Summary
Background
Three-dimensional transoesophageal echocardiography (3D-TOE) is a new noninvasive tool for quantitative assessment of left ventricular (LV) volumes and ejection fraction.
Aim
The objective of this pilot study was to evaluate the feasibility and accuracy of 3D-TOE for the estimation of cardiac output (CO), using transpulmonary thermodilution with the Pulse index Contour Continuous Cardiac Output (PiCCO) system as the reference method, in intensive care unit (ICU) patients.
Methods
Fifteen ICU patients on mechanical ventilation prospectively underwent PiCCO catheter implantation and 3D-TOE. 3D-TOE LV end-diastolic and end-systolic volumes were determined using semi-automated software. CO was calculated as the product of LV stroke volume (end-diastolic volume − end-systolic volume) multiplied by heart rate. CO was also determined invasively by transpulmonary thermodilution as the reference method.
Results
Among 30 haemodynamic evaluations, 29 (97%) LV 3D-TOE datasets were suitable for CO calculation. The mean 3D-TOE image acquisition and post-processing times were 46 and 155 seconds, respectively. There was a correlation ( r = 0.78; P < 0.0001) between PiCCO and 3D-TOE CO. Compared with PiCCO, the 3D-TOE CO mean bias was 0.38 L/min, with limits of agreement of −1.97 to 2.74 L/min.
Conclusions
Noninvasive estimation of CO by 3D-TOE is feasible in ICU patients. This new semi-automated modality is an additional promising tool for noninvasive haemodynamic assessment of ICU patients. However, the wide limits of agreement with thermodilution observed in this pilot study require further investigation in larger cohorts of patients.
Résumé
Contexte
L’échocardiographie trans-œsophagienne tridimensionelle (ETO-3D) est une nouvelle modalité non invasive d’évaluation des volumes et de la fraction d’éjection du ventricule gauche (VG).
Objectif
Évaluer la faisabilité et la performance de l’ETO-3D comparativement à la thermodilution transpulmonaire par méthode PiCCO pour la mesure du débit cardiaque (DC).
Méthodes
Dans cette étude pilote, 15 patients sous ventilation mécanique admis en réanimation et bénéficiant d’un monitorage hémodynamique invasif par le système PiCCO ont été prospectivement évalués par ETO-3D. Les volumes télé-diastolique et télé-systolique du VG ont été mesurés en utilisant un logiciel semi-automatique spécifique. Le DC a ensuite été calculé en multipliant le volume d’éjection systolique du VG (volume télé-diastolique − volume télé-systolique) par la fréquence cardiaque. Le DC a également été mesuré de façon invasive par thermodilution transpulmonaire.
Résultats
Parmi les 30 évaluations hémodynamiques effectuées, 29 (97 %) acquisitions ETO-3D étaient exploitables. Les temps moyens nécessaires pour l’acquisition et l’analyse des données ETO-3D étaient respectivement de 46 et 155 secondes. Les mesures de DC effectuées par ETO-3D et par méthode invasive étaient corrélées ( r = 0,78 ; p < 0,0001). Le biais moyen entre les 2 méthodes de mesure était de 0,38 L/min, les limites d’agrément étaient de −1,97 à 2,74 L/min.
Conclusions
L’évaluation non invasive du DC par ETO-3D est faisable. Cette nouvelle modalité ultrasonore est un outil prometteur pour l’évaluation hémodynamique des patients admis en réanimation. Les limites d’agrément relativement larges observées dans cette étude pilote comparativement à la theromdilution nécessitent toutefois d’être évaluer sur de plus larges populations de patients.
Background
Cardiac output (CO) measurement is an important variable required for the haemodynamic management of critically ill mechanically ventilated patients. Thermodilution by the Swan-Ganz catheter is still considered as the reference method in the clinical setting . More recently, transpulmonary thermodilution was established as an alternative to the pulmonary catheter . However, this method is also invasive, and requires artery and central venous catheterization, which can lead to severe complications .
Transoesophageal echocardiography (TOE) is increasingly used for the haemodynamic management of patients admitted to the intensive care unit (ICU) . While the transthoracic examination of mechanically ventilated patients may be challenging, TOE is safe, and provides accurate heart imaging in most of these patients . TOE training is an essential part of advanced critical care echocardiography learning .
Recently, a new generation of TOE probes has been introduced, allowing real-time three-dimensional (3D) imaging of the heart . Currently, 3D-TOE is available for routine practice in several echocardiography machines, integrating both traditional modalities and 3D imaging. In addition, image acquisition has been simplified, and recent software that uses a semi-automated approach has dramatically reduced the 3D dataset off-line post-processing time .
The accuracy of 3D-ultrasound for left ventricular (LV) systolic and diastolic volume measurements and for LV ejection fraction calculation is superior to a conventional two-dimensional (2D) approach when using magnetic resonance imaging as the gold standard method . However, the value of this technology for the estimation of CO in the ICU setting has been poorly investigated. This new semi-automated ultrasound modality, offering the advantage of providing the most reliable ultrasound quantitative assessment of LV ejection fraction, may have a suitable role in CO assessment, especially in cases where the conventional ultrasound Doppler method is not applicable (e.g. beam misalignment).
The main objective of this pilot study was to evaluate the feasibility and accuracy of 3D-TOE for the estimation of CO, using transpulmonary thermodilution as the reference method, in ICU mechanically ventilated patients.
Methods
Patients
Fifteen ICU patients on mechanical ventilation who had received a Pulse index Contour Continuous Cardiac Output (PiCCO; PULSION Medical Systems, Munich, Germany) haemodynamic monitoring device were prospectively included in the study. Exclusion criteria were: age < 18 years; non-sinus rhythm; contraindication for TOE ; tricuspid, aortic or mitral valve regurgitation > 2/4; extracorporeal membrane oxygenation support; and mechanical mitral valve prosthesis.
In addition to haemodynamic measurements, the following data were recorded: age; sex; simplified acute physiology score (SAPS II) ; and primary reason for ICU admission.
All patients were on continuous intravenous sedation combined with neuromuscular blocking agents during haemodynamic and TOE measurements.
The study was approved by the institutional committee for human research at our institution. Written informed consent was obtained from an appropriate designee for each patient before participation.
Invasive CO measurements
A 5F thermistor-tipped catheter was placed in the femoral artery and connected to the PiCCO system. From a central venous catheter positioned in the internal jugular or subclavian vein, 20 mL of cold saline solution (< 8°) were injected using the distal lumen, and CO was calculated with the transpulmonary thermodilution method ; the mean of three consecutive CO measurements was used. PiCCO measurements were performed independently by an intensivist who was unaware of the echocardiography results.
TOE
First, the ultrasound oesophageal probe was introduced. Immediately after PiCCO measurements, TOE examination was acquired in all patients by two experienced physicians (> 1000 examinations) using the IE33 system (Philips Medical Systems, Andover, MA, USA), equipped with a 3D-TOE probe (X7-2t). The images were transferred to a workstation equipped with QLAB software version 8.1 for post-processing. All the examinations were analysed off-line, blinded to invasive measurements. All projections were obtained according to echocardiography guidelines . All measurements were averaged over three cardiac cycles. Ectopic and post-ectopic beats were disregarded.
Doppler method
From a 120° mid-oesophageal two-dimensional long-axis zoom of the aortic valve, LV outflow tract diameter was measured. Using pulsed-wave Doppler mode, the LV outflow tract time-velocity integral was recorded from the transgastric view. Guided both by the 2D image and colour Doppler, the Doppler sound beam was placed as parallel as possible to the LV outflow tract flow. CO was calculated as recommended (Doppler CO) .
3D method
From a 2D LV mid-oesophageal four-chamber view at 0°, the real-time biplane imaging modality (X-plane mode) was used to obtain a simultaneous visualization of two orthogonal LV views, and therefore to adjust the probe position and acoustic window. During a temporary interruption of ventilator support at end-expiration (5–10 seconds, depending on the patient’s heart rate), a 3D LV full-volume dataset was then reconstructed, using the R-wave gated method over four to six consecutive cardiac cycles, and acquired. Sector size and depth were adjusted to achieve optimal visualization of the LV at the highest possible frame rate (mean value 28 ± 5 frames/s). During post-processing, the full-volume LV dataset was organized off-line into four-chamber, two-chamber and short-axis views. Mitral annular and apical points were placed manually on these images in end-diastole and end-systole. LV endocardial borders were then detected automatically by the software, and adjusted manually if needed. The software then used sequence analysis to track the endocardium in all frames, and to determine LV end-diastolic volume, LV end-systolic volume and LV ejection fraction. CO was calculated as the product of LV stroke volume (end-diastolic volume − end-systolic volume) multiplied by heart rate ( Fig. 1 ).
