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Hemodynamic Data
Box 4 -6 Formula for Assumed Oxygen Consumption
1. Dehmer formula:
2. LaFarge formula:
HR) × BSA (male);
HR) × BSA (female)
3. Bergstra formula:
+ 4.8 (male);
(female)
BSA, Body sur face area; HR, heart rate;
V
O2 (mL/min) = 125 × BSA
V
O2 (mL/min) = 138.1 − (11.49 × log age) + (0.378 ×
V
O2 (mL/min) = 138.1 − (17.04 × log age) + (0.378 ×
V
O2 (mL/min) = 157.3 × BSA + 10 − (10.5 × log age)
V
O2 (mL/min) = 157.3 × BSA − (10.5 × log age) + 4.8
V
O2, assumed oxygen consumption.
consumption difference is calculated (in milliliters of oxygen) as the
difference in left ventricular oxygen (LVO
bin × LVO
venous) oxygen (PAO
saturation × 10) minus the pulmonary arterial (mixed
2
) content (1.36 × hemoglobin × PAO2 saturation
2
) content (1.36 × hemoglo-
2
× 10).
With accurately measured oxygen consumption, the Fick technique is the most accurate method of assessing CO, particularly in
patients with low CO. Supplemental oxygen is often administered to a
patient during diagnostic cardiac catheterization. Mixing the supplemental oxygen with room air makes determination of the oxygen
content of the inspired air difficult (if not impossible) to calculate.
Supplemental oxygen therapy should be discontinued at least 10 to 15
minutes before determination of CO by the Fick technique.
There are several formulae used to compute assumed oxygen
consumption (VO
). Studies have shown that measured VO2 differed
2
significantly using values derived from formulae of Dehmer, LaFarge,
and Bergstra (Box 4-6) with median absolute differences of 28, 38,
and 32 mL/min, respectively (P < 0.0001 for each). The measured and
estimated values differed by >25% in 17% to 25% of patients, depending
on the formula used. Median absolute differences were greater in
severely obese patients (body mass index >
40 kg/m2) but were not
affected by sex or age (Narang et al).
Compared to measured oxygen consumption, the assumed Fick
at 3 mL O2/kg correlated poorly. This assumption using the LaFarge
equation is based on pediatric measurements and is specifically not
recommended for use in adults, despite the fact that it is common
practice. At the Mayo Clinic (B. Bourlag, personal communication),
VO
is measured directly at the time of PA and arterial blood sampling
2
using a metabolic cart made by MCG Diagnostics (see Fig. 4-3, B) and
is the same device that is used in cardiopulmonary exercise testing.
For most accurate assessment of cardiac function and reserve function, cardiopulmonary exercise testing with measured VO
Estimates of resting
O2 derived from conventional formulae are inac-
V
is optimal.
2
curate, especially in severely obese individuals. When accurate hemodynamic assessment is important for clinical decision making (Box
4-7), VO
should be directly measured.
2
Indicator Dilution Cardiac
Output Principle
The indicator dilution technique is based on the principle that a single
injection of a known amount of an indicator (e.g., cold saline for the
TD technique) injected into the central circulation mixes completely
with blood and changes concentration as it flows to a more distal
location. The change in the indicator concentration (or temperature)
is plotted over time; the area under the curve is used to calculate CO.
Thermodilution Indicator Method
The TD indicator method requires a PA balloon flotation catheter
(Swan-Ganz) with a thermistor at the tip. The TD Swan-Ganz catheter
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Hemodynamic Data 183
Box 4 -7 Clinical Scenarios Favoring Invasive
Hemodynamic Assessment
1. Aortic Stenosis and/or Regurgitation
a. Poor Doppler signature by echocardiography
b. Discrepancy between symptoms and noninvasive testing
c. Severe anemia
d. Concomitant subvalvular/supravalvular stenosis
e. Low- output/low-gradient AS with angiographic assessment for
concomitant coronary disease before dobutamine challenge
f. Low-output/low-gradient AS with preserved left ventricular (LV)
ejection fr action; vasodilator challenge if high peripheral vascular
resistance
2. Mitral stenosis and/or regurgitation
a. Discrepancy between symptoms and transmitral gradient or
pulmonary pressures by noninvasive testing
3. Pulmonary hypertension (primary and secondary)
4. Heart failure with preserved ejection fraction (HFpEF) without
valvulopathy
5. Candidacy evaluation for LV assist device or orthotopic heart
transplantation (OHTx)
6. Right ventricular (RV) failure
7. Hyper trophic cardiomyopathy (HCM)
8. Constrictive and/or restrictive cardiomyopathy
is a triple lumen design. The proximal port, located 30 cm from the
tip, is used for RA pressure measurement and rapid infusion of
the saline indicator during CO determination, the distal end hole is
used for pressure measurement, and the lumen is used to inflate the
balloon. A thermistor at the distal tip measures blood temperature.
The balloon ser ves two purposes: (1) a positioning aid and
(2) facilitation of PCWP. The inflated balloon helps to direct the catheter into the PA by “floating” with the flow of blood through the rightheart chambers. When positioned in the PA, the catheter can be
advanced to the distal pulmonary vasculature, “wedging” in a small
branch. The balloon forms a seal that isolates the tip from PA flow to
measure LA pressure. The thermistor at the end of the catheter is
positioned in the PA when the proximal port is in the RA.
Measuring Cardiac Output
CO is determined by rapid-injection 10 mL of iced (4° C) or room
temperature (20° C) saline injected as the indicator through the proximal port of the PAC. An external thermistor measures the temperature
of the saline injectate. Complete mixing of the injectate with blood
causes a decrease in blood temperature, detected by the distal thermistor. The CO computer calculates the change in indicator concentration (temperature over time) to determine CO in liters per minute. To
obtain consistent, reliable results, the physician and technical staff
must use appropriate technique. The following steps contribute to TD
CO accuracy.
1. Position the catheter properly in the PA. Excessive coiling of the
catheter in the RA or RV can result in poor positioning of the dual
thermistor in relation to the injectate port. This problem is sometimes encountered with a large, dilated right side of the heart. In
these instances, a 0.025-inch guidewire can be used to give additional stiffness to the catheter. A wedged catheter does not register
the appropriate temperature change.
2. Most TD systems require input of a specific computation constant
(in the output computer) depending on the system used. Some
computers provide a table that gives a constant corresponding
to the injectate volume and temperature and size and type of catheter used.
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Hemodynamic Data
3. Deliver the precise amount of injectate, which is relatively cool in
temperature relative to the patient. In adults, 10 mL is the commonly used volume, and 5 mL is typical in children. In adults for
whom fluid restriction is important, a smaller bolus of injectate may
be desired. If so, remember to change the computation constant
on the output computer to reflect the change in injectate volume.
4. Coordinate the start button and bolus injection. Press the start
button on the computer followed within a few seconds by the bolus
injection. Injection before the release of the start button is a
common error. When this happens, the computer does not recognize the full bolus of injectate delivered.
5. Deliver the bolus rapidly at a constant flow rate. Use two hands to
inject.
6. Securely connect the catheter to the computer. The interface cable
from the CO computer is nonsterile so care should be taken not to
contaminate the catheterization table.
7. If room temperature saline is used, there should be at least a 10° C
difference between the injectate and body temperature. Most computers have built-in sensors that enable the technician to check
these parameters before the start of the procedure.
8. Obtain three to five CO values. Erroneous values are ignored in the
final averaging of results.
9. Keep in mind that TD technique is inaccurate with tricuspid regurgitation or low CO. Both conditions interfere with the normal flow
of injectate past the sensing thermistor. Intracardiac shunts further
reduce the accuracy of TD.
Left-Sided Heart Catheterization
The protocol for left-sided heart catheterization is summarized in Box
4-2 and indications for left-sided heart catheterization are summarized
in Chapter 1. A combined right-side and left-side heart protocol is a
precise and complete method for addressing the most common hemodynamic problems in the catheterization laboratory (see Box 4-3).
Computations for Hemodynamic
Measurements
When hemodynamic data have been obtained, specific computations
quantify cardiac function. In this section, some of the most common
computations and standard formulae are provided for measurement
of cardiac work, flow resistance, valve areas, and shunts. Complete
derivations and applications of these formulae can be found
elsewhere.
1. CO using the Fick principle and O2 consumption is:
O consumption mL/min
CO
=
A V difference mL O / mL blood
2
O
2 2
( ) ××10
Oxygen consumption is best measured from a metabolic “hood.”
(This device is widely used except in a pulmonar y function laboratory.) It is more commonly estimated as 3 mL O2/kg or 125 mL/min/
m2. The true O2 consumption for an adult based on the preceding
estimate has been strongly questioned. AVO
from arterial–mixed venous (PA) O
ration × 1.36 × hemoglobin.
For example, if arterial saturation is 95%, O
1.36 ×
13.0 g = 16.7 mL, PA saturation 65%, O2 consumption 210 mL/
min (70 kg × 3 mL/kg) or measured value, CO is determined as follows:
( )
100
difference is calculated
content, where O2 content is satu-
2
2
content 0.95 ×
2
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Hemodynamic Data 185
2. Cardiac index (CI; L/min/m2) is
where CO is cardiac output; BSA is body surface area.
3. Stroke volume (SV; mL/beat) is
where HR is heart rate.
4. Stroke index (SI; mL/beat/m2) is
5. Stroke work (SW; g • m) is
SW mean LV systolic pressure mean LV diastolic pressure= −
6. Pulmonary arteriolar resistance (PAR; Wood units) is
7. Total pulmonary resistance (TPR; Wood units) is
8. Transpulmonar y gradient (TPG; mm Hg) is
pressure
9. PA capacitance (PAC: mL/mm Hg) is
10. Systemic vascular resistance (SVR, Wood units) is
where R is resistance; Δp is mean pressure differential across the
vascular bed, and
are also called hybrid resistance units or Wood units. To convert Wood
units to metric resistance (dynes × s × cm
0 95 0 65 1 36 13 0 10
( . . ) . .
( )
×
SSV × 0 0144.
=
TPR
TPG = mean pulmonary artery pressure (mPAP) − mean RA
PAC = stroke volume/PA pulse pressure
SVR
=
Resistance calculations follow the form of Ohm’s law, where
210
− × × ×
CO mL/beat
CI
SV
SI
mean pulmonary arterial pressure
=
mean systemic mean right arterial pressure
is blood flow. Resistance units (mm Hg/L/min)
Q
( )
=
BSA m
CO mL/min
( )
=
HR bpm
SV mL/beat
( )
=
BSA m
− ( )
−
R p/Q= ∆
210
= = L/min
53
2
( )
( )
2
( )
CO
CO
−5
), multiply by 80.
3 96
.
Computations of Valve Areas
from Pressure Gradients and
Cardiac Output
A pressure gradient is the pressure difference across a valvular or
vascular obstruction, such as a stenosis or narrowed valve. Figure 4-3,
C, is a diagram of a coronary stenosis with higher pressure proximal
to the stenosis and lower pressure beyond the stenosis. The same
principle applies to heart valves with higher pressure proximal to the
stenotic valve and lower pressure distal to the stenotic valve. Several
methods are used to measure transvalvular gradients.
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Hemodynamic Data
Box 4 -8 Techniques to Measure Left Ventricular—Aortic
Pressure Gradients (from Least to Most Accurate)
Single catheter left ventricular (LV) -aortic (Ao) pull back
LV and femoral sheath
LV and long Ao sheath
Bilateral femoral access
Double lumen pig tail catheter
Trans-septal LV access with ascending Ao
Pressure guidewire with ascending Ao
Multitransducer micromanometer catheters
Box 4 -9 Artifactual Variables Influencing Hemodynamic
Accuracy
Miscalibrated pressure transducers
Pressure leaks on catheter manifold or connecting tubing
Pressure tubing type, length, and connectors
Air in system
Catheter sizes (especially those with small diameters)
Fluid viscosity (viscous contrast material tends to damp pressure wave)
Position of catheter side holes moving across aortic (Ao) valve (see Fig. 4-16)
Techniques to measure LV-aortic (Ao) pressure gradients are
listed in Box 4-8.
All pressure gradients are affected by a number of physiologic,
anatomic, and artifactual variables (Box 4-9). Physiologic variables
include (1) rate of blood flow (e.g., CO, coronary blood flow),
(2) resistance to flow, and (3) proximal chamber pressure and compliance. Anatomic variables include (1) shape and length of valve orifice;
(2) tortuosities of the vessels (for arterial stenosis), folding of coronary
artery by guidewire (i.e., pseudostenosis); and (3) multiple or serial
lesions (for cardiac valves and arterial stenosis).
Valve Area Calculations
Area cm
=
K C MVG
value flow mL/s
2
( )
where MVG is mean valvular gradient (mm Hg), K (44.3) is a derived
constant by Gorlin and Gorlin, C is an empirical constant that is 1 for
semilunar valves and 0.85 for the mitral valve, and valve flow is measured in milliliters per second during the diastolic or systolic flow
period.
For mitral valve flow,
CO mL/min
diastolic filling period HR
( )( )
( )
For Ao valve flow,
CO mL/min
systolic ejection period HR
( ) ( )
( )
where systolic ejection period (sec/min) is systolic period (sec/beat)
× heart rate (HR).
( )
× ×
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Hemodynamic Data 187
Examples of Aortic and Mitral
Valve Area Calculations
Most modern physiologic recording systems use computer-generated
waveforms and gradient-producing valve areas automatically. When
using femoral artery (FA) and LV pressure, time shifting the FA back
to match upstroke of the LV will underestimate the true Ao valve gradient (Fig. 4-4). When calculating from the formula, convert CO to milliliters per minute, not liters per minute. When computing flow, convert
the ejection period and filling period to fractions of the period in
seconds.
Ao [200] LV [200]
Ao [200]
Sheath
LV
Ao
A
Ctrl
RT
Ctrl
[0]
200
180
160
140
120
100
80
60
40
20
0
12:04:29 PM 12:04:31 PM 12:04:33 PM 12:04:35 PM
200
180
160
140
120
100
80
60
40
20
0
B
Figure 4 -4 A, Aortic (Ao)-left ventricular (LV) pressure gradient for comput-
ing valve area by shifted and unshifted femoral artery (FA) pressure compared with central Ao pressure. The unshifted additional gradient area
(orange) is big ger than the central area, overestimating the severity of the
valve area. The shifted area is smaller than the true central area, underestimating valve area. B, Computer-shifted LV-Ao pressure ar tificially makes
the gradient area smaller and hence overestimates the Ao valve area.
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Figure 4-5 Lef t ventricular (LV) and central aortic (Ao) pressure as mea-
sured by dual lumen pigtail catheter. Note immediate upstroke of Ao pressure measured just above Ao valve with no delay as seen with FA pressure.
(Scale 0 to 200 mm Hg.) EDP, End-diastolic pressure.
Hemodynamic Data
Data obtained at catheterization for Ao stenosis (Fig. 4-5) are:
1. CO = 4 L/min = 4000 mL/min
2. HR = 60 beats/min
3. Pressure waves of LV-Ao pressures are displayed at 100-cm/sec
sweep speed. If FA pressure is used, greater accuracy is associated
with unshifted Ao upstroke and LV pressure.
Step 1. Obtain the Ao-LV gradient area (if in AF, the average is 10 beats).
Gradient area cm=12 202.
Step 2. Measure systolic ejection period (SEP).
SEP cm next convert to time sec/ cm second= = × =4 1 4 1 1 10 0 41. ( ) . .
Step 3: Compute mean systolic pressure gradient (Mean valve gradient
(MVG) in systole).
mm Hg/ cm
19 6 1
.
MVG cm
2
.
cm
4 1
. .
239
4 1
58
mm Hg= × = =12 2
Step 4: Compute Ao valve flow.
Flow
4000
24 6
CO
.
=
SEP HR
=
126 6
=
×
0 41 60
mL/m
. iin Note: SEP now in sec/beat( .)
mL/min
4000
s/beat bpm
.
×
Step 5: Compute aortic valve area (AVA).
AVA
1 0 44 3
. .
162 6
=
44 3 7 6
. .
MVG
×
16
.
=
×
336 6
Ao value flow
=
22 6
=
.
.
162 6
.
×
44 3 58
.
0 48
.= cm
2
Notes on the Aortic Valve Gradient
The mean pressure gradient (MPG) is the area of superimposed Ao
and LV pressure tracings. Peak-to-peak pressure difference is easily
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Hemodynamic Data 189
measured and often used as an estimate of valve stenosis severity. Peak-to-peak pressure difference is not equivalent to the mean
gradient for mild and moderate stenosis but is often close to the
mean gradient for severe stenosis.
The delay in pressure transmission and pressure wave reflection
from the proximal aorta to the FA artificially increases the mean gradient. If the pressure tracing is shifted to match the upstroke of the LV,
femoral pressure overshoot (amplification) reduces the true gradient.
In patients with low gradients (i.e., <
35 mm Hg), more accurate valve
areas were obtained with unshifted LV-Ao pressure tracings (see Fig.
4-4, A). Consider changing the default option on the computer to auto-
matically shift Ao pressure to correspond with LV pressure (see Fig.
4-4, B). A dual lumen pigtail catheter will eliminate the delay in pres-
sure transmission and femoral pressure amplification. Figure 4-5
shows LV-Ao pressure measurement from the central Ao catheter
without delay in the upstroke of Ao pressure.
Simplified formulae provide quick in-laboratory determinations of
AVA. AVA can be accurately estimated as CO divided by the square
root of the LV-Ao peak-to-peak pressure difference.
Peak-to-peak gradient mm Hg
Quick valve area Hakke formula
==65
CO L/min
5
L/min
5
L/min L/min
5
65
5
8
0 63
2
cm( ) .= = =
Note: The quick formulae for valve area differ from the Gorlin
formula by 18 ± 13% in patients with bradycardia (<65 beats/min) or
tachycardia (>100 beats/min). The Gorlin equation at low-flow states
overestimates the severity of valve stenosis.
Use of Valve Resistance for
Aortic Stenosis
Although not commonly computed, valve resistance (a measure of
valve obstruction) has been shown to have clinical value. Valve resistance has not been used because the units of dynes/second/cm
not well related to clinical outcomes.
Valve areas of <
important clinical syndromes, and areas of >
0.7 cm2 are almost always associated with an
1.1 cm2 are usually not
associated with significant symptoms. Areas between these two measurements are in a gray zone. One of the most common clinical situations is found in a patient with a valve area of 0.9 to 1.0 cm2, a low
transvalvular pressure gradient, low CO, and poor LV function. There
is a greater risk of surgery and a high mortality rate if ventricular function does not improve after surgery.
Valve resistance is calculated using the same variables for valve
area measurement. In contrast to valve area, the MPG is considered
to be a linear variable rather than taken as a square root term. The
contribution of pressure gradient to the magnitude of valve resistance
is greater. Resistance has also been shown to be more constant than
valve area under conditions of changing CO. Figure 4-6 shows resistance and area calculated in a group of patients before and after
balloon Ao valve dilation. Resistance rises sharply above a valve area
of 0.7 cm2. The shoulder of this curve is 0.7 to 1.1 cm2, which is the
common area of indeterminate significance of Gorlin AVA. Some
patients in this gray zone tend to have higher valve resistance than
others. In this setting, it has been shown that patients with resistance
>250 dynes/sec/cm
than patients with resistance <200 dynes/sec/cm
−5
are more likely to have significant obstruction
–5
. There is also a gray
−5
were
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Hemodynamic Data
1250
)
–5
1000
750
500
Valve resistance
250
(dynes • sec • cm
0
0.0 0.5 1.0 1.5 2.0
Valve area (cm2)
Figure 4- 6 Comparison of valve area by Gorlin formula vs. valve resistance
before (pre) and after (post) aortic (Ao) valvuloplasty. Valve resistance <200
dynes/sec/cm−5 is associated with minimal obstruction and >250 dynes/
sec/cm−5 with significant obstruction. This measure complements and
refines valve area decision making. (From Feldman T, Ford L, Chiu YC, et al:
Changes in valvular resistance, power dissipation and myocardial reserve
with aortic valvuloplasty. J Heart Valve Dis 1:55– 64, 1992.)
zone in using this index; some patients may have resistance <250
dynes/sec/cm
−5
despite a planar valve area of 0.7 to 0.8 cm2.
Pre
Post
Resistance is a complementary index, not a replacement for
valve area.
Catheter Selection for Aortic Stenosis
Initial catheter selection is a matter of operator choice and experience.
In most cases, dual lumen pigtail ventriculography catheter is a good
initial choice. Of note, the side port of the dual lumen catheter is prone
to damping, should be flushed routinely, and must be carefully positioned to ensure its placement above the Ao valve in the central aorta.
The Ao valve is usually crossed using a 0.038-inch straight-tipped
safety guidewire, extending the wire out of the catheter, straightening
the pigtail, and directing the wire into the area of highest turbulence
as detected by jet impact motion of the wire seen on the fluoroscopy
monitor. Manipulation of the wire and catheter allows wire positioning
in various directions to cross the valve. After wire crossing, advance
the LV catheter over the wire and position correctly for ventriculography and hemodynamic studies. Other catheter choices for crossing
the Ao valve include the left and, occasionally, right Amplatz catheter,
left and right Judkins catheters, multipurpose catheter, and specially
designed catheters. If the left and right Judkins catheters are unsuccessfully used to cross the Ao valve, before exchanging catheters they
can complete coronary angiography. Only the pigtail catheter is suitable for LV angiography (see the Januar y 2014 commentary in Cath
Lab Digest for “The end of the end-hole ventriculogram”). It should not
require more than several minutes to cross an Ao valve, even if severely
stenosed, and if great difficulty is encountered, a trans-septal approach
should be strongly and rapidly considered.
Points to Remember When Crossing the Aortic Valve
with Guidewires
1. Give adequate heparinization (a 40- to 50-U/kg bolus) anticipating
10 minutes of work. Perform frequent catheter flushing (approximately every 2 to 3 minutes) on removal of the guidewire.
2. To prevent guidewire clotting, a maximum of 3 minutes per crossing attempt is a good rule before wire withdrawal, wiping, and
flushing of the catheter.
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3. A 0.035-inch guidewire may be insufficiently stiff to support the
catheter on crossing severely deformed, calcific Ao valves. Substitute a 0.038-inch guidewire. A 5-F or larger catheter accepts this
size wire.
4. To prevent dissection of the coronary arteries, avoid positions of
the catheter and guidewire that point to the coronary ostia.
5. Practice gentle manipulation of the wire to avoid damaging the
valve, lifting atheromas, or causing a perforation of the cusps or
Ao root.
6. After valve crossing and during catheter exchanges, obser ve the
distal guidewire position in the ventricle to avoid wire perforation.
Once the catheter is in the ventricle, the tip of the exchange guidewire can be shaped in a curve to prevent perforation during further
catheter exchanges.
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Hemodynamic Data 191
When Do You Need to Cross the Aortic Valve for
Hemodynamic Assessment?
An example of a class-I recommendation is when experts recommend
a procedure for direct hemodynamic measurement in symptomatic
aortic stenosis (AS) patients for whom noninvasive tests are inconclusive or in whom a discrepancy exists between the noninvasive test and
clinical findings regarding the severity of Ao stenosis (level C).
It is a class-III recommendation (experts agree procedure provides no benefit or may be harmful) not to cross the Ao valve during
cardiac catheterization when noninvasive tests are adequate and
concordant with clinical findings (level C) or for the assessment of
LV function and severity of Ao stenosis in asymptomatic patients
(level C).
If the echocardiogram can provide precise, accurate, reproducible, high-quality, and high-confidence information, crossing of
the Ao valve is relatively superfluous. This would also apply to any
routine catheterization, because LV function can readily be assessed
by echocardiography, and the contribution of left ventricular enddiastolic pressure (LVEDP) over the years in most routine patients
has been minimally helpful in decision making. However, in the
patient with Ao stenosis in whom there is a question about the adequacy of the noninvasive testing, certainly retrograde cannulation
with catheter-obtained hemodynamics of the Ao valve is important
in determining the true transvalvular gradient as the standard of
care. Some with an experience in which the echocardiogram has
registered mitral regurgitation and the finding has been confused
with Ao stenosis have questioned the decision for Ao valve replacement based on echocardiography alone. Thus, the decision of
whether to cross the valve or not remains patient, echocardiographer, and hemodynamic-operator specific.
Data from the Catheterization Laboratory
for Calculation of Mitral Valve Area
Figure 4-7 shows a hemodynamic tracing to calculate the MVA.
1. CO = 3.5 L/min = 3500 mL/min
2. HR = 80 beats/min
3. Scale factor = 1 cm = 3.9 mm Hg (40-mm Hg full scale)
4. Tracing of LV-PCW pressure at 100-mm/sec paper speed = (10-cm/
sec paper speed) (align PCW v wave with downstroke of LV
pressure)
Step 1. Planimeter five LV-PCW areas (10 if in AF).
Area = 9.46 cm
2
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