Customer Solutions
A Virtual Instrument for Critical Care Monitoring of Newborn Patients with Hypoplastic Left Heart Syndrome Following Norwood Operation Using LabVIEW
Author(s):
Mark Schroeder, Jewish Heart Hospital and Lung Institute
Industry:
Life Science
Product:
Data Acquisition, LabVIEW
The Challenge:
Provide pediatric cardiac surgeons with a virtual instrument to monitor the delicate balance of flow from a single ventricle through the systemic and pulmonary vasculature for the post-operative care of neonates born with hypoplastic left heart syndrome recovering from a Norwood procedure.
The Solution:
We created a user-friendly interface enabling physicians to monitor the balance of flow through the parallel systemic and pulmonary vasculature using a laptop PC based clinical data acquisition system with a National Instruments 16-channel A/D board (model: DAQCARD-AI-16XE-50) and LabVIEW software.
Abstract
Hypoplastic left heart syndrome is a condition that occurs in newborns in which the left ventricle is unable to support the systemic circulation. It has the highest mortality among cardiac defects in newborns. Untreated, none will survive beyond 4 months. Pediatric surgeons treat these patients with the Norwood Operation, a complex surgical procedure in which a shunt is connected from the right subclavian artery to the pulmonary artery so that the single (right) ventricle can pump blood to both the systemic and pulmonary vasculature. Post-operative care is managed by measuring arterial and venous oxygen saturations which are used to estimate the systemic (Qs) to pulmonic (Qp) flow ratio. In order to optimize survival, blood flow through these parallel circulations must be balanced (Qp/Qs ratio = 1). National Instruments A/D hardware and LabVIEW software allowed us to integrate these multiple measurements into a real-time virtual instrument to monitor flow ratio trends and other physiologic signals, which otherwise would not be possible.
Background
Optimal postoperative survival following the Norwood procedure requires that one ventricle pump blood to the lungs and to the body in a balanced fashion. The Norwood procedure is te first of three surgical procedures and is usually performed within the first 2 weeks of life. To ensure survival, particularly during the critical first 48-hours after surgery, balanced flow through the systemic and pulmonary circulations must be maintained to ensure adequate oxygenation of blood and perfusion of end-organs. The standard technique for determining the balance of systemic (Qs) and pulmonary (Qp) flows is the calculation of a recently reported flow ratio (Qp/Qs) based upon oxygen saturation measurements that include systemic arterial (SaO2), systemic venous (SvO2), and pulmonary venous (SpvO2) shown below (Equation 1). Additionally, an oxygen excess factor (W), which is a function of systemic arterial (SaO2) and systemic venous (SvO2) oxygen saturations (Equation 2), has been proposed as a helpful tool in managing infants with critical cardiovascular problems.
Pediatric surgeons rely heavily upon these oxygen saturation measurements from multiple instruments, which then must be processed manually to determine the flow ratio (Qp/Qs) and/or oxygen excess factor (W), in order to select an appropriate treatment and to monitor post-operative recovery. The current problem lies in the delay of small, yet critical, changes in blood flow ratios and trend information that are used for expeditious management decisions. National Instruments A/D hardware and LabVIEW software allowed us to develop a virtual instrument that enables pediatric surgeons to monitor real-time flow ratio trends and other physiologic signals that would otherwise not be possible, thereby improving surgeon’s ability to manage this life-threatening condition.
Virtual Instrument Design
The pediatric surgeons requested that the virtual instrument be physically small in size, monitor and acquire data continuously in real-time, and display long-term history of pertinent parameters. Specifically, the virtual instrument needed to acquire and display the flow ratio, oxygen excess factor, and other physiological parameters (aortic pressure, central venous pressure, aortic flow, and ECG). The requirements were met by developing a virtual instrument using a DAQCARD-AI-16XE-5016-channel A/D board (National Instruments, Austin, TX) and LabVIEW v5.1 software (National Instruments, Austin, TX) housed on a laptop computer (Gateway Pentium III, Sioux Falls, SD). Oxygen saturation and other physiological measurements are made from multiple clinically approved instruments (i.e. HP model 66, pulse oximeter, etc.).
The virtual instrument is initiated by completing front panel documentation (Figure 1), in which the user enters the study name, subject number, DAQ operator, and the base file name. The base file name is automatically incremented and appended to include the correct date and time saved.
Once documentation data have been appropriately entered, the user selects the ‘continue’ button to initiate the monitoring and data acquisition virtual instrument (Figure 2). The virtual instrument support five primary functions: (1) tank indicator for monitoring systemic and pulmonary oxygen saturation levels; (2) flow ratio and oxygen excess factor meters; (3) continuous real-time physiological parameterwaveform charts; (4) flow ratio and oxygen excess factor 4-hour waveform chart; and (5) save options control buttons. The tank indicator and meters are updated every second and the real-time waveform charts are updated every 0.5 seconds. The 4-hour waveform chart provides the surgeon with a history of flow ratio and oxygen excess factor parameters from meters, which allows them to observe the long-term trends associated with post-operative recovery and/or therapeutic interventions. The save control buttons provide the user the option of saving data in three different ways: (1) continuous recording, (2) data epoch save where the length is specified by the user, or (3) automatically save the specified data epoch at a specified time interval. Data is saved to a new time and date stamped spreadsheet file for every save. The saved data can be easily imported into numerous spreadsheet programs or the user can view the data in the custom designed program HPLP_Viewer.VI, which allows the user to easily access and view any of the previously saved data.
Clinical Application
Our virtual instrument provides pediatric surgeons with a valuable tool for post-operative management of newborns following a Norwood procedure for the treatment of hypoplastic left heart syndrome (Figure 3). Specifically, the surgeon can now continuously track changes in the flow ratio and oxygen excess factor in real-time, which they were previously unable to do. They can also observe trends in the flow ratio and oxygen excess factor over a 4-hour timecourse, which provides extremely important information in deducing the effects of therapeutic interventions for maintaining a balanced flow ratio (Qp/Qs =1). For example, if the flow ratio is out-of-balance (i.e. Qp/Qs > 5), the surgeon would be interested in the effects of vasodilators/vasoconstrictors that adjust the balance of flow, but may take an extended period of time to have an effect (i.e. 20 minute timecourse). The surgeon may also be interested in how stable the flow ratio is over a 4-hour timecourse, where large fluctuations in the flow ratio may be indicative of an unstable system that may require aggressive treatments and close monitoring. Finally, the data acquisition feature allows surgeons and researchers to post-process these data and identify promising therapeutic treatments, develop post-operative management strategies, and elucidate critical physiologic control mechanisms.
Conclusion
A medical instrument for monitoring the flow ratio and oxygen excess factor or acquiring these data does not exist. National Instruments A/D hardware and LabVIEW software provided the platform to construct a virtual instrument that integrated oxygen saturation and physiological measurements from multiple medical devices for calculating, displaying, and saving these vital parameters. We believe this integrated virtual instrument will allow pediatric surgeons to better treat these patients and improve clinical outcomes.
schroede.pdf
View the entire user solution in Adobe Acrobat PDF format.
