Putting Hemodynamics at the Center: A Systemic Approach to Teaching Vascular Ultrasound

Abstract

A systemic framework for teaching vascular sonography that is centered on hemodynamics is useful to unify the initial test-by-test approach that developed with the pioneering and expansion of the field. Vascular sonography education is most powerful when taught with a system-level view of the circulation that connects how vascular anatomy (form) and physiology (function) are inextricably linked with hemodynamics (flow). Furthermore, understanding hemodynamic principles is key to using waveform morphology as a diagnostic tool. Teaching a system-based, hemodynamic-centered perspective aligns with the objectives of the recent Consensus Statement on Doppler Waveforms by the Society for Vascular Medicine and the Society for Vascular Ultrasound and equips the vascular sonography student to recognize, understand, and appropriately document vascular findings.

Keywords

vascular sonography education vascular technology

In the early 1950s, physicians who sought a non-invasive alternative to angiography began developing novel techniques to assess their patients. Groundbreaking studies catalyzed an interest in the application of ultrasound for vascular testing, resulting in tools such as continuous wave (CW) Doppler, duplex ultrasound imaging with color, and spectral Doppler.^1,2 These techniques, were developed one at a time, initially focused on application to a single vascular bed. In each instance, technology deepened hemodynamic understanding (see Table 1).

Table 1.

Vascular Ultrasound Application Development by Year.

Ultrasound Examination Type	Date
Lower extremity arterial	1972³
Extracranial cerebrovascular	1972¹
Lower extremity venous—for DVT	1975⁴
Renal artery	1984⁵
Penile circulation	1985⁶
Mesenteric arteries	1986⁷
Renal transplantation	1986⁸
Intracranial cerebrovascular	1986⁹
Hepatoportal circulation	1987¹⁰
Uterine circulation	1988¹¹
Liver transplantation	1988¹²
Scrotal circulation	1990¹³
Upper extremity venous	1990¹⁴
Hemodialysis access	1991¹⁵
Abdominal aorta	1991⁷
Lower extremity venous—for insufficiency	1991¹⁶
Ovarian circulation	1992¹⁷

Abbreviation: DVT, deep vein thrombophlebitis.

Development of this array of technology provided both detailed renderings of a patient’s anatomy and previously unobservable physiologic characteristics of real-time human blood flow. Doppler waveforms offered insightful maps of hemodynamic properties that were unobtainable by surgical or cadaveric exploration of vessels and impossible to simulate in laboratory settings.^18-20 These tools were capable of acquiring information that was beyond the contemporary understanding in the field at the time and fueled the modern era of clinical hemodynamics, with the waveform as its centerpiece, providing a written language for the translation of vascular physiology.²⁰

Proposed Systemic Approach: Vascular Form and Function Are Inextricably Linked to Flow

Since the advent of vascular testing, the modern field of clinical hemodynamics has solidified principles of arterial and venous flow patterns, offering explanations for the underlying phenomena that govern vascular flow. Vascular sonography education is most powerful when it places these hemodynamic principles at its core, providing a cohesive framework for understanding flow within the vascular system as a whole, as well as the significance of its individual parts. A foundational, system-level view of the circulation that connects how vascular anatomy (form) and physiology (function) are inextricably linked with hemodynamics (flow) provides a fundamental approach to explaining normal and pathologic states, rather than just describing them. Furthermore, this perspective can be used to synthesize how these elements are captured in duplex ultrasound and indirect vascular testing modalities. This approach provides a framework that supports critical thinking and skillful imaging of the vascular system.

Foundations: Anatomy, Physiology and Hemodynamics (Form, Function, and Flow)

The vascular system is not a disconnected collection of unique vessels that operate in isolation from one another. Rather, it is a complex interconnected system whose parts work in balance, with each vessel’s effectiveness arising from its position and structure, as well as the health of the other mechanisms of circulation (heart, other blood vessels, organs, and blood^18,21). A systemic view recognizes that all these attributes contribute to hemodynamic patterns and, subsequently, normal and abnormal waveform morphology within any given vessel.

Because the vascular system is a closed pressurized circuit, flow at any given location is a product of conditions both up and downstream from the point of interest. Specifically, all flow characteristics are derived from the interplay of pressure and resistance , with flow morphology and velocity profiles of arterial and venous vessels reflecting both pressure from the source of flow, and resistance at its destination.^18-20 This is the foundation of all hemodynamic phenomenon and is captured in an analogous form of Ohm’s law, where Q is flow, ΔP, is pressure gradient, and R is resistance to flow:

Q = Δ P / R

From this formula, it is noted that the main driver of flow is pressure gradient and the main detractor is resistance. The interplay between pressure and resistance is reflected in fluctuation of flow velocities, which are mapped over time to yield waveforms.^20,21

Arterial Hemodynamics

For the arteries, the primary source of flow is the dynamic pressure of the cardiac cycle, while resistance arises largely from the distal arteriolar beds.

Arterial pressure: during the systolic phase of the cardiac cycle, dynamic pressure from ventricular contraction is the dominant source of flow and greatly exceeds resistance in healthy patients. This dynamic pressure drives brisk, laminar acceleration to normal peak velocities.¹ It is for this reason that the systolic phase of an arterial waveform indicates the status of the health of the source structures responsible for arterial “supply,” namely the heart and more proximal arteries (see Figure 1).

Figure 1.

Systolic phase components of arterial waveforms and their hemodynamic significance.

Assessment of the systolic phase of a waveform reveals the efficacy of source pressure and provides information on the health of the upstream components of circulation: the more proximal arteries and the heart.

Arterial resistance: during the diastolic phase of the cardiac cycle, dynamic pressure from the heart plummets and the variable state of distal resistance, as determined by the vasodilation or vasoconstriction of the arterioles, becomes the dominant factor in velocity of arterial flow. For arteries feeding dependent vascular beds with low demand, arteriolar vasoconstriction generates high resistance, demonstrating little to no forward flow in diastole. Conversely, for arteries feeding dependent vascular beds with high demand, arteriolar vasodilation offers low resistance, as reflected in significant antegrade flow through diastole.^18,21 Therefore, in the diastolic phase of the cardiac cycle, flow levels reflect distal conditions, which are determined by “demand” of the dependent vascular bed (see Figures 2 and 3).

Figure 2.

Diastolic phase components of arterial waveforms and their hemodynamic significance.

Figure 3.

Varying resistivity levels of arterial waveforms.

Assessment of the diastolic phase of a waveform provides information on the level of resistance created by the physiologic state of downstream arteriolar beds.

Venous Hemodynamics

Venous pressure: rather than sharing a single uniform source of pressure that drives flow (like the heart for arteries), venous flow is convergent and gains pressure as it accumulates from distal tissues in significant enough volumes to overwhelm opposing forces of resistance.¹⁹ Venous volumes are dependent on the arterial inflow to the capillary bed of the tissues being drained. If arterial output is high, venous outflow is constant; if arterial output is low, venous outflow is low and more likely to be sporadic (see Figure 4).

Figure 4.

Arterial inflows as compared with their venous outflows.

Forces that alternate between sources of venous pressure and resistance: as venous flow converges from distal vascular beds to form centripetal flow, various forces exist that can either augment or impede flow, depending on their relationship to the venous vessel of interest. These sources of pressure or resistance include position-dependent hydrostatic pressure, motion of the diaphragm, and the heart.^19,22,23

Much different than its high-pressure arterial counterpart, the low-pressure venous system is a delicate balance between source pressure and downstream resistance, with direction and velocity of flow indicating which is the dominant force in any given moment. When source pressure is dominant over downstream resistance, centripetal (toward the heart/antegrade) flow ensues. When downstream resistance is dominant over source pressure, centripetal flow ceases and centrifugal (away from the heart/retrograde) flow may occur.

Venous waveforms are meaningful maps of the pressure balance within the venous system, demonstrating normal flow volumes and responsiveness to patterns of cardiac and respiratory motion based on their position within the vascular system (see Figures 5 and 6).

Figure 5.

Influences of venous waveform flow velocity and direction in a middle hepatic vein waveform.

Figure 6.

Effects of respiration on a venous waveform of inferior vena cava flow.

A hemodynamic-centered approach to teaching vascular ultrasound offers, by nature, a systemic perspective of the vascular system because it recognizes that the blood within the circulatory system forms a single continuous body of fluid, with flow at any given point being dependent on the conditions both up and downstream, namely, at the source of blood flow and its destination.

Discussion

A hemodynamic-centered approach to vascular sonography education, as introduced in the previous section, extends beyond teaching traditional anatomic and physiologic characteristics of the vascular system and seeks to integrate how particular waveform appearances result from flow dynamics created by these. It defines and describes the varying elements that characterize normal flow and constructs an explanation of what underlying anatomic and physiologic characteristics support them: both at the point of interest and systemically. Vascular educators can achieve a hemodynamic-centered approach by (1) creating an initial foundation of knowledge that demonstrates how the three key content areas of anatomy, physiology, and hemodynamics are interdependent and broadly responsible for subsequent waveforms and (2) referencing this hemodynamic foundation when teaching individual vascular tests using a standardized framework for explaining specific flow characteristics within the vessels of any vascular subsystem.

The hemodynamic foundation initially taught should provide a cohesive framework to explain both normal and pathologic flow states. Using Ohm’s law, all normal anatomic and physiologic attributes can be explained as contributing to either the pressure that contributes to antegrade flow or the resistance that opposes it. To emphasize the dependent nature of flow at any point within a given vessel, students should be taught to think in terms of the source of flow and its destination. For arteries, this can be framed in terms of supply and demand, whereas for veins, the terms (centripetal) source pressure and (centrifugal) resistance are more useful. Sources of pressure and resistance that should be explored are noted in Table 2.

Table 2.

Foundational Drivers of Hemodynamic Profiles of Arterial and Venous Vessels: Sources of Vascular Pressure and Resistance.

	Sources of Pressure	Sources of Resistance
Arterial	Cardiac systole Cardiac diastole Hydrostatic pressure	Blood viscosity Vessel wall composition Intraluminal diameter Arteriole vasoconstriction/dilation Hydrostatic pressure
Venous	Capillary outflow Calf muscle pump Respiratory motion Cardiac motion Hydrostatic pressure	Blood viscosity Vessel wall composition Intraluminal diameter Calf muscle pump Respiratory motion Cardiac motion Hydrostatic pressure

The difference between pressure and resistance at any given point is demonstrated in velocity fluctuations over time, which are mapped by waveforms. A thorough comprehension of how anatomy and physiology contribute to hemodynamics paves the way for a firm understanding of waveform morphology. After normal states are explained, all vascular pathology, by contrast, should then be framed as processes that disrupt anatomy and physiology and subsequently alter the normal balance of flow and resistance, which is reflected in abnormal waveforms (See Figure 7).

Figure 7.

Pedagogical approach to hemodynamic-centered vascular education.

Once this initial foundation has been laid, the same approach should also be applied in standardized fashion to teaching vascular testing of specific structures. This involves identifying all vessels that collectively serve a dependent vascular bed and then identifying the sources of pressure and resistance among them. This methodology captures the many factors that may influence flow and waveforms in any individual vessel, which can range from localized pathology (like stenosis in the vessel of interest), proximal pathology, distal pathology, or pathology in an associated vessel (such as renal vein thrombosis impacting renal artery waveform; subclavian occlusion impacting vertebral artery waveform). Table 3 uses the example of the left common carotid artery to demonstrate how this systemic, hemodynamic perspective can be applied to map fundamental anatomic and physiologic attributes of vascular structures that are traditionally taught, alongside an explanation of how pathologic change impacts waveform morphology.

Table 3.

Anatomic and Physiologic Attributes of a Vessel Which Impact Hemodynamics, as Applied to the Left Common Carotid Artery.

	Attribute	Normal Hemodynamics	Pathologic Hemodynamics
Extracranial cerebrovascular system and left common carotid artery	Source of flow: Heart	Presence of a sharp systolic upstroke, systolic peak, spectral window and diastolic flow are driven by cardiac health. Frequency and rhythm of peak systole is determined by heart pace and rhythm.	Bilateral changes in waveform morphology suggest cardiac pathology, including aortic insufficiency, aortic valve stenosis, and congestive heart failure.²⁴ Change in pace or rhythm of systolic peaks can indicate tachycardia, bradycardia or arrhythmia; presence of intra-aortic balloon pump or left ventricular assistive device.²⁵
	Destination of flow:Low resistive (high demand) BrainEyesThyroid gland²⁶ Salivary glands²⁷ Lacrimal glands²⁸ High resistive (low demand) MSK structures of the head/neck	High flow volumes are necessary to meet the high demands of the combined dependent vascular beds.Resistivity index is intermediate, reflecting the combined characteristics of the high and low resistive flow of the dependent vascular beds.	A unilateral increase in RI/decrease in flow volume suggests distal stenosis.²⁹ A unilateral decrease in RI/increase in flow volume suggests possible compensatory flow for contralateral disease.³⁰
	Vessel type/hierarchical positionMedium-sized artery	Normal diastolic flow volume is dependent on the mechanical property of vessel wall compliance.	As atherosclerotic disease increases, RI and carotid intima-media thickness both rise.²⁹
	Parent/proximal vessel:Aortic arch	Normal systolic waveform morphology, as well as flow volume is dependent on the health in the aortic arch.	Aortic stenosis or aneurysm will change waveform morphology in the systolic phase, impacting upstroke, shape of systolic peak and spectral window.²⁴
	Branch/distal vessels:Left internal carotid arteryLeft external carotid artery	Normal diastolic flow levels are determined by the health of branch/distal vessels.	Significant distal stenosis or occlusion in the ECA will result in lower RI. Significant distal stenosis or occlusion in the ICA will raise RI.³¹
	Supplemental vesselsRight carotid arteryVertebral arteries	Normal flow volume and waveform morphology is dependent on healthy vertebral arteries and contralateral carotid artery.	Significant vascular disease in the vertebral, subclavian or contralateral CCA increase flow volume.³¹
	Companion vesselsInternal jugular vein, external jugular vein	Normal jugular vein flow is dependent on carotid inflow.	Jugular vein pathology has not been demonstrated to typically impact carotid flow.

Recently, a consensus document by the Society for Vascular Medicine and the Society for Vascular Ultrasound has clearly defined and standardized descriptive terms for normal and pathologic Doppler waveforms in both the peripheral arterial and venous systems. This important achievement not only codifies a shared language for discussing waveform morphology but also identifies the necessary skill level for vascular sonographers to possess. The authors state,

All interpreting physicians and sonographers should be able to describe Doppler waveforms, be able to identify the changes which occur with physiologic and disease states, and effectively communicate these waveform characteristics to interpreting physicians so that consistent information is given to referring providers.³²

It should be the goal of vascular educators to provide necessary instruction in waveforms and the hemodynamic principles underlying them so that students can achieve competence in these areas. The list of major descriptors and modifying terms for waveform morphology that were defined in the consensus document serve to establish the scope of necessary knowledge for vascular sonographers. Vascular students, therefore, will need foundational instruction in anatomy, physiology, and hemodynamics to support an understanding of the significance of these key terms, as listed in Table 4.

Table 4.

Key Major Descriptors and Modifier Terms Used as Scope of Necessary Information for Hemodynamic-Centered Vascular Education.

Arterial	Key major descriptors	• Flow direction (antegrade, retrograde, bidirectional, absent) • Phasicity (multiphasic, monophasic) • Resistance (high, intermediate, low)
Arterial	Modifier terms	• Upstroke (rapid, prolonged)• Sharp peak• Spectral broadening• Staccato• Dampened• Flow reversal
Venous	Key major descriptors	• Flow direction (antegrade, retrograde, absent)• Flow pattern (respirophasic, decreased, pulsatile, continuous, regurgitant)• Spontaneity (spontaneous, nonspontaneous)
Venous	Modifier terms	• Augmentation (normal, reduced, absent)• Reflux• Fistula flow

Adapted from Society for Vascular Medicine / Society for Vascular Ultrasound (SVM/SVU) Consensus Statement on Doppler Waveforms.³²

The systemic, hemodynamic-centered framework offered above provides foundational reasoning about the significance of varying anatomic and physiologic attributes of vascular structures, as well as patterns of waveform morphology. This is a useful educational tool because it offers a cohesive perspective for teaching students to understand the causes of normal and abnormal findings of vascular testing, beyond recognizing their appearances. Although key hemodynamic principles were only applied to basic normal waveform morphology in this article, they can also be readily applied to additional normal and pathologic characteristics.

While the normal appearance of vascular structures and waveforms vary from vessel to vessel, findings for each are in line with the principles of hemodynamics. When combined with the well-documented sonographic appearances of waveforms for many circulatory subsystems, these explanations provide students with a central framework that is meaningful for understanding the significance of the waveforms they are recording.

Footnotes

Declaration of Conflicting Interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Miriam Teft

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