blood has a lower oxygen content than normal. Therefore, the AV content widens and when this exceeds 8 ml of oxygen per 100 ml of blood, cardiac function can be assumed to be inadequate. Therapy in these circumstances is directed toward the use of cardiotonic drugs if wedge pressure is high, or volume replacement if it is low. Similarly, the circulation may be hyperdynamicas might be true in compensated septic shock or when defective red cell function such as that which follows massive transfusion is present. In this circumstance, the fractional oxygen extraction from arterial blood is lost, and the mixed venous blood returns with a higher oxygen saturation. In this situation the AV difference narrows. The normal content difference as noted is 5 ml of oxygen per 100 ml of blood but in the hyperdynamic patient it may narrow to 2 to 3 ml of oxygen per 100 ml of blood. A simultaneous arterial blood gas assessment
also allows determination of the fraction of blood shunted through the lungs without being oxygenated. This shunt calculation is the single best indicator of pulmonary decompensation in patients in whom "shock lung" develops. It parallels the arterial oxygen tension but is a better guide than that number because it is independent of cardiac output. I'he details of the calculation are readily available.' Suffice it to say that this calculation uses the mixed venous saturation, the arterial oxygen saturation and a knowledge of the inspired oxygen concentration which the patient is breathing. In a normal person, the shunt fraction is no more than 3 to 5 percent. In a patient with significant respiratory dysfunction from the adult respiratory distress syndrome, it may be 40 to 50 percent. The consequence of this is severe hypoxemia with which clinicians are familiar. In Chart 1, the relationship between the arterial oxygen pressure (pO2) and the shunt fraction is indicated with a patient breathing 100 percent oxygen. The purpose of the figure is to show the pronounced difference that occurs in arterial oxygen tension with differences in cardiac output, as reflected in the arteriovenous oxygen difference. The central curve is a normal situation while the curve on the right is the hyperdynamic state and that on the left is the situation with a failing circulation. In Chart 2, a vertical line has been drawn to represent a patient with a 20 percent shunt, a moderate degree of pulmonary dysfunction. The points at which it intersects each of the three curves indicates what the arterial pO2 would be in each of those patients. The horizontal lines which are projected out to the X-axis indi700
100% 02 HGB-14.5gms
100% 02 HGB-14.5gms
0
2.5--
A-V
02 DIFFERENCE
02 DIFFERENCE
-a
ULJ
:
I
0.1
0.2
0.3
0.4
0.5
0.
0.9
SHUNT FRACTION
Chart 2.-Different arterial oxygen pressure (pO2) values obtained at three levels of cardiac function in a patient with a 20 percent shunt fraction.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
SHUNT FRACTION
Chart 3.-The relationship between a fixed arterial oxygen pressure (pO2) and cardiac function on the shunt fraction. THE WESTERN JOURNAL OF
MEDICINE
343
CARDIOVASCULAR MONITORING
cate the values. It can be seen that in a patient in whom there is a failing circulation, the arterial pO2 would be only 120 while in a normal patient it is approximately 275, and in a patient with hyperdynamic circulation it would be 465. In all three instances the shunt fraction, and therefore the relative degree of pulmonary impairment, would be the same. However, the arterial P02 could obviously be improved significantly by improving the cardiac output. In a patient with failing circulation, this would be a clear indication to give cardiac inotropic agents in order to improve oxygenation and allow the inspired oxygen concentration to be reduced. In Chart 3, another use for the shunt determination is evident. A horizontal line has been drawn to represent an arterial P02 of 150. The points at which this intersects the three lines indicates the relative shunt in each of three situations mentioned previously and shows how the actual shunt fraction varies depending on the AV difference in a given patient. A patient with
344
APRIL 1975 * 122 * 4
hyperdynamic circulation who had a PO2 of 150 might actually have a shunt of 42 percent, and therefore severe pulmonary dysfunction, whereas a patient with failing circulation who had a pO2 of 150 would have only an 18 percent shunt, a moderate degree of impairment. The arterial P°2 alone, therefore, is an inadequate indicator of a patient's pulmonary status because it depends, to a considerable extent, on the state of the circulation. In an average patient the arterial P02 serves well and a detailed analysis of the pulmonary and cardiac systems is not needed. However, in a severely traumatized patient or one in whom there is a combination of cardiac'and respiratory failure, or who appears to be in refractory'shock with poor perfusion, it becomes essential to dissect out the relative contributions of the cardiac and pulmonary lesions. In this situation the SwanGanz catheter is a valuable adjunct and permits the precise titration of therapy. REFERENCE 1.
Nunn JF: Applied Respiratory Physiology. London, Butter-
worth & Co., 1969, pp 243-247
Trauma Rounds Chief Discussant FRANK R. LEWIS, MD Editors DONALD D. TRUNKEY, MD F. WILLIAM BLAISDELL, MD
1975
Current Concepts in Cardiovascular Monitoring
This is one of a series of Conferences on Trauma at San Francisco General Hospital
CAROL RAVIOLA, MD: * The patient we would like to discuss today is a 63-year-old woman who was admitted 12 hours ago, having been injured when her runaway automobile crushed her against the door of her garage. She was admitted within thirty minutes of injury and was in shock with poor peripheral skin perfusion, and a blood pressure of 80/60 mm of mercury. The injuries appeared to be limited to the lower abdomen and pelvis. Her abdomen was diffusely tender and the pelvis was grossly unstable. Findings on x-ray studies included a fracture of the right iliac crest and superior and inferior pubic rami and a right sacroilic separation. On laparotomy, one hour after admission, a large retroperitoneal hematoma was noted, which filled the pelvis and extended upward on the right nearly to the diaphragm. This was not entered. After thorough exploration of the abdominal cavity showed no additional injuries, the abdomen was closed. The patient received 5 units of blood during the induction of anesthesia and the surgical procedure. Immediately after operation, peripheral perfusion was poor. The patient's toes appeared blue and mottled, the blood pressure was 140/90 mm of mercury and the pulse rate was 100 beats per minute. Central venous pressure was 10 cm of *Chief Resident, Surgery Department, San Francisco General Hospital. Sponsored in part by NIH Grant GM18470 and the Northern California Trauma Committee, American College of Surgeons. Reprint requests to: D. D. Trunkey, MD, Department of Surgery, San Francisco General Hospital, San Francisco, CA 94110.
water. One unit of blood was given over the first two hours after operation and the central venous pressure rose to 25 cm of water. Urine output which was 30 ml the first hour, fell to 20 ml the second hour. A Swan-Ganz catheter was threaded into the pulmonary artery after 11/2 hours of manipulation. The wedge pressure was 7 to 8 cm of water and additional volume was given with an increase in urine output and stability of vital signs. The patient's condition is still critical at this time and we would like to ask Dr. Lewis if he would discuss the indications for pulmonary artery catheterization and discuss its use in monitoring the critically injured patient. FRANK LEWIS, MD: t In patients admitted with serious injuries, the problem of monitoring can be divided into cardiovascular and respiratory components. Today, as requested by Dr. Raviola, I will limit my discussion to the newer sophisticated cardiovascular monitoring techniques used in the Intensive Care Unit at San Francisco General
Hospital. Two questions arise when patients are in clinical shock. One is, what is the patient's volume status relative to the capacity of the vascular tree? A second is, how well is the heart pumping the load of fluid which is being delivered to it? All of the forms of shock can be defined in terms of alterations in volume status or cardiac action: for tAssistant Professor of Surgery, University of California, School of Medicine, San Francisco General Hospital.
THE WESTERN JOURNAL OF MEDICINE
339
CARDIOVASCULAR MONITORING
example, hemorrhagic shock is due to the lack of adequate circulating volume with a relatively normal intravascular capacity. Septic shock results from an expanded intravascular capacity with a blood volume that is inadequate to fill it. Cardiogenic shock results from ineffective pumping action even though the vascular capacity and volume are matched. The problem of assessment of cardiovascular function can be divided into two different periods. The first consists of the initial assessment of cardiovascular function in the emergency rooms and during surgical procedures, and the second consists of the assessment of complicated problems such as the patient today presented in the postoperative period. The initial period of assessment and resuscitation of the critically injured patient has been covered in a previous "Trauma Rounds" and I will confine my remarks to the venous pressure and more sophisticated measurements. In the initial evaluation in the emergency room, the status of the neck veins will give a gross indication of venous pressure. This is often all that is needed in making immediate decisions about management (for example, a patient who is hypotensive because he "bled out" will have flat neck veins while the one who is hypotensive from a cardiac problem such as tamponade will have bulging veins). Quantitative assessment of venouts pressure is obtained using central venous pressure (cvp). A central venous pressure line is inserted in the emergency room through an antecubital vein. A long percutaneously placed commercial catheter can be used or, if veins are difficult to find, a cutdown on an antecubital vein can be carried out. A catheter is threaded upward into the great veins of the chest. If a cutdown is done, a long largebore catheter such as a pediatric feeding tube should be used. This will permit rapid transfusion as well as the measurement of central venous pressure. Once the cvp line has been placed, it is connected to a three way stopcock and a saline manometer. Readings should be made with the patient absolutely supine, and with the zero reference point chosen at the anterior axillary line. In order to prevent error from one observer to the other, it is convenient to mark the zero reference level on the patient so that discrepancies in readings do not arise from variations in selecting the base line zero. The errors in cvp readings result from
340
APRIL 1975 * 122 * 4
three major sources: (1) the patient's position is flat, (2) the zero reference point is not maintained the same from one reading to the next and (3) the position of the tip of the catheter is incorrectly located-either being outside the chest or else too far central and in the right ventricle. The former of these two difficulties can be avoided by watching the manometer level and noting the presence of good respiratory fluctuations. The second one can be avoided by noting the presence of ventricular pulsations when the cvp is being measured or by noting the location of the catheter tip on an x-ray film of the chest. A pressure below 3 to 5 cm of water is taken generally to indicate hypovolemia and the normal range is considered to be approximately 5 to 10 cm of water. Above the range of 12 to 15 cm of water, the pressure is clearly elevated. With tamponade, pressures of 30 to 40 cm of water are seen. The central venous pressure has been used since the early 1960's as an indicator of volume status. It replaced nuclear medicine techniques for blood volume assessment which were relatively cumbersome and inaccurate. It has come under considerable criticism in recent years as to its accuracy. In many conditions, studies have shown that left atrial pressure or left ventricular end diastolic pressure do not always move in parallel with the central venous pressure. Particularly in instances where there is myocardial dysfunction (for example, mycardial infarction), the filling pressures on the two sides of the heart may differ. Individual reports have emphasized the variance which exists in many conditions-including sepsis, pneumonia and cirrhosis-but the main problem arises when there is myocardial or cardiac valvular disease or severe respiratory failure such as "shock lung." In a study that compared central venous pressure (cvp) and left atrial pressure (LAP) in a group of normal people, there was a strong correlation between these two values. This is true in a patient with trauma in most instances as well. The central venous pressure is a valuable measurement, and its value should be not overlooked in the discussions which have gone on about subtle differences between the cvp and LAP. It is not as precise as the left atrial pressure in determining volume status, but in the average patient, and when it is carefully done, it is entirely adequate as a volume indicator in most clinical situations. In the emergency room and in the operating room it has not yet proved practical or necessary not
CARDIOVASCULAR MONITORING TABLE 1.-Comparison of Swan-Ganz Catheters Showing Their Cost, Uses and Features. Type of Lumen
Sizes
2 ...
5,7F
Cost $18
7F
35
7F
65
3 4
...
Uses and Features
Pulmonary artery and wedge pressures. Aspiration of mixed venous blood Same as above plus side hole in right atrium for central venous pressure measurement All of features of three-lumen type plus thermodilution cardiac outputs
to consider more sophisticated monitoring. The central venous pressure therefore is the mainstay of volume monitoring in these two situations. Once the patient is out of the operating room and in the intensive care unit, it may become necessary to have a more precise measurement of volume status in order to have better "fine tuning" of his fluid replacement. To obtain this, measurement of left atrial pressure is necessary. The most feasible means of obtaining this is to use the pulmonary capillary wedge pressure. In the last few years, the Swan-Ganz catheter has become available from Edwards Laboratories. It allows measurement of pulmonary artery and wedge pressures without the need to use x-ray or fluoroscopic control. Instead, the Swan-Ganz catheter can be placed while the patient is in the intensive care unit, using only a pressure monitor to determine the position of the tip. A PHYSICIAN: Would you describe the catheter?
DR. LEWIS: The Swan-Ganz catheter comes in two sizes, SF and 7F. The small catheter has a diameter of 1.6 mm and the larger, 2 mm. They may be introduced either through a cutdown or through a large bore needle. The catheters come with two, three or four lumens depending upon monitoring requirements. Table 1 gives a comparison of the various types of catheters, their cost and the features available in each. The basic two lumen type is available in two sizes, and may be passed to the wedge position in the pulmonary artery so that pulmonary artery pressures and pulmonary wedge pressures can be measured with the end hole depending whether or not the balloon is inflated. In addition, mixed venous blood can be aspirated from the tip of the catheter to determine arteriovenous (Av) oxygen content differences. The three lumen catheter has
essentially the same features but in addition a side hole is added approximately 20 cm proximal to the tip, which places it in the right atrium when the catheter is in the wedge position. This allows simultaneous measurement of the cvp, as well as the pulmonary artery pressure. The four lumen version incorporates the same features as the three but in addition has a small temperature sensing device (thermistor) at the tip of the catheter. With the use of this device the cardiac output can be determined by the thermodilution method. This technique requires the injection of cold glucose solution through the right atrial side port and then measuring the temperature change between the site of injection and the thermistor at the catheter tip. A time temperature curve so obtained can be easily analyzed to give direct reading of cardiac output. When the cardiac output as well as the arteriovenous oxygen difference is known, the patient's oxygen consumption can be ascertained. DONALD TRUNKEY, MD:* Could you describe the procedure for inserting a pulmonary artery line? DR. LEWIS: The method of insertion of a SwanGanz catheter is the following. Through a cutdown or introducer the catheter is advanced into the superior vena cava. Markings on the catheter at 10 cm intervals from the tip facilitate the placement. When the site of insertion is an antecubital vein, passage to the 35 cm mark will place the catheter in the superior vena cava. The balloon should then be inflated with the required amount of air, which in the case of the 5F catheter is 0.8 ml and in the 7F catheter is 1.2 ml. The catheter should also be connected to a strain gauge so that pressure at the tip can be continuously monitored. With the balloon inflated, the catheter is advanced while watching the pressure tracing. As the catheter tip reaches the right atrium, the flow of blood tends to drag the inflated balloon through the atrium and the tricuspid valve into the right ventricle. These locations can be assessed by noting the irregular phasic tracing with respiratory fluctuations when the catheter is in the vena cava, the accentuated and very regular atrial waves when the catheter enters the heart and then the high spiking ventricular pressure waves when the catheter passes through the tricuspid valve. Further advancement results in the catheter balloon being carried out the ventricular outflow *Assistant Professor of Surgery, University of of Medicine, San Francisco General Hospital.
California, School
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CARDIOVASCULAR MONITORING
track into the pulmonary artery where the high spiking systolic waves are accompanied by a measurable diastolic pressure. From the main pulmonary artery the catheter tip is advanced into progressively smaller branches of the artery until it reaches a wedge position. At this point the catheter position is adjusted so that wedge readings reflecting the left atrial pressure can be taken with the balloon inflated while pulmonary artery pressures can be obtained with the balloon deflated. The normal procedure is to advance the catheter with the balloon expanded to the wedge position, deflate the balloon and look for pulmonary artery waves. The catheter is pulled back if necessary until the waves are seen on the monitor. The balloon is then reinflated to determine if a wedge pressure reading results. If not, the catheter is advanced slightly and the process repeated. Ultimately a position can usually be found where the wedge and pulmonary artery pressures are both obtainable. Should this not be possible, the catheter should be left in the free pulmonary artery position and not in the wedge position, since it will occlude the vessel when wedged and may produce thrombosis in the pulmonary artery distal to the catheter. It is sometimes difficult to tell when the catheter passes from the right ventricle into the pulmonary artery, because the systolic pressure is the same in both and the upsweep of the pressure wave form is the same. However, the down slope of the pressure wave in the ventricle falls rapidly to the range of the central venous pressure. In contrast the downslope in the pulmonary artery is slower and there is usually a valve closing artifact visible along the downslope. In addition, the pressure does not fall to central venous levels but rather to pulmonary artery diastolic levels which is usually higher than the central venous pressure. A PHYSICIAN: Is it possible to place the catheter in all patients? DR. LEWIS: Our success in passing the catheter has been similar to that reported in the literature. We have approximately a 95 percent success rate in passing the catheter into the pulmonary artery and can pass it into the wedge position in about three quarters of patients. In approximately 20 percent of patients, there will be premature ventricular contractions. The primary problem is the time required to pass the catheter and place it properly in the pulmonary artery. With experience
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this can often be accomplished in five to ten minutes but there are often times when it may take 30 minutes to an hour to place the catheter. In general it is easier to place the 5F catheter because it is smaller and more flexible and seems to negotiate the curve in the ventricle more easily than the 7F. In certain instances failure to pass the SF has been followed by success with the 7F and there is no definite way of predicting which will drop in most easily. We usually use the two lumen Swan-Ganz catheter. This allows the drawing of mixed venous blood as well as the sampling of pulmonary artery pressures, the most essential data that we require. In certain circumstances it may also be desirable to obtain cardiac output and in this situation the four lumen Swan-Ganz with the thermistor at the tip is used. Once the catheter is in place, wedge pressure measurements can be obtained as often as needed to monitor the patient's volume status. This reading is a direct reflection of the filling pressure of the left atrium, therefore is the most accurate single number we have for determining the patient's relative volume status and myocardial function. The same pressure ranges as noted for the cvp also apply to the wedge except that they average 2 to 3 cm of water higher. Once the catheter is in place it can be left in for several days and is managed the same as any other indwelling venous line. In order to keep the lumen free of thrombus, a continuous slow infusion of heparinized saline or dextrose solution should be maintained. The other advantage of using a Swan-Ganz catheter is that it allows mixed venous blood to be ob`ained. A cautionary note is in order, in that this blood must be withdrawn slowly with the 700
100% 02 HGB-14.5gms
600
500 c0o
fi 400 j-
A- V
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DIFFERENCE
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200
0.4 0.5 0.6 SHUNT FRACTION
.3
Chart 1.-Relationship between arterial oxygen pressure (pO2) shunt fraction and cardiac outputs.
CARDIOVASCULAR MONITORING
balloon deflated in order to prevent arterialized blood from being pulled back across the pulmonary capillaries, making the mixed venous sample appear more oxygenated than it actually is. The mixed venous blood allows two physiologic measurements to be obtained; the arteriovenous oxygen content difference and the pulmonary shunt fraction. By measuring the oxygen saturation in arterial and mixed venous blood simultaneously, the content difference may be easily established. In a normal person the difference is 5 ml of oxygen per 100 ml blood. When the heart is pumping ineffectively, the fractional oxygen extraction from arterial blood increases and the mixed venous blood has a lower oxygen content than normal. Therefore, the AV content widens and when this exceeds 8 ml of oxygen per 100 ml of blood, cardiac function can be assumed to be inadequate. Therapy in these circumstances is directed toward the use of cardiotonic drugs if wedge pressure is high, or volume replacement if it is low. Similarly, the circulation may be hyperdynamicas might be true in compensated septic shock or when defective red cell function such as that which follows massive transfusion is present. In this circumstance, the fractional oxygen extraction from arterial blood is lost, and the mixed venous blood returns with a higher oxygen saturation. In this situation the AV difference narrows. The normal content difference as noted is 5 ml of oxygen per 100 ml of blood but in the hyperdynamic patient it may narrow to 2 to 3 ml of oxygen per 100 ml of blood. A simultaneous arterial blood gas assessment
also allows determination of the fraction of blood shunted through the lungs without being oxygenated. This shunt calculation is the single best indicator of pulmonary decompensation in patients in whom "shock lung" develops. It parallels the arterial oxygen tension but is a better guide than that number because it is independent of cardiac output. I'he details of the calculation are readily available.' Suffice it to say that this calculation uses the mixed venous saturation, the arterial oxygen saturation and a knowledge of the inspired oxygen concentration which the patient is breathing. In a normal person, the shunt fraction is no more than 3 to 5 percent. In a patient with significant respiratory dysfunction from the adult respiratory distress syndrome, it may be 40 to 50 percent. The consequence of this is severe hypoxemia with which clinicians are familiar. In Chart 1, the relationship between the arterial oxygen pressure (pO2) and the shunt fraction is indicated with a patient breathing 100 percent oxygen. The purpose of the figure is to show the pronounced difference that occurs in arterial oxygen tension with differences in cardiac output, as reflected in the arteriovenous oxygen difference. The central curve is a normal situation while the curve on the right is the hyperdynamic state and that on the left is the situation with a failing circulation. In Chart 2, a vertical line has been drawn to represent a patient with a 20 percent shunt, a moderate degree of pulmonary dysfunction. The points at which it intersects each of the three curves indicates what the arterial pO2 would be in each of those patients. The horizontal lines which are projected out to the X-axis indi700
100% 02 HGB-14.5gms
100% 02 HGB-14.5gms
0
2.5--
A-V
02 DIFFERENCE
02 DIFFERENCE
-a
ULJ
:
I
0.1
0.2
0.3
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0.5
0.
0.9
SHUNT FRACTION
Chart 2.-Different arterial oxygen pressure (pO2) values obtained at three levels of cardiac function in a patient with a 20 percent shunt fraction.
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
SHUNT FRACTION
Chart 3.-The relationship between a fixed arterial oxygen pressure (pO2) and cardiac function on the shunt fraction. THE WESTERN JOURNAL OF
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cate the values. It can be seen that in a patient in whom there is a failing circulation, the arterial pO2 would be only 120 while in a normal patient it is approximately 275, and in a patient with hyperdynamic circulation it would be 465. In all three instances the shunt fraction, and therefore the relative degree of pulmonary impairment, would be the same. However, the arterial P02 could obviously be improved significantly by improving the cardiac output. In a patient with failing circulation, this would be a clear indication to give cardiac inotropic agents in order to improve oxygenation and allow the inspired oxygen concentration to be reduced. In Chart 3, another use for the shunt determination is evident. A horizontal line has been drawn to represent an arterial P02 of 150. The points at which this intersects the three lines indicates the relative shunt in each of three situations mentioned previously and shows how the actual shunt fraction varies depending on the AV difference in a given patient. A patient with
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APRIL 1975 * 122 * 4
hyperdynamic circulation who had a PO2 of 150 might actually have a shunt of 42 percent, and therefore severe pulmonary dysfunction, whereas a patient with failing circulation who had a pO2 of 150 would have only an 18 percent shunt, a moderate degree of impairment. The arterial P°2 alone, therefore, is an inadequate indicator of a patient's pulmonary status because it depends, to a considerable extent, on the state of the circulation. In an average patient the arterial P02 serves well and a detailed analysis of the pulmonary and cardiac systems is not needed. However, in a severely traumatized patient or one in whom there is a combination of cardiac'and respiratory failure, or who appears to be in refractory'shock with poor perfusion, it becomes essential to dissect out the relative contributions of the cardiac and pulmonary lesions. In this situation the SwanGanz catheter is a valuable adjunct and permits the precise titration of therapy. REFERENCE 1.
Nunn JF: Applied Respiratory Physiology. London, Butter-
worth & Co., 1969, pp 243-247
