Phasic Waveforms Research Articles

Alterations in Venous Flow Patterns in a Pediatric Patient: Impact of Mechanical Ventilation

Lower extremity venous waveforms are normally phasic with respiration and not pulsatile. The following case study describes abnormal venous spectral Doppler waveforms encountered during a pediatric ultrasound examination. A four-year-old female patient presented to the hospital with acute respiratory distress. The patient was immediately intubated and placed on an oscillating ventilator. A lower extremity venous duplex was ordered to rule out a deep venous thrombosis (DVT) due to the presence of lower extremity swelling and a fever. Thrombus was observed in the left common femoral vein surrounding a central venous catheter. The thrombus extended into the external iliac vein. This thrombus appeared hypoechoic and spongy in nature and was interpreted to be acute. Additionally, pulsatile waveforms were noted bilaterally through both lower extremities. There was no history of congestive heart failure or pulmonary hypertension. Subsequently, the patient was found to have Factor V Leiden. A repeat venous ultrasound was ordered 10 days later at which time the patient was no longer intubated. The venous waveforms in the left leg were now continuous while the right leg displayed normal, phasic waveforms. No venous pulsatility was present in either extremity. Continuous venous Doppler signals are expected in the presence of non-occlusive DVT. This case illustrates the unusual occurrence of pulsatile lower extremity venous Doppler signals. These were the result of respiratory pressure changes due to the oscillating ventilator. It is important to consider the effects of mechanical ventilation on venous blood flow patterns.

Relationship between coronary blood flow velocity waveform and transmural distribution of myocardial blood flow in coronary artery.

It is important to know the transmural distribution of myocardial blood flow in assessing the severity of ischemia in coronary heart disease. We analyzed the relation between phasic waveform of epicardial coronary flow velocity with a Doppler flow probe in the left anterior descending artery in dogs and regional myocardial blood flow using a colored microsphere technique. Time-velocity integral in an average of 5 cardiac cycles was measured as an index of coronary blood flow during diastole (TVId) and systole (TVIs). The diastolic fraction of coronary blood flow (%DF) was defined as TVId/(TVId + TVIs). Myocardial specimens were divided into inner (subendocardial), middle, and outer (subepicardial) layers, and the inner layer to outer layer myocardial blood flow ratio (endo/epi ratio) was used as an index of transmural distribution of myocardial perfusion. The mean endo/epi ratio and the mean %DF decreased as the pressure gradient increased. There was a moderate but significant correlation (r = 0.57) between the endo/epi ratio and the %DF. In conclusion, analysis of the phasic pattern of coronary blood flow velocity provides some information about the transmural distribution of blood flow in the myocardium. The %DF may be a useful index for evaluating subendocardial ischemia.

A model of phasic arterial and venous blood flow in the coronary circulation

A numerical model of the coronary circulation was developed which simultaneously addresses both the arterial and venous sides of the circulation. The model is based upon the concept of an intramyocardial pump rather than the vascular waterfall mechanism and utilizes only phasic pressures as input. The left ventricular myocardium was modelled as three equal and parallel compartments, each consisting of fixed arterial and venous resistances on either side of a lumped arterial and venous intramyocardial capacitance. A time-varying intramyocardial pressure, P i ( t), different for each compartment, was applied through the capacitance. Input pressures were obtained from direct measurements of phasic aortic, right atrial and intramyocardial pressures in six open-chest anesthetized dogs. The model accurately predicted the phasic waveforms of coronary arterial inflow and venous outflow and the nearly 180° phase shift between coronary inflow and outflow. The predicted magnitudes of mean systolic coronary arterial inflow (4.7 ± 1.0 ml/min), mean diastolic coronary inflow (38.4 ± 0.5 ml/min) and mean total coronary arterial inflow (27.7 ± 1.0 ml/min) in the left anterior descending coronary artery were not significantly different from those measured in dogs. The measured values were 5.2 ± 1.0, 28.1 ± 4.2, and 22.2 ± 3.0 ml/min, respectively. The model also predicted a transmural gradient of coronary arterial blood flow which favors the subepicardial layers during systole and the subendocardial layers during diastole. The latter is consistent with experimental observations in the beating canine left ventricular myocardium.

Two components of muscle activation: scaling with the speed of arm movement.

1. The temporal waveform of muscle activity was related to the speed of arm movement. Speed was expressed in terms of the duration of a fixed amplitude movement or the "movement time." 2. Human subjects moved their arms to targets in three-dimensional space. The right arm started at a standard initial position and moved directly to the target in a single stroke. The targets were placed in various directions in a vertical plane. The arm movements consisted of shoulder and elbow rotations. 3. Subjects were required to vary the speed of their movements. In most of the experiments, trials with different movement times were randomly ordered. One of the experiments also included randomly interspersed static trials, in which the subject held the arm still at the initial posture, the final posture, or halfway between the two extremes. 4. Electromyographic (EMG) activity was recorded from several superficial elbow and/or shoulder muscles. The time base of rectified EMG records was normalized for movement time such that records from movements with various speeds were compressed to align the ending times of the movements. 5. A principal component (PC) analysis revealed that the compressed EMG waveforms could be described by a summation of PC1 and PC2 waveforms; each individual EMG waveform was approximated by a weighted sum of these two components. 6. The PC1 weighting coefficients scaled down in a monotonic relationship with movement time such that the fastest movement corresponded to a large positive weighting coefficient and the slowest movement corresponded to a small positive weighting coefficient. The PC2 weighting coefficients exhibited a similar monotonic scaling, but the values ranged from positive to negative. Further analysis demonstrated that these two components can be mathematically transformed into a tonic waveform with a constant mathematically transformed into a tonic waveform with a constant weighting coefficient and a phasic waveform with positive weighting coefficients that scale down with movement time. 7. The amplitude scaling of EMG records cannot be described by a single component, but instead requires a summation of two separate components. The tonic component may correspond to the force element needed to counteract gravity, because the magnitude of this element does not scale with movement speed. The phasic component may correspond to the force element that scales quadratically to produce a linear increase in velocity.

Doppler ultrasound in the evaluation of experimental microvascular grafts

A study was made to evaluate the accuracy of Doppler ultrasound in assessing patency in experimental microvascular grafts in rats. In 20 animals, pre-operative assessment produced a phasic wave form and audio signal consistent with arterial flow when the Doppler probe was placed over the groin crease. Ten animals then had a sham operation exposing the femoral vessels through a groin crease incision. The remaining 10 had a 10 mm segment of femoral artery excised to simulate an occluded graft. These animals were re-assessed 18 to 20 days later and the wave form and audio signal of arterial flow were detectable in the sham operated animals with the probe placed below the groin incision. No signal was elicited at this site in the animals whose femoral arteries had been excised. When these principles were applied to 20 rats with microvenous grafts and 20 with 1 mm PTFE grafts in the femoral artery we predicted graft patency with 100% accuracy. This non-invasive technique is inexpensive and highly accurate.

The velocity profile in the canine ascending aorta and its effects on the accuracy of pulsed Doppler determinations of mean blood velocity.

The velocity profile in the ascending aorta, which has been variously reported as parabolic, skewed or flat, was measured using an intraluminal probe equipped with a miniature piezoelectric crystal activated by a 20 MHz pulsed Doppler with a 1 mm3 sample volume size. Phasic velocities, determined via spectral analyses, were obtained at 2 mm intervals across the aortic lumen of anaesthetised open-chest dogs by sequentially moving the crystal between posterior and anterior walls, maintaining the ultrasound beam parallel to the flow axis. Range-gating allowed data acquisition 3.5 mm proximal to the crystal, thus eliminating probe induced turbulence. Time and spatial distributions of velocities over the cross-section were computed. Expected errors in vessel mean velocity estimates based on point measurements were calculated. Analysis of phasic waveforms showed increased posterior velocities during systole but third order regression analysis of normalised point mean velocities across the lumen showed the best fit was a statistically flat straight line. Centreline referenced analyses indicated the expected difference between point mean velocity and vessel mean velocity was maximal (19.3 +/- 10.1%) 4 mm toward the posterior wall and minimal (4.7 +/- 9.1%) 5 mm toward the anterior wall. Anterior posterior wall referenced analyses indicated the most reliable position for estimating vessel mean velocity from point mean velocity along this axis was 5 mm from the anterior wall.