wLV = LVd/AOw wLV = 4.1/.795(W1/3) wLV = 4.1/.795(451/3) wLV = 4.1/.795(.356) wLV = 4.1/2.83

wLV = 1.45

The normal range for weighted left ventricular size is 1.305 to 1.861, indicating that this dog has a normal diastolic left ventricular chamber size.

Using the same dog and applying the M-mode aortic measurement without consideration for weight, the following calculations are made:

aLV = LVd/AOm aLV = 41/30 aLV = 1.37

The calculated aortic root adjusted left ventricular size of 1.37 falls within this method’s normal reference range of 1.117 to 1.992 for the left ventricle in diastole.

This method of evaluation seems superior to raw M-mode measures when compared to studies that used linear regression analysis of parameter to body weight or body surface area. The study was published prior to logarithmic correlation of M-mode parameters to weight in dogs. In dogs and horses, the weight-based aortic indices seem superior to raw aortic root ratios in that smaller ranges and standard deviations are found. Using aortic root ratio indices in cats did not add any more accuracy to measurements, presumably because of the narrow weight range, but weight-based ratios did result in good adjustments for size (89).

Some advantages that ratio indices probably have over linearly correlated M-mode parameters is that they may be more accurate for different somatotypes and even within breeds that vary greatly in weight but not body type (i.e., Irish Wolfhounds). They may also allow direct comparison between patients of differing size and body type in the same way that fractional shortening and volume indices are used. This method may also be more accurate in overweight animals and in the highly trained athletic dog and horse. Reference ranges for these ratio indices are included in the appendices.

Measurement and Assessment of Spectral Doppler Flow

Doppler has dramatically increased the diagnostic capabilities of cardiac ultrasound. Its ability to provide information about direction, velocity, character, and timing of blood flow allows definitive diagnostics in most cardiac examinations.

Several good reference articles reviewing the principles of Doppler echocardiography and its uses in veterinary medicine are published (9,90–98). Although the measurements may appear intimidating, the equipment performs most of the calculations. Remember to interrogate multiple views when possible for all valvular flows since the imaging planes for best Doppler alignment varies from animal to animal. What follows are directions for measurement and how to apply them.

Measurement

Peak and Mean Velocity

Peak velocity is simply measured by placing a measurement caliper at the apex of maximal upward or downward motion (Figure 4.40). The velocity is displayed in centimeters per second (cm/sec) or meters per second (m/sec). Tracing the flow profile provides a measure of mean velocity throughout the flow period and is called the velocity time integral (VTI), flow velocity integral (FVI), or time velocity integral (TVI) (Figure 4.41).

Figure 4.40 Peak velocity for aortic or pulmonary flow is made by identifying the maximum downward point. The peak velocity of pulmonary flow in this image is marked by point A and is calculated to be 92.1 cm/sec.

Figure 4.41 Tracing the aortic or pulmonary flow profile provides a measure of mean velocity (V) throughout systole. Here mean velocity is calculated to be 63.3 cm/sec. Peak velocity (MAX V) is automatically calculated from flow traces as is the velocity time integral (VTI) of 14.9 cm.

Flow Velocity Integral

The flow velocity integral is directly proportional to stroke volume (98,99). The flow integral is calculated by tracing the flow profile with a trackball or joystick (Figure 4.41). Once the entire flow profile is traced, the FVI is displayed on the monitor in cm. The area under the flow velocity curve represents the distance a volume of blood travels. It is used with the area of the vessel or valve the blood is flowing through in order to calculate stroke volume.

Systolic Time Intervals

Systolic time intervals can be measured from aortic and pulmonary flow profiles. Left and right ventricular ejection times (LVET and RVET) are measured from the onset of flow to the end of flow at the baseline (Figure 4.42) (95,99,100). This is also called flow time (FT) (95). Time to peak (TTP) flow (also called acceleration time [AT]) is measured from the onset of flow to the point of maximal velocity (Figure 4.43) (95,99,100). These two systolic time periods are then divided to yield a variable that indicates what fraction of time is spent in reaching maximal velocity (TTP/FT). Pre-ejection periods are measured from the onset of the QRS complex to the onset of systolic flow (95,99,100).

Figure 4.42 Systolic time intervals can be measured from aortic and pulmonary flow profiles. Here aortic flow is used to measure ejection time (AVET) from the onset of flow to the end of flow at the baseline and pre-ejection period (PEP) from the start of the QRS complex to the beginning of aortic flow.

Figure 4.43 Acceleration rate is measured from the start of flow to the point of peak velocity (A) while deceleration rate is measured from the point of peak velocity to the end of flow (B). The equipment will automatically determine the rates. Time to peak (TTP) flow corresponds to the time interval represented by A.

Diastolic Time Intervals

The isovolumic relaxation time (IVRT) is measured by placing a CW or PW signal in the left ventricular outflow tract on apical fouror five-chamber imaging planes near the mitral valve and recording part of both the aortic flow profile and the transmitral flow profile (Figure 4.44). The time interval from cessation of aortic flow to the beginning of transmitral flow corresponds to the isovolumic relaxation time period (Figure 4.45). When left ventricular pressure drops below left atrial pressure, the mitral valve opens.

Figure 4.44 (A) Isovolumic relaxation time (IVRT) may be recorded by placing the Doppler cursor through the left ventricular outflow tract on left parasternal apical four-chamber views or (B) on apical five-chamber views. This can be done with pulsed wave or continuous wave Doppler. LV = left ventricle, RV = right ventricle, VS = ventricular septum, AO = aorta, MV = mitral valve, LA = left atrium, RA = right atrium.

Figure 4.45 Doppler flow of isovolumic relaxation time (IVRT) includes part of aortic systolic flow and part of diastolic transmitral flow. The time period from the end of systolic flow to the start of mitral flow corresponds to IVRT.

Evaluation

There is little or no correlation between peak velocities for flow across the four valves with age, sex, or breed in the dog (95,99,101). Several studies have found that heart rate and weight do affect flow velocities while others have not (95,101–103). The studies that showed an effect of body mass and heart rate on flow velocities reveal that in general, decreases in mass and increases in heart rate increased flow velocity.

Physical Factors That Increase

Doppler Flow Velocities

Increasing HR

Inspiration

Decreasing weight

Age, sex, and breed have no effect

Aortic Flow

Aortic flow profiles are negative and have rapid acceleration compared to the slower deceleration rate (91,94–102,104,105). This gives the normal aortic flow profile an asymmetric appearance (Figure 3.45). Peak velocity should be reached during the first third of systole.

Peak velocities obtained from PW and CW examinations are only slightly different. The discrepancy may be secondary to Doppler angle (95). Most normal healthy dogs have aortic flow velocities less than 200 cm/sec. There is agreement that flows above 250 cm/sec are abnormal, but flows that fall within the 200to 250-cm/sec range are equivocal. Other aspects of the echocardiographic exam will have to be used in order to determine whether disease is present. Flow velocities are affected by heart rate. Fast heart rates will increase peak and mean velocity (95,101,105).

Doppler flow profiles are difficult to obtain in the horse primarily due to the large angles of incidence that are encountered. A study of 30 normal standard-bred horse had a range of 60–280 cm/sec with a mean of 101 ± 29 (102). The value of 280 was much higher than the rest of the population; the closest value to it was 170 cm/sec. It is postulated that the angle correction used during flow evaluation overestimated the velocity. Normally Doppler cannot overestimate a velocity

unless inaccurate angle correction is applied (99,105).

Aortic Flow

Rapid acceleration

Early peaking maximum velocity

Slower deceleration

Shorter ejection time than PA

Longer PEP than PA

Pulmonary Artery Flow

More symmetrical profile

Peak velocity in middle third of ejection period

Pulmonary Artery Flow

Pulmonary artery flow profiles are also negative in all the views that can be obtained in animals. It has a very symmetrical shape with very similar acceleration and deceleration rates (Figure 3.48) (91,94,96–102,105,106). Often it displays a rounded peak as opposed to the pointed peak velocity of aortic flow (95). Peak velocity is reached approximately half way through ejection, and in the dog the mean ratio of time to peak velocity to total ejection time is .43 (95).

Peak pulmonary flow velocity in the dog is typically less than 130 cm/sec. This is lower than aortic flow presumably because of lower resistance within the pulmonary vascular system (94,95,100,101,105). It is interesting to note that this difference is not reported in horses (102). This is thought to be secondary to erroneous flow velocity readings secondary to inaccurate angle correction. Pulmonary flow has a slightly longer ejection time and reduced pre-ejection period compared to aortic flow because of the reduced afterload (95,105). Peak pulmonary artery flow velocity in the horse is less than 160 cm/sec (96,107,108).

Respiration affects flow within the right side of the heart (94,95,99–102,105). Increased venous return with inspiration increases pulmonary flow velocity during inspiration. Heart rate affects maximal velocity as it does aortic flow. Faster heart rates in the dog increase velocity (95). There is also a body mass effect on the right side of the heart. Increased body mass decreases the mean velocity and is speculated to be secondary to decreased heart rate in larger dogs (95).

Systolic Time Intervals

Heart rate affects LVET, but the effect is minimized by normalizing the interval. The heart rate is multiplied by the slope of the regression line for heart rate versus the LVET, and this value is added to the measured LVET (109). This in effect allows LVET to be extrapolated to a heart rate of zero. The slope for heart rate versus LVET is .55, and the resulting equation for heart rate corrected LVET, left ventricular ejection time index (LVETI) is as follows in Equation 4.6:

Equation 4.6

where heart rate should be measured from the R to R interval on the beat preceding the measured LVET.

The pre-ejection time period is very similar to the isovolumic contraction period where both the aortic and mitral valves are closed and the ventricle is building up enough pressure to open the aortic valve. A ratio of PEP to LVET is usually calculated in order to reduce the effects of heart rate on LVET. This is considered to be a more accurate indicator of left ventricular function.

Velocity of circumferential fiber shortening is a calculation that incorporates the ejection time into the fractional shortening equation. This is a measure of how fast the left ventricle shortens and is calculated as shown in Equation 4.7:

Equation 4.7

where LVET is measured in seconds and chamber sizes are measured in centimeters. Velocity of circumferential shortening (VCF) may be divided by heart rate and multiplied by 100 in order to reduce the effects of rate on this systolic time interval.

Because of the strong effect of heart rate on systolic time intervals, several beats should be measured and averaged. In animals with atrial fibrillation or marked sinus arrhythmia, many beats should be measured and more than 10 is recommended (109). Time intervals measured from the longest cardiac cycles tend to be the most accurate indicators of left ventricular function when heart rates are variable (109,110). Avoid measuring during ventricular or supraventricular premature complexes or the beats that follow them (109).

Mitral Valve Flow

Transmitral valve flow profiles in all planes are positive, and when heart rates are slow enough, the two phases of left ventricular filling are displayed (Figure 3.52). Once heart rates exceed approximately 125, the two phases begin to overlap and rates greater than 200 beats per minute show no separation of filling phases (95,111). The early phase of ventricular filling extends from mitral valve opening to peak ventricular filling and is called the E peak just as it is on M-mode images of the mitral valve. Flow into the left ventricle during atrial contraction is represented by the A point of the mitral inflow profile (112).

The E peak corresponding to rapid ventricular filling should have a higher velocity than the A peak in the normal heart. The E:A ratio is always greater than one in the normal canine heart, but both slow heart rates and high heart rates bring the E:A ratio closer to 1 (94,95,103,113,114). Increased volume associated with the atrial contraction in animals with slow heart rates increases the A flow velocity minimizing the difference in E and A velocities. Rapid heart rates decrease the E velocity secondary to decreased early ventricular filling volume and increases flow associated with the atrial contraction. As opposed to dogs, the normal E:A ratio in sheep may be less than 1 with a mean value of .96 (105).

Transmitral flow velocity is measured at the peak of both E and A points (Figure 4.46). Transmitral flow is affected by preload, myocardial relaxation, and heart rate (115,116). Deceleration time after rapid ventricular filling is the time from the point of maximal E velocity along its deceleration slope to the baseline (Figure 4.46). The peak filling rate and flow deceleration are influenced by several factors including isovolumic relaxation, the pressure gradient from left atrium to left ventricle, and ventricular compliance (116).

Figure 4.46 Peak E and A velocities are measured from mitral inflow profiles. Deceleration time is also measured by identifying peak E velocity and following the slope of the flow to the baseline. Calculations are automatically performed.

Changes in relaxation affect early filling of the left ventricular chamber while changes in compliance affect late diastolic filling of the ventricle. E wave velocity is increased with increased left atrial pressure, decreased left ventricular pressure secondary to increased rate of relaxation, and small mitral valve area. Early filling is decreased by low atrial pressure, decreased rate of relaxation, and increased compliance, or a large valve area. Conditions that decrease early filling of the left ventricular chamber, decreasing transmitral E wave amplitude, usually result in increased A wave velocity because late diastole contributes more to total left ventricular filling (117,118).

MV E Velocity Increases With

Increased LA pressure

Increased rate of LV relaxation

Decreased LV compliance

Small mitral valve area

MV E Velocity Decreases With

Decreased LA pressure

Impaired relaxation

Increased LV compliance

Increased heart rate is one of the factors resulting in a greater contribution to left ventricular filling in late diastole during the atrial contraction (94,95,113). Faster heart rates can also result in superimposed E and A waves as the diastolic time period becomes shorter. Body weight and body surface area do not affect transmitral flow, but age is a major factor in altering transmitral flow profiles (111).

While E deceleration time does not change with age in cats as it does in man, there are weak correlations with age and other transmitral flow parameters. Mitral valve E wave velocity decreases and A wave velocity increases with age and the E:A ratio decreases as a result (111,119). Transmitral valve E dec time increases and E:A decreases in dogs over the age of 10 when compared to dogs less than 6 years of age. Body weight increases transmitral valve E deceleration time and A duration. Increases in heart rate increase mitral valve E deceleration time in dogs. Sex has no effect on mitral valve flow velocity or time intervals (113).

Tricuspid Valve Flow

Trans tricuspid flow profiles are similar to mitral flow profiles with two phases to ventricular filling and is always positive in the planes used for Doppler interrogation. Tricuspid flow velocities are lower than mitral flow velocities, probably because of the reduced pressure drop from right atrium to right ventricle when compared to the pressure change from the left atrium to the left ventricle (94,95,99,105).

Peak tricuspid E velocity varies with respiration. Inspiration increases peak flow velocity while expiration decreases E flow velocities (95,105). The E:A ratio therefore increases with inspiration and decreases with expiration. The ratio can even be less than 1 under appropriate conditions in a normal heart with ratios ranging from .69 to 3.08 in the dog, and from .75 to 1.91 in sheep (94,105). As with left ventricular inflow, rapid heart rates increase A velocity (95).

TV E Velocity

E increases with inspiration.

E decreases with expiration.

Pulmonary Vein Flow

Pulmonary venous flow is continuous and phasic into the left atrial chamber (Figure 3.59). Left atrial filling occurs predominantly during systole when the mitral valve is closed. The velocity of this systolic flow is directly related to mean left atrial pressure. During diastole when the mitral valve is open, blood flow into the left atrium is directly related to flow moving into the left ventricle and occurs at the same time as early transmitral valve flow (E wave). During atrial contraction there is reverse flow into the pulmonary veins, and the degree of this is affected by end diastolic left atrial pressure, left atrial function, left ventricular compliance, and heart rate and rhythm (Figure 4.47) (111,112,116–118,120,121). The ratio of transmitral valve A duration to pulmonary vein Ar is correlated to left ventricular filling pressure and ventricular compliance. Partial fusion of the mitral valve E and A waves does not invalidate this correlation (116).

Pulmonary Vein

D occurs at the same time as MV E.

Ar occurs at the same time as MV A.

Figure 4.47 During atrial contraction there is reverse flow into the pulmonary veins (Ar), and the velocity and duration of this flow are affected by end diastolic left atrial pressure, left atrial function, left ventricular compliance, and heart rate and rhythm. Here Ar is increased in duration and velocity. S = systolic pulmonary vein flow, D = diastolic pulmonary vein flow.

There is a significant positive correlation between pulmonary venous S wave peak velocity and systolic fraction with age in cats (111). There is also a significant positive correlation between age and heart rate and pulmonary vein S velocity in cats (111,119). There is no association between age and pulmonary vein D or Ar velocity in cats (111). There is however a positive correlation between age and pulmonary vein Ar duration in the cat (119). Dogs have a significant positive association between age and pulmonary vein Ar velocity and a negative correlation between age and Ar duration. These relationships were only present in dogs greater than 10 years. Increased heart rate increases S and Ar velocity, and increased body weight increases Ar duration in dogs (113).

Pulmonary Vein Flow—Feline

S wave velocity

Increases with ↑ HR and ↑ age

Ar wave duration

Increases with ↑ age

Pulmonary Vein Flow—Canine (>10 yrs)

Ar velocity

Increases with ↑ age

Ar duration

Decreases with ↑ age

Isovolumic Relaxation Time

Ventricular relaxation is indirectly measured from IVRT. Delayed relaxation is reflected in longer IVRT. As atrial pressure increases however this parameter will become less useful as it becomes “normalized” (114,116). It is also affected by increased systolic aortic pressure and decreased left atrial pressure, both of which will prolong the IVRT and not truly reflect impaired relaxation (111,122,123). Heart rate is positively correlated with IVRT in man, but its effects on IVRT are variable in dogs and cats (111,113,116). Body weight and age do not significantly affect this parameter in cats (111). The relationship between increasing age and increasing IVRT in cats has an r 2 value of .18 (119).