BASES DE HEMODINAMIA.docx

8/11/2019 BASES DE HEMODINAMIA.docx

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BASES DE HEMODINAMIA

Introduction

Many of the mechanical processes inherent to cardiac physiology can be understood bymeasuring changes in blood pressure and blood flow; the term hemodynamics refers to

this discipline.

Currently, hemodynamics is considered indispensable to the clinician managing patientswith cardiovascular disease and forms the foundation of invasive diagnostic cardiology.

History

The first cardiac catheterization and pressure measurement performed on a living animalis attributed to the English physiologist Stephen Hales early in the 1700s and reported inthe book Haemastaticks in 1733.

As early as 1844, the famous French physiologist Claude Bernard performed numerous

animal cardiac catheterizations designed to examine the source of metabolic activity.Using a thermometer inserted in the carotid artery, Bernard2 compared the temperature ofblood in a living horses left ventricle to blood in the right ventricle, accessed from theinternal jugular vein, and showed slightly higher right-sided temperatures, indicating thatmetabolism occurred in the tissues, not in the lungs.

Later in the 1800s, in an attempt to address the controversy regarding the nature andtiming of the cardiac apex beat, the French veterinarian Jean Baptiste Auguste Chauveauand physician E tienne Jules Marey performed catheterization using rubber cathetersplaced from a horses jugular vein and carotid artery. These meticulous scientistsrecognized the importance of obtaining the highest quality data and recorded pressures invarious cardiac chambers with clever mechanical devices invented by others but modified

to suit their needs.

From these early explorations of cardiac pressure measurement evolved an interest toquantify blood flow. In 1870, the German mathematician and physiologist Adolph Fick3published his famous formula for calculating cardiac output (oxygen consumption dividedby arteriovenous oxygen difference). Fick3 also contributed to the emerging field ofhemodynamics with his valuable work of refining early pressure recording devices.

Despite numerous animal studies over many years, the placement of a catheter into thedeep recesses of a living human heart would have to wait for an accurate method to imagethe course and position of the catheter. This would, ultimately, be feasible only afterWilhelm Roentgens discovery of X-rays in 1895.

Although the historical record bestows acclaim for the first human cardiac catheterizationto Werner Forssmann (performed on himself in 1929), his accomplishment may have beentrumped by the little known, often disputed, and poorly documented efforts of fellowGermans Fritz Bleichroeder, E. Unger, and W. Loeb1,2 in 1905. During one attempt madeon his colleague Bleichroeder, Unger may have actually gotten into the heart becauseBleichroeder reported the development of chest pain. They could not prove this theorybecause they failed to document the catheter position by x-ray or pressure recording andnever published their observations, attempting to gain credit only after


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Forssmann received his in 1929. Later he was awarded the Nobel Prize in Medicine andPhysiology in 1956, along with Andre Frederic Cournand and Dickinson WoodsonRichards.

Great turmoil and controversy followed Forssmanns publication. He failed to gain support

from the medical community, and, while he continued investigations in cardiaccatheterization (including at least six more self-experiments), 2 he became increasinglydiscouraged by the rigid, hierarchical nature of German academic medicine and became aurologist in private practice.

Nearly a decade would pass before there emerged a systematic discipline of right-heartcatheterization exemplified by the classical work of Andre F. Cournand and Dickinson W.Richards at Columbia Universitys First Medical Division of Bellevue Hospital.Development of right-heart catheterization arose out of Cournand and Richardss interestin pulmonary function, measurement of blood flow, and the interactions between the heartand lungs in both health and disease.

In the early 1930s, the group desired to measure pulmonary blood flow using the directFick method; however, this would require measuring mixed venous blood from the rightheart, a feat considered too dangerous. Aware of Werner Forssmanns act, the group firstdemonstrated safety in animals and then placed modified urethral catheters in the rightatrium of humans, sampling blood for oxygen content and making determinations of bloodflow using Ficks principle.

By the early 1940s, a safe and valuable methodology of right-heart catheterization hadbeen established and Columbia became recognized as the first cardiopulmonarylaboratory capable of applying these techniques to the study of cardiac and pulmonarydiseases.

Growing confidence and experience in right-heart catheterization techniques led to interestin catheterization of the left heart.

Henry Zimmerman et al.9 reported the first series of retrograde left-heart catheterizationsfrom a left ulnar artery cutdown. This report noted failure to pass a catheter across theaortic valve from a retrograde approach in five normal subjects, theorizing that the normalaortic valve prevented against the streampassage of the catheter so they turned theirattention to patients with aortic insufficiency. Zimmerman successfully entered the leftventricle in 11 patients with syphilitic aortic insufficiency. However, in a single patient withrheumatic aortic insufficiency, the attempt proved fatal. However, perseverance improvedthe safety and success at retrograde left-heart catheterization to its currently recognizedform.

By the end of the 1950s, right- and left-heart catheterization had become firmly establishedclinical techniques for the evaluation of valvular, structural, and congenital heart disease.

The invention of the balloon flotation catheter exemplified by the Swan-Ganz catheterrepresented the innovation leading to the universal acceptance and widespread practicalapplication of hemodynamic assessment.


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The balloon flotation catheter became a clinical reality from the desire of Dr. Harold JCSwan, professor ofmedicine at the University of California, Los Angeles, and director ofcardiology at Cedars-Sinai Medical Center, to apply cardiac catheterization techniques tostudy the physiology of acute myocardial infarction.

Swan became aware of the work of Ronald Bradley,10 who reported the use of very small

tubing to safely instrument the pulmonary artery and measure pressures in severely illpatients. WhenSwan attempted this technique, however, he found little success in passing the flimsy,small-caliber catheters from a peripheral vein to the pulmonary artery. In addition to thedearth of techniques to access a central vein, the most likely explanation for Swans lackof success related to the low output state of his patients compared to those of Bradley,preventing flotation of the catheter along the blood flow stream.

Edwards Laboratories worked with Swan to create the first five prototype catheters thatrelied on a balloon to accomplish flotation rather than parachutes or sails.

Swan had previously hired William Ganz, an immigrant from the former Czechoslovakia

and survivor of the World War II labor camps, to work in the experimental laboratory atCedars of Lebanon Hospital. The first animal experiments performed by Ganz with theprototype catheters were a brilliant success. Once the catheter was advanced into the rightatrium and the balloon inflated, the catheter quickly migrated across the tricuspid valveand out the pulmonary artery to the wedge position, confirming Swans notion. Thecatheters were tried in humans with similar success and led to the landmark publication inthe New England Journal of Medicine.

The group further refined the catheters design, andGanz added a thermistor to measurecardiac output by the thermodilution technique.

The core elements of diagnostic cardiac catheterization and hemodynamic assessment

have changed little since the 1970s.

Central venous pressure

CVP measures RA pressure and is affected by circulating blood volume, venous tone, andRV function.

1. Indicationsa. Monitoring. Monitoring of CVP is indicated for all cardiac surgical patients.b. Fluid and drug therapy. CVP can be used to infuse fluid or blood products; as a port foradministering vasoactive drugs; and for postoperative hyperalimentation.c. Special uses. One may elect to place a CVP catheter, with delayed PA catheter

placement in selected patients. Placement of a PA catheter can be difficult in patients withnumerous congenital cardiac disorders, in those with anatomic distortion of the right-sidedvenous circulation, or in those requiring surgical procedures of the right heart orimplantation of a right heart mechanical assist device.


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Relationship between the electrocardiogram (top) and central venous pressure (CVP)

(bottom). The normal CVP trace contains three positive deflections, known as the A, C,and V waves, and two negative deflections, the X and Y descents. The A wave occurs inconjunction with the P wave on the ECG and represents atrial contraction. The C waveoccurs in conjunction with the QRS wave and represents the bulging of the tricuspid valveinto the right atrium with right ventricular contraction. The X descent occurs next as thetricuspid valve is pulled downward during the latter stages of ventricular systole. The finalpositive wave, the V wave, occurs after the T wave on the ECG and represents right atrialfilling before opening of the tricuspid valve. The Y descent occurs after the V wave whenthe tricuspid valve opens and the atrium empties into the ventricle.

Interpretation

a. Normal waves. The normal CVP trace contains three positive deflections, termed the A,C, and V waves, and two negative deflections termed the X and Y descents (Fig. 3.10).b. Abnormal waves.

Atrial fibrillation: absent a wave (no effective atrial contraction)

Tricuspid stenosis: Canon a wave

1st degree heart block: a-c interval is prolonged

Complete heart block: normal number of a waves but has no relation to cxv waves,

cannon a waves appear (atrium contracts with ventricle at the same time


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c. RV function. CVP offers direct measurement of RV filling pressure.d. LV filling pressures. The CVP can be used to estimate LV filling pressures. Parametersthat distort this estimate include LV dysfunction, decreased LV compliance (i.e., ischemia),pulmonary hypertension, or mitral valvular disease. In patients with coronary arterydisease

PA catheter1. Parameters measureda. PA pressure reflects RV function, pulmonary vascular resistance, and left atrial (LA)filling pressure.b. PCWP is a more direct estimate of LA filling pressure. With the balloon inflated andwedged in a distal branchPA, a valveless hydrostatic column exists between the distalport and the LA at end-diastole (Fig. 3.11).c. CVP. A sampling port of the PA catheter is located in the RA and allows measurementof the CVP.d. Cardiac output. A thermistor located at the tip of the PA catheter allows measurement ofthe output of the RV by the thermodilution technique. In the absence of intracardiacshunts, this measurement equals LV output.e. Blood temperature. The thermistor can provide a constant measurement of bloodtemperature, which is an accurate reflection of core temperature.f. Derived parameters. Several indices of ventricular performance and cardiovascular

status can be derived from parameters measured by the PA catheter. Their formulas,physiologic significance, and normal values are listed in Table 3.6.


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g. Mixed venous oxygen saturation. Oximetric PA catheters can measure real time PAvenous blood oxygen saturation providing information on end-organ oxygen utilization.h. RV performance. New PA catheter technology allows for improved assessment of RVfunction distinct from LV dysfunction.

2. Indications for PA catheterization. A consensus among cardiac anesthesiologistsregarding PA catheter use has not been reached. In some institutions, cardiac surgery withCPB represents a universal indication for PA pressure monitoring in adults; other


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institutions rarely use PA catheters. Particular indications are listed in Table 3.7.Differentiation of left versus RV function and assessment of intracardiac filling pressuresduring cardioplegia administration (enhanced myocardial protection) are two indicationsthat can not be performed with CVP alone. Discordance in right and left heart functionoccurs with variable frequency where pressures measured on the right side (i.e., CVP) donot adequately reflect those on the left side [4].

3. Catheter

This model has four ports consisting of a proximal lumen (a), a distal lumen (b), and theballoon port (c), which inflates the balloon mounted at the tip of the catheter. There is anextra infusion port (d) on this model. The thermistor for performance of thermodilution

cardiac outputs connects to the computer via a connecting plug (e). The catheter has 10-cm increments marked by lines (arrow).


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4. Wave forms

+ The superior vena cava pressure is identified by a venous pressure waveform, whichappears as small amplitude oscillations. This pressure remains unchangedafter the catheter tip is advanced into the right atrium.+ When the catheter tip is advanced across the tricuspid valve and into the right ventricle,a pulsatile waveform appears. The peak (systolic) pressure is afunction of the strength of right ventricular contraction, and the lowest (diastolic) pressureis equivalent to the right-atrial pressure.


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+When the catheter moves across the pulmonic valve and into a main pulmonary artery,the pressure waveform shows a sudden rise in diastolic pressure with nochange in the systolic pressure. The rise in diastolic pressure is caused by resistance toflow in the pulmonary circulation.

+ As the catheter is advanced along the pulmonary artery, the pulsatile waveform

disappears, leaving a nonpulsatile pressure that is typically at the same levelas the diastolic pressure of the pulsatile waveform. This is the pulmonary artery wedgepressure, or simply the wedge pressure, and is a reflection of the fillingpressure on the left side of the heart (see the next section).

+ When the wedge pressure tracing appears, the catheter is left in place (not advancedfurther). The balloon is then deflated, and the pulsatile pressurewaveform should reappear. The catheter is then secured in place, and the balloon is leftdeflated.

5) Thermistor

The ability to measure cardiac output increases the monitoring capacity of the PA catheterfrom 2 parameters (i.e., central venous pressure and wedge pressure) to at least 10parameters (see Tables 8.1 and 8.2), and allows a physiologic evaluation of cardiacperformance and systemic oxygen transport.

The indicator-dilution method of measuring blood flow is based on the premise that, whenan indicator substance is added to circulating blood, the rate of blood flow is inverselyproportional to the change in concentration of the indicator over time. If the indicator is atemperature, the method is known as thermodilution.The thermodilution method is illustrated in Figure 8.5. A dextrose or saline solution that iscolder than blood is injected through the proximal port of the catheter in the right atrium.The cold fluid mixes with blood in the right heart chambers, and the cooled blood is ejected

into the pulmonary artery and flows past the thermistor on the distal end of the catheter.The thermistor records the change in blood temperature with time; the area under thiscurve is inversely proportional to the flow rate in the pulmonary artery, which is equivalentto the cardiac output in the absence of intracardiac shunts. Electronic monitors integratethe area under the temperaturetime curves and provide a digital display of the calculatedcardiac output.


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Steps for insertion


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BASES DE HEMODINAMIA.docx