Introduction to Hypertonic Saline Volume Calibration
In CD Leycom’s video series number five, we explore an technique for calibrating pressure–volume (PV) loop catheter data using hypertonic saline. While previous modules focused on real‑time, imaging‑based methods, this calibration method is typically left for offline post‑procedure workflows so as not to interrupt live data acquisition.
Essential Inputs: Cardiac Output and Saline Boluses
This calibration method hinges on two critical inputs. First, you need an average cardiac output value obtained from a reliable external reference—such as thermodilution or indirect Fick estimation—measured during the case. Although this value is not used during live PV loop recording, it provides the anchor for scaling conductance‑derived volumes to absolute units. Second, you must acquire at least two 10 mL bolus injections of 5% hypertonic saline per dataset. The saline’s high conductivity transiently alters measured conductance, enabling the algorithm to isolate true ventricular volume from catheter‑related artefacts.
Understanding Datasets and Procedural Phases
A “dataset” in BiV Conduct NT corresponds to each distinct ventricular deployment of the PV loop catheter, requiring its own calibration. For example, in a transcatheter aortic valve replacement (TAVR), Dataset 1 holds pre‑TAVR recordings, while Dataset 2 captures post‑TAVR data following catheter re-delivery. Since catheter re-delivery may change volumetric measurements separate from true, physiological responses after valve implantation, each dataset demands a separate cardiac output reference and hypertonic saline injections to maintain accuracy.
Visualizing the Saline‑Induced Conductance Shift
During a hypertonic saline bolus (10 mL of 5% NaCl delivered over approximately three seconds), clinicians perform an end‑expiratory breath hold to minimize respiratory artefacts. In the resulting PV loop display, you’ll observe a pronounced rightward shift in the loops—a hallmark of the saline’s higher conductivity relative to blood. This shift is not a genuine increase in ventricular volume but rather an acute conductance change. By quantifying this change in conductance, the software delineates the true blood‑filled cavity from surrounding tissues, providing the basis for accurate volume calibration.
Parallel Conductance Correction via Saline Calibration
Conductance catheters inherently measure both blood and surrounding myocardial or structural conductance (“parallel conductance”), leading to systematic overestimation of true intraventricular volume. The hypertonic saline method exploits the hypertonic saline‑induced conductance shift to define the ventricular boundaries. As the Inca system processes the conductance changes, it automatically computes and subtracts the parallel conductance component, yielding a corrected volume signal that faithfully tracks end‑diastolic and end‑systolic values in absolute terms.
Organizing Calibration Files During Data Acquisition
To streamline later calibration, it is best practice to save recordings with specific file types during live data acquisition. In BiV Conduct NT, hemodynamic recordings are saved as standard PV loop “Data” files by default. When acquiring a separate recording representative of your external cardiac output reference—often derived by injecting cold saline for thermodilution—save that file as an “SVcal” file. Similarly, save each hypertonic saline bolus recording as an “EFcal” file. Thoughtful naming conventions (e.g., “CO = 4.02 L/min” for the SVcal file) ensure you’ll recall calibration values without rechecking external reports.
Accessing the Volume Calibration Interface
Once your case concludes, navigate to the Volume Calibration window, which will display Dataset 1 by default. Here, you’ll see lists of recordings that were saved as “Data” files. If you saved your cardiac output and saline recordings correctly, they’ll appear within their respective SVcal and EFcal location, which can be accessed via the dropdown menu. If your cardiac output and saline recordings were not saved correctly, you may need to move these files to their appropriate location before proceeding.
Performing the SVCal Step
Begin with the SVCal (stroke‑volume calibration) file. Within the SVcal dropdown, select your cardiac output reference file—named for its measured value if you followed naming guidelines. In the “Cardiac Output Reference” column, enter the average flow rate (e.g., 4.02 L/min). Then, check the box beside the file so that the software populates the SVcal value under “Calculated Values”. This step aligns the global conductance signal to a known absolute flow, setting the stage for hypertonic saline‑based correction.
Analyzing EFCal Files
Within the EFcal dropdown, double‑click each hypertonic saline (EFcal) file to review its beat‑by‑beat conductance response. The software auto‑detects the saline‑induced volume increase, highlighting those beats in red. It then computes two correlation coefficients: EF, reflecting the rise in end‑diastolic volume, and R, reflecting the rise in end‑systolic volume. Targets for both metrics exceed 0.8 for reliable calibration. If either falls short, you can manually adjust the calibration range by adding or removing beats using the toolbar. Each edit triggers live recalculation, allowing you to fine‑tune the volume calibration range until EF and R achieve acceptable thresholds. Once optimized, check the box beside the file so that the software populates the EFcal value under “Calculated Values”
Finalizing Calibration and Verifying Results
Check that both SVcal and EFcal values appear in the “Calculated Values” section at the bottom of the calibration window. Remember, these values serve as calibration coefficients, not direct measures of stroke volume or ejection fraction. Click “Apply” to commit the volume calibration for the entire dataset. The software confirms success, and any file opened thereafter will display volume traces and PV loops in blue—visually indicating calibrated signals.
Preparing for Analysis and Future Datasets
Once the calibration is applied, all data files within the same dataset inherit the calibration automatically. If you plan to analyze post‑intervention loops—such as in Dataset 2 of a TAVR case—you must repeat the calibration process: record fresh SVcal and EFcal files, input the new external cardiac output, and apply a separate hypertonic saline correction. By compartmentalizing calibration per dataset, BiV Conduct NT ensures that each phase of your procedure rests on precise, context‑specific volume scaling.
Key Takeaways and Best Practices
Hypertonic saline calibration offers a robust means of correcting conductance‑derived PV loop volumes by combining an external cardiac output reference with at least two 5% saline bolus recordings. To optimize your workflow:
- Plan early – designate SVcal and EFcal file types during live acquisition and use clear naming.
- Inject strategically – perform saline boluses during end‑expiratory holds to minimize respiratory artefacts.
- Validate rigorously – aim for EF and R correlation coefficients above 0.8, editing beats as needed.
- Isolate datasets – recalibrate after each catheter redeployment.
By adhering to these guidelines, you’ll harness CD Leycom’s hypertonic saline method to produce accurate, reproducible ventricular volume measurements—transforming raw conductance signals into clinically meaningful insights.