[ 0001 ] The present invention relates to Doppler ultrasound monitoring of a patients total cardiac output and in particular to transoesophageal Doppler probes for use during surgery and anaesthesia.

[ 0002 ] The invention has been developed primarily for use as a transoesophageal Doppler probe with improved spatial stability with respect to a monitored blood flow for more accurate determination of a patient's total cardiac output and will be described hereinafter with reference to this application. However it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND OF THE INVENTION

[ 0003 ] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field. [ 0004 ] Doppler ultrasound is an established clinical method of measuring blood flow in animals and humans. The ultrasound signal is usually acquired using a transducer placed on the skin surface, from either the apical, parasternal or suprasternal acoustic access, or transoesophageal access, where the transducer is located at the end of an endoscope probe and placed down the subject's throat to lodge in the oesophagus or stomach as depicted in Fig. 1. This design is primarily for imaging the heart during surgery and is usually for monitoring cardiac morphology and ventricular size. Another iteration is for evaluation of blood flow in the descending thoracic aorta (DTA), where the transducer is turned away from the heart to access blood flow in the DTA. Using a known angle of insonation, an estimation of DTA cross sectional area and assuming a fixed proportion of CO flowing down the DTA (usually 70%), then a calculation of CO can be made.

[ 0005 ] Example transoesophageal ultrasound probes are disclosed in US Patent 4757821 to Snyder, US Patent 6645149 to Smith and PCT Application PCT/GB93/00856 to Tregoning.

[ 0006 ] Despite the general acceptance of the transoesophageal procedure among most cardiac anaesthetists and surgeons, it is significantly limited by the inability of the ultrasound transducers to maintain a constant spatial relationship with the Descending Thoracic Aorta (DTA). The orientation of the ultrasound beam moves with the transducer within the oesophagus producing a variation in the measured ultrasound signal and calculated flow volume and rendering the method inaccurate. This source of significant error in practice is understandable with even a rudimentary knowledge of oesophageal anatomy. The oesophagus is a hollow muscular tube which is a conduit for ingested materials from the mouth to the stomach. When empty the oesophagus is collapsed and characterised by mucosal folds 32 as demonstrated in Fig. 2.

[ 0007 ] Additionally, the muscular tube functions to propel ingested material using waves of contraction which deform the oesophageal cross section. These peristaltic waves cause the ultrasound transducer to move within the oesophagus and, of course, in relation to the DTA, resulting in changes in the detected Doppler signal which has no relation to changes in the patient's haemodynamics.

[ 0008 ] In the design of currently used probes, the diameter of the endoscope has progressively been reduced for ease of insertion and to reduce the risk of damage to the oesophagus. However, as the probe diameter is reduced, there is an increased tendency for the probe, and hence, the transducers, to move within the oesophageus. This increased movement increases the variability of the spatial relationship between the probe and the DTA and hence increases the noise in the Doppler signal.

DISCLOSURE OF THE INVENTION

[ 0009 ] It is an object of the invention in its preferred form to provide a transoesophageal Doppler echocardiography probe with improved spatial stability for more accurate signal collection.

[ 0010 ] According to a first aspect of the invention there is provided a transoesophageal Doppler probe including: an elongate flexible endoscope for insertion into the oesophagus of a patient; a transducer for the transmission and reception of ultrasonic signals at the distal end of the endoscope;

securing means for securing the distal end of the probe in the patient's oesophagus thereby to retain the probe in a substantially constant spatial relationship with the descending thoracic artery of the patient.

[ 0011 ] Preferably, the distal end of the probe is secured substantially between mucosal folds of the patient's oesophagus. Preferably, the securing means is an inflatable variable size balloon. In other embodiments the securing means is a dilated portion located at the distal end of the endoscope. Preferably, the dilated portion is detatchable, such that one of a plurality of dilated portions may be selectively attached to the endoscope in accordance with requirements, particularly in relation to the age and oesophageal dimensions of the patient.

[ 0012 ] Preferably, the transducer is a plurality of multi-element phased-array transducer.

[ 0013 ] According to a second aspect of the invention, there is provided a method of measuring blood flow in a patient including the steps of: inserting a transoesophageal probe into the oesophagus of a patient, the probe including a ultrasonic transducer at the distal end; aligning a beam of ultrasonic signals from said transducer at the distal end of the probe substantially with the descending thoracic aorta of the patient; securing the distal end of the probe in the patient's oesophagus; measuring the blood flow velocity in the patient's descending thoracic aorta from reflected Doppler-shifted ultrasonic signals; and utilising the Doppler-shifted signal to determine said patient's total cardiac output.

[ 0014 ] Preferably, the patient's total cardiac output is monitored substantially continuously.

[ 0015 ] Preferably the distal end of the probe is secured in the mucosal folds of the patient's oesophagus by a remotely inflatable balloon located at the distal end of the probe.

BRIEF DESCRIPTION OF THE DRAWINGS

[ 0016 ] A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[ 0017 ] Fig. 1 is a cross-sectional view of a human patient showing a transoesophageal Doppler probe inserted into the patient's oesophagus according to the invention;

[ 0018 ] Fig. 2 is a top MRI cross-sectional view of the patient taken generally along line 2-2 of Fig 1, showing the mucosal folds of the oesophagus and their relationship to the descending thoracic artery;

[ 0019 ] Fig. 3 is a side view of a first embodiment of the invention including a dilated portion on the tip of the probe;

[ 0020 ] Fig 4 illustrates a side sectional view of a second embodiment having an inflatable balloon around the distal end of the probe, the balloon shown in a deflated form;

[ 0021 ] Fig. 5 illustrates a side sectional view of the second embodiment with the balloon shown in an inflated position;

[ 0022 ] Fig 6 illustrates a top sectional view of a second embodiment inside the oesophagus, the balloon shown in a deflated form;

[ 0023 ] Fig. 7 illustrates a top sectional view of the second embodiment inside the oesophagus, with the balloon shown in an inflated position; [ 0024 ] Fig. 8 is a side view of a third embodiment of the invention including an inflatable balloon at the tip of the probe, where the balloon is shown in the deflated position; and

[ 0025 ] Fig. 9 is a side view of the embodiment of Fig. 8 where the balloon is shown in the inflated position in communication the patient's oesophagus. DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

[ 0026 ] With reference to the drawings, Fig 1 shows the transoesophageal Doppler probe 10 which includes an elongate flexible endoscope 12 for insertion into the oesophagus 3 of a patient 5. A transducer 14 for the transmission and reception of ultrasonic signals is incorporated into the tip of the endoscope 12. The endoscope is flexible and typically provides anterior, posterior and lateral flexion of the tip for

guiding the endoscope during insertion, and positioning of the transducers 14 and aiming of the beam 20 when the endoscope is in place. In preferred embodiments, the transducer incorporates a plurality of multi-element phased-array transducers.

[ 0027 ] From the oesophageal access the ultrasonic beam 20 from the transducers 14 is aligned with major flow of the Descending Thoracic Aorta (DTA) 30 shown in Fig. 2 to measure blood flow velocity through the DTA. During the alignment, the tip of the endoscope is positioned within the mucosal folds 32 of the patient's oesophagus 3.

[ 0028 ] The detected signal from the ultrasonic beam reflected from the DTA is then optimised and the transducers 14 are secured in place by a securing means located in the tip of the endoscope 12. The securing means secures the probe in the patient's oesophagus substantially between the mucosal folds 32. This ensures that the transducers 14 in the tip of the probe maintain a substantially constant spatial relationship with the patient's descending thoracic artery 30, and thereby substantially eliminating the error in the Doppler signal resulting from movement of the probe within the patient' s oesophagus 3.

[ 0029 ] The securing means may be an endoscope with a dilated portion 40 at the tip of the probe as shown in Fig. 3. The dilated portion may be interchangeable to enable one of a plurality of similar portions of varying dimensions to be attached to the probe 12. The different sized portions may then be chosen in accordance with differences between patients of different ages and oesophageal dimensions.

[ 0030 ] In other embodiments, the securing means can comprise a balloon structure formed around the tip of the probe. One such arrangement is illustrated in Fig. 4 wherein the probe 12 is surrounded by a balloon 50 which is deflated upon insertion into the oesophagus. Once appropriately positioned, fluid is pumped into the balloon 50 via tube 51. The balloon is thereby inflated as shown in Fig. 5, with the inflation holding the balloon in a stable position within the oesophagus. The difference being illustrated in Fig. 6 and Fig. 7, with Fig. 6 illustrating a sectional view through the oesophagus 51 showing the balloon 50 in a collapsed state and Fig. 7 illustrating a sectional view with the balloon in an expanded state. [ 0031 ] In other embodiments the securing means is an inflatable balloon 44 mounted in the tip of the probe as shown in Figs 4 and 5. The inflatable balloon is remotely inflatable and its inflated volume may be varied by the operator in accordance with the

oesophageal dimensions of the patient. The balloon 44 is typically inflated by the introduction of water or air through the body of the endoscope. An advantage of the inflatable balloon securing method is that the probe may be inserted into the patient with the balloon in the deflated state. This increases the ease of the insertion and decreases the amount of discomfort to the patient. After insertion and once the orientation of the ultrasound beam 20 has been optimised, the balloon 44 may then be inflated to secure the tip of the probe into position. Removal of the probe after the procedure may be facilitated by first deflating the balloon 44 to reduce the amount of discomfort to the patient and minimise the risk of injury to the patient by the probe. [ 0032 ] Once the transducers 44 have been secured in place in the patient's oesophagus, the patient's total cardiac output is continuously monitored. The reflected Doppler shifted ultrasonic beam from the DTA as detected and, using an algorithm based on the height, weight and age of the patient, an estimate is made of the cross sectional area of the blood flow in the DTA to calculate descending thoracic aortic flow volumes. An alternative method uses continuous real time M-Mode imaging to measure the DTA diameter from a flow volume calculation. An assumption is made that this flow volume represents a constant proportion of total Cardiac Output (CO), such that the DTA signal may be multiplied to calculate true CO. For example, if it is assumed that DTA flow represents 70% of true CO, then the DTA flow volume is multiplied by 100/70 to determine an estimate of CO. This method has been validated for measuring CO at baseline and after interventions, and for optimising stroke volume (SV) in a variety of conditions.

[ 0033 ] It will be appreciated that the illustrated transoesophageal Doppler probe may be used for improved spatial stability with respect to a monitored blood flow for more accurate signal collection.

[ 0034 ] Although the invention has been described with reference to a specific example, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.