Why Measure Flow: The Oft Neglected Vital Sign
To Measure Is to Know
By measuring flow, one is quantifying or placing a number value to blood flow. Rather than relying on subjective impressions or qualitative images, real numbers provide functional information. Just as a pilot must learn to fly by instruments to avoid crashing, the researcher or clinician must rely on real quantitative data in order to draw objective conclusions.
The Quintessential Vital Sign
Flow, the amount of fluid passing a certain point during a defined time frame (ml/min, L/min), is life’s quintessential vital sign. By delivering oxygen and nutrients to be metabolized by cells, and carrying away metabolic wastes, blood flow enables life. When blood flow stops, life stops, as attested to by heart attacks or carotid artery occlusion strokes.
Accurate Flow Measurement Technology Now Available
Yet, why is measuring flow often ignored? Simply put, devices and technologies to precisely measure blood flow were unavailable until the mid-twentieth century. In their stead, surrogate measurement modalities were and are still being used. Pressure, used by the body to control flow, is routinely measured to assess hemodynamics. Pulses are felt. Doppler technology measures the velocity of blood flow, not true flow. All are surrogates for absolute volume flow, flow which can only be accurately measured with transit-time ultrasound at this time. Transonic’s founder, Cornelis Drost, was the original inventor of the transit-time ultrasound measurement principle and the first flowmeter which was used to measure flow in animal models in the 1983. The first flow measurements on humans in didn’t begin until 1988 at Harvard (again using Transonic equipment) so the widespread conversion to the use of absolute volume flow for intraoperative patency assessment is still ongoing.
Case Reports Illustrate How Intraoperative Flow Measurements Provide CABG Patency Assurance.
RIMA-RCA graft flow suppressed by
competitive RCA Flow
This is one of the cases included in Transonic’s new four-page Case Reports brochure. Seven CABG cases with accompanying waveforms showcase the invaluable information that intraoperative flow measurements provide the surgeon following anastomoses of coronary artery bypass grafts. Intraoperative graft flow measurements allow for the surgical correction of a problem before the patient leaves the operating room. Two LIMA-LAD cases graphically demonstrate that, despite good Pulsatiity Indices, poor mean flows indicate that the grafts do need revision. Another case shows a LIMA-Cx graft with zero mean flow. After revision, mean flow increased to 30 mL/min. Intraoperative flow measurements revealed a clotted graft.
High Arteriovenous Access Flow & Cardiac Complications
Please Complete the Form to Download Transonic's White Paper on Arteriovenous Access Flow and Cardiac Complications
High arteriovenous (AV) access flow is often overlooked as a source of life threatening cardiac dysfunction. By bypassing the customary arteriole/capillary beds and establishing a direct high flow connection between the arterial and venous systems, an AV access causes a precipitous drop in peripheral arterial resistance that significantly affects blood flow. In order to maintain blood pressure and improve cardiac output, the body compensates for this immediate drop in resistance by increasing its heart rate and stroke volume, which, over time can lead to the development of congestive heart failure (CHF).
The flow level that will induce high-output failure or extremity ischemia will vary with each patient, based on co-morbidities, and especially the degree of cardiac disease and peripheral arterial disease. For patients at risk based on such pre-existing conditions, the evidence-based consensus is that patients with access flows of 2L or higher should be tested and followed for these complications--and have a flow-reduction procedure performed at the earliest signs of cardiac complications or extremity ischemia.
MacRae JM et al, “Arteriovenous Fistula-associated High-output Cardiac Failure: A Review of Mechanisms,” Am J Kidney Dis 2004; 43(5): 17-22. (Transonic Reference # HD408A)
Toronto Study Uses Aortic Flowprobe to Measure Superior Vena Cava Flow as Indicator of Successful Bidirectional Cava-pulmonary Shunt (BCPS) in Children
COnfidence Flowprobes® consist of a Flowprobe shell and a single-use soft, flexible Ultrafit liner. The form-fitting Ultrafit Liner slips into the transducer shell to encircle the vessel and keep the vessel in place and protect it during a flow measurement.
Overall outcomes for three-staged surgical repair of children with a functional single ventricle have improved. However, there is still a high mortality rate after the first repair, mainly due to “in parallel circulation.” Clinicians at the Hospital for Sick Children, Toronto evaluated the effect of a new measurement-superior vena cava (SVC) flow along with anatomic factors on postoperative arterial oxygen saturation and clinical outcome in patients who underwent bidirectional cavopulmonary shunt (BCPS).
SVC blood flow was measured in 19 children (age range, 2 to 29 months, median 6 months) who underwent a BCPS between January 2009 and May 2011. Body weight was 7.0 ± 1.7 kg. Bilateral SVCs were present in four patients. SVC flow was measured at the time of BCPS by a Transonic four-crystal Aortic Flowprobe. SVC flow, preoperative hemodynamics, pulmonary artery size, and clinical outcome were analyzed to determine risk for morbidity and death.
After analyzing the absolute and indexed data, the clinicians concluded that low SVC flow may be a marker for BCPS failure or death. They suggest that SVC flow is more important that SVC size in predicting successful BCPS.
Kotani Y, Honjo O, Shani K, Merklinger SL, Caldarone C, Van Arsdell G. “Is Indexed Preoperative Superior Vena Cava Blood Flow a Risk Factor in Patients Undergoing Bidirectional Cavopulmonary Shunt?,” Ann Thorac Surg. 2012 Nov;94(5):1578-83. Transonic Reference # 9735AH
AureFlo Offers Diastolic/Systolic Ratio (D/S), & Diastolic Filling Percentage (DF%) As Quality Assurance Indicators
Dual-channel (HT364) Optima Flowmeter
as part of the AureFlo® system.
Request a Technical Not on D/S Ratio or DF Percentage
Transonic’s new FlowTrace software uses ECG signals to analyze and display D/S Ratio and DF%, two expressions used to represent the amount of blood flow through a bypass graft that occurs during diastole. A D/S Ratio compares diastolic flow to systolic flow, and a DF% compares diastolic flow to flow occurring during both systole and diastole.
The D/S Ratio is a ratio of average diastolic flow to average systolic flow. It’s an extension of Transonic’s FlowSound analysis. Surgeons are encouraged to listen to FlowSound while simultaneously examining the corresponding flow waveform. FlowSound and waveform analyses correlate with Transonic’s coronary artery graft assessment where:
- A high D/S Ratio indicates a diastolic-dominant flow profile.
- A D/S Ratio between 1 and 2 indicates a balanced, diastolic-systolic, flow profile.
- A D/S Ratio <1 indicates a systolic-dominant flow profile.
Diastolic Filling is expressed as a percentage of average diastolic flow to total (diastolic and systolic) flow.
- A DF% >50% indicates a diastolic-dominant flow profile.
- A DF% approximating 50% indicates a balanced,
diastolic-systolic, flow profile.
- A DF% <50% indicates a systolic-dominant flow profile.
Both the D/S ratio and DF% help determine whether a patient’s graft flow has diastolic or systolic dominance. A suggested surgical decision tree for graft patency assurance might include in order of importance: mean flow assessment; waveform morphology assessment, pulsatility index, D/S or DF% assessment in the context of the flow waveform.
Hemodynamic Assessment Webinar Series by Transonic Part I: PV Loop Case Study
Swine left ventricle (light green) and right ventricle (dark green) pressure-volume loops during inferior vena cava (IVC) occlusion
Date: Starting In March 2014
Join Dan Burkhoff, Transonic® and other industry leaders for a comprehensive web series all about the pressure-volume loop and how various technologies can be used to record these measurements from the beating heart.
Known as a gold standard for hemodynamic research and cardiac function, PV loops are an essential tool for many research applications, including models of heart failure, pharmacological studies, and stem cell research. This is an essential web series for the cardiovascular scientist – register today for more information
- Essential Left Ventricular PV Loop Measurements
- The utility of Right Ventricular PV Loops
- Understanding PV data in drug discovery and safety pharmacology
- PV Loops in Biomedical Device Assessment
- Utilizing PV Loops in a Core Lab
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