Transcutaneous Microcirculatory Imaging in Preterm Neonates

Summary

Microcirculatory imaging (MI) is used to monitor peripheral perfusion in critically ill or preterm neonates. This manuscript and video demonstrates the optimal approach for obtaining high-quality images.

Abstract

Microcirculatory imaging (MI) is a relatively new research tool mainly used in the intensive care setting. MI provides a clear view of the smallest capillaries, arterioles and venules. The magnifying effect visualizes the flow pattern of erythrocytes through these vessels.

It's non-invasive character makes it suitable to apply in (preterm) neonates, even in cardiorespiratory unstable patients. In adults and children, MI is mainly performed sublingually, but this is not possible in preterm infants as these cannot cooperate and the size of the probe is problematic. In preterm infants, MI is therefore performed transcutaneously. Their thin skin makes it possible to obtain high quality images of peripheral microcirculation.

In this manuscript we will demonstrate the method of transcutaneous MI in preterm infants. We will focus on the different techniques and provide tips to optimize image quality. The highlights of software settings, safety and offline analysis are also addressed.

Introduction

Hemodynamic diagnostics in critically ill preterm neonates has always been difficult. Most diagnostic tools used in adults cannot be applied in these tiny preterm infants; and then there is a problem of the sensitivity of the outcome parameters. But most of all, these infants are so vulnerable, that the risks of diagnostic procedures do not outweigh the benefits. As a result, in the field of neonatology, hemodynamics has been neglected and therefore there is a lack of knowledge on this topic.

An interesting option for handling these problems might be visualizing the microcirculation. The introduction of handheld microscopes in the late 1990s made it possible to visualize the microcirculation in a non-invasive manner. Three generations of devices have been introduced: Orthogonal Polarization Spectral (OPS) imaging¹, Sidestream Dark Field (SDF) imaging², and Incident Dark Field (IDF) imaging³. They all use more or less the same technique in which green light with a specific wavelength (548nm) stroboscopic illuminates the microcirculation. The green light is absorbed by oxy- and deoxyhemoglobin and mostly reflected by the surrounding tissue. This property of green light therefore creates visible contrast. The reflected light passes a magnification lens and is projected on a camera sensor. Hereby it is possible to visualize the flowing red blood cells at a depth of approximately one millimeter of mucosal tissue or directly at solid organs.

Over the past 15 years, the microcirculation has been mainly studied in adults, especially in patients with septic shock^4-6. These observational studies found that persistent microcirculatory alterations were associated with organ failure and mortality. This observation cannot be extrapolated directly to (preterm) infants however, as in the adults the microcirculation was measured sublingually. High quality images of the sublingual microcirculation cannot be obtained in preterm infants because they are unable to cooperate. In term infants the buccal microcirculation has been the area of interest⁷. Fortunately, in preterm infants the thin skin allows transcutaneous microcirculatory imaging. This approach has been applied in neonatal studies focusing on blood transfusion⁸, therapeutic hypothermia⁹ and hypotension¹⁰.

In this manuscript we present our protocol for transcutaneous microcirculatory imaging using Incident Dark Field imaging in preterm neonates. We will focus on different strategies to acquire the highest quality images. Technical details and differences between the SDF and IDF devices can be found elsewhere¹¹.

Protocol

This protocol follows the guidelines of the local human research ethics committee.

1. Preparation

1. Schedule the microcirculatory measurement so that it does not coincide with other procedures such as blood sampling. In term neonates it is best performed after feeding. This prevents agitation and will ease the measurement.
2. Ensure that a nurse or a parent attends to support and comfort the neonate during the examination, using the principles of Newborn Individualized Developmental Care and Assessment Program¹².
   Note: Although measurements can be performed by a single person, it is highly recommended to have a second person assist. One holds the camera and is focused on the neonate whilst the other operates the computer and software. In our experience, this results in higher quality images and a shorter duration of the procedure.
3. If the clinical condition of the neonate permits, place the neonate in supine position. Microcirculatory imaging can be performed in prone position, but this requires more skill and patience.
4. Make sure the body temperature of the preterm infant is within appropriate range (36.5 - 37.5 degrees Celsius).

2. Procedure

1. Install the device along the incubator. Make sure the incubator is at the right height.
2. Put the disposable cap on the camera.
3. Apply gel, oil or saline on the tip of the probe; this will help smoothen the contact between probe and skin.
4. Place the camera on the ventromedial side of the infants upper arm. To prevent focus-artefacts, make sure the probe is perpendicular to the skin. This may require repositioning of the infants arm.
   Note: The ventromedial side of the upper arm is the primary location to measure the cutaneous microcirculation. This location has little lanugo hair and is therefore less prone to artefacts. It is most easily reached if the patient is positioned in supine position.
5. To minimize the total length of procedure, gain time by finding the optimal depth of focus (Figure 3) while searching the location with the fewest artefacts.
   Note: Depth of focus depends primarily on postnatal age rather than gestational age. The average depth of focus in the first week of life is 0 - 80 µm. Hereafter, due to maturing of the skin, the focus depth rapidly increases with average values of 80 - 200 µm between 1 - 4 weeks of postnatal age (Figure 2). In term born neonates the average depth of focus is 80 - 160 µm at birth.
6. Stabilize the probe to avoid movement artefacts. To do so, rest the elbow on the incubator window and the wrist beside the neonate. Alternatively, position the probe alongside the neonate on a pillow.
7. Avoid pressure artefacts by letting the camera only have the slightest contact with the skin. Pressure artefacts can be recognized during image capturing if there is back-and-forth flow in vessels or if large vessels are non-perfused while there is good flow in small vessels. Also if the flow pattern is identical throughout the whole screen, beware of pressure artefacts.
8. Record videos for a minimum duration of 5 sec.
9. After a successful capture, move the camera to another spot on the upper arm.
   Note: It is recommended to capture in total 5 - 10 videos at 3 - 5 different locations, as some artefacts are only recognized at offline analysis, which means that the video in question is not usable for analysis.
10. Gently remove the gel, oil or saline from the skin with a small gauze.

3. Offline Analysis

1. Crop the video if there is a significant movement that impedes analysis. Go to the 'Tools' section and use the button 'Editor'. Select the frame interval eligible for analysis and click the 'Crop Video' button. Note: Videos are acceptable if movement is within ½ of the field view¹³.
2. Select the cropped video and stabilize it. Go to the 'Tools' section and use the button 'Analysis'. Click the 'Stabilize' button.
   Note: All movies must be stabilized before automatic analysis can be performed.
3. Select the stabilized video. Go to the section 'Analysis' and click the 'Detect' Button. Make sure the options 'Capillaries' and 'Vessels' are highlighted.
4. After detection (Figure 4), click the 'CNA' or 'De Backer' button for a full microcirculatory report. This report includes the most used outcome parameters like the total vessel density (TVD), perfused vessel density (PVD) and proportion of perfused vessels (PPV).
   Note: As an alternative, videos can be exported offline to be manually analyzed. This option can be found in the section 'Tools'. Select the option 'Export' and click the 'AVA export' button.

Results

Figures 1 and 2 show representative still images of high-quality MI videos. These examples demonstrate the difference in skin thickness in the same infants between day 1 (Figure 1) and day 28 (Figure 2) of postnatal age. On day 1, there is a bright illumination, adequate focus on the micro vessels and minimal presence of artefacts. On day 28 it is more difficult to find the right balance between focus on the micro vessels and artefacts du...

Discussion

In this manuscript we describe and demonstrate the approach for transcutaneous microcirculatory imaging in preterm neonates. Visualizing this method will help researchers overcome two of the biggest challenges in research: reproducibility and the time and labor intensive nature of learning new techniques. This technique can provide useful information of peripheral microcirculation in preterm infants in a non-invasive manner. Serial measurements can help clinicians evaluate the effects of therapeutic interventions. The mi...

Materials

Name	Company	Catalog Number	Comments
Cytocam	Braedius	http://www.braedius.com/magnoliaPublic/braedius/products.html	Other well known handheld microscopes to visualize the microcirculation are MicroScan (Microvision Medical) using SDF technique or the CytoScan (CytoMetrics) using OPS technique
Disposable Lens Cover	Glycocheck	http://www.glycocheck.com/lenscovers.php
CCTools	Braedius	http://www.braedius.com/magnoliaPublic/braedius/products.html	Another well known offline analysis programme is AVA (Microvision medical).