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CLINICAL INSIGHTS JULY/AUGUST 2018 61 the middle of the upward systolic deflection. Normal systolic velocities in the aorta reach 1.0 to 1.5 m/sec. Caudal Vena Cava The caudal vena cava can be seen most easily in the abdomen, right of midline, where it runs parallel and adjacent to the aorta in the caudal abdomen, and in the cranial abdomen where it traverses the right side of the liver before crossing the diaphragm. At the level of L3, the caudal vena cava begins its ventral course (away from the aorta and dorsal position in the abdomen) to the level of a mid-diaphragmatic position in the cranial abdomen, as seen on thoracic radiographs. The cranial aspect of the abdominal caudal vena cava, at the level of the caudate process of the caudate lobe of the liver, is used as a landmark to find the right adrenal gland, as the right adrenal gland is located immediately dorsolateral or lateral to the caudal vena cava at the level of the renal hilum. Cats deposit fat between the caudate lobe of the liver (renal fossa) and the right kidney (located in a more caudal position than in dogs). In cats, the right adrenal gland is located cranial to the right kidney along the lateral border of the caudal vena cava at the level of the liver. The pulsed-wave Doppler spectral waveform is complex and is influenced by cardiac cycle and the phase of respiration as well as patient motion ( FIGURE 5B ). If imaging the vessel so that blood flow is toward the transducer, there will be an initial positive waveform at the time of ventricular systole. Before the waveform reaches baseline, there will be another positive spike during late diastolic filling as the right ventricular and right atrial pressure increases; during atrial contraction, there will be a negative deflection as blood flows from the right atrium into the right ventricle. Because the flow velocity varies in the caudal vena cava, a laminar flow pattern will not be seen. The peak velocities in the caudal vena cava (at the time of right ventricular systole) are typically around 20 to 35 cm/sec. Portal Vein The portal vein has a characteristic flat venous pulsed- wave Doppler profile ( FIGURE 5C ) and is located within the porta hepatis, dorsal to the body of the pancreas. Moving cranially, the portal vein is formed by the confluence of cranial and caudal mesenteric portal veins, the splenic portal veins, and the gastroduodenal portal vein. The portal vein proper is a very short vessel that travels into the porta hepatis and gives off the right divisional branch (right liver lobes) and the central divisional branch (quadrate and central hepatic lobes) and continues on as the left divisional branch (left hepatic lobes). The flow pattern is nonlaminar and the typical flow velocities are 15 to 25 cm/sec. FIGURE 5. Pulsed-wave Doppler evaluation of a 1-year-old mixed-breed dog under sedation (Dexdomitor 0.25 mcg/kg). (A) Long- axis view of the abdominal aorta from a right-sided window that documents the typical triphasic appearance of the pulsed-wave Doppler spectral tracing. The peak velocity is typically < 1 m/sec. The dog had a sinus arrhythmia so that after the long diastolic pause, there was a "stronger" systolic contraction (increased stretch of myocardial fibers due to increased ventricular filling; S); the first contraction is taller (higher velocity) than the other 2 beats during inspiration. (B) Long-axis view of the caudal vena cava as imaged from the right. Due to the decreased heart rate from the sedation, there are 4 peaks on the pulsed-wave Doppler spectral tracing of the caudal vena cava instead of the standard three. The initial positive deflection (note that the spectral trace has been inverted as the position of the sampling gate; flow would be away from the transducer) represents ventricular systole. The diastolic filling phase has been divided into 2 parts (d1 and d2), and there is flow reversal at atrial contraction (A). (C) Long-axis view of the portal vein as imaged from the right. The pulsed-wave Doppler spectral tracing of the portal vein is typical in appearance, with a flat profile and not under the influence of the cardiac cycle or respiration. B C A

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