FIELD

The present invention relates to a non-invasive method and apparatus for monitoring the respiration of a medical or surgical patient, in particularly using ultrasound.

BACKGROUND

Measurement and monitoring of respiration is essential to treatment of a wide range of medical conditions where mistakes have grave consequences for the patients and are associated with considerable economic cost for the society. The thoracic diaphragm is the main breathing muscle, and its dysfunction can be symptomatic of many respiratory disorders and conditions.

WO 2004/049951 discloses a respiration monitor comprising an ultrasound transducer array having a plurality of individual transducer elements positioned in an intercostal space so as to span at least part of the region of thoracic diaphragm movement of a patient. Because air has a much lower acoustic impedance than tissue, the reflection of the ultrasound beam is much more pronounced when the lung is insonated. By measuring the strength of the receiving signal, it is possible to determine the presence of the patient's lung, and hence the degree of inspiration by using several transducers located along the direction of the motion of the lower lung border.

In accordance with another existing technique, the motion of the diaphragm can instead be measured by conventional ultrasound imaging techniques. A beam of ultrasound pulses is aimed from a transducer onto the skin surface towards the diaphragm from below, via the liver. A distinct, strong echo can then be detected from the diaphragm since it is a smooth, specular surface. The variations in the distance between the ultrasound transducer and this echo can then be used for measuring the magnitude of the excursions. Instead of using only a narrow beam (M-mode), a real-time two-dimensional ultrasound image (B-mode) might also be used, with the additional advantage of securing a better anatomical orientation.

SUMMARY

It is desirable to provide further methods for monitoring of respiration.

The present invention provides a method of non-invasively monitoring the respiration of a patient comprising: transmitting ultrasound into the body toward an internal structure of the patient's body, the internal structure being one of the liver, the spleen or a kidney; selecting a depth range; measuring the phase of ultrasound echo signals from the internal structure at multiple points along the depth range for at least a first and a second echo signal, the first and second echo signals being received at different times; detecting the motion of the internal structure within the patient's abdomen by reference to differences in the measured phase between the first and the second echo signals; and thereby monitoring the respiration of the patient by associating movement of the internal structure with movement caused by respiration.

It has been found that movement of the internal structures closely mirrors movement caused by respiration, such as movements of the thoracic diaphragm, which in turn can be used to monitor the respiration of a patient. The diaphragm is the major muscle of inspiration, and continuous monitoring may support and add information to decision makers in a variety of settings therefore making it a “technology platform” for applications in respiratory diseases, and operating room to emergency room settings. Surprisingly, the movement of internal structures as in the first aspect can be reliably mapped to the patient's respiration. The depth range may be selected based on the internal structure of interest and/or based on prior assessment of the patient, including earlier non-invasive imaging, for example. For the liver a depth range of 2-5 cm may be selected. The depth range is selected to cover a sufficient section of the internal structure so that if for some reason the signal from a part of the range is weak, as might happen if the beam is aimed through a low echo region such as a blood vessel or a bile duct, then there will always be neighbouring tissue regions within the range with adequate echo intensity that can be used instead.

The internal structures of interest are composed of generally solid tissue and move generally as a solid body. They are not liquid or gas filled such as blood vessels, the gallbladder or intestines. They are further of a sufficient size that movement caused by respiration does not cause the tissue to move entirely out of a field of view. Ultrasound is non-invasive, effective and can be used for prolonged periods of time without harm to the patient. Thus, for example, a transducer could remain on the patient for hours, days or even weeks if necessary, while continuously monitoring respiration.

The size of the ultrasound sample volume along the beam direction is preferably in the range from 2 to 5 cm. This will improve the amplitude stability of the signal and avoid drop-outs where a smaller sample volume might happen to be completely inside a vessel or a bile duct.

The method may use an unfocused, or only slightly focused, ultrasound beam. Doing so increases the time of observation of individual scatter elements in the tissue when motion is in a direction that deviates from the direction of the sound beam, and will improve the accuracy of the estimated motion and velocity.

The motion of the internal structure is calculated based on the measured phase at the multiple points along the depth range and the differences in phase for at least two echo signals received at different times. This may be implemented by calculating the displacement of the tissue as an average along the ultrasound sound beam, where the observations of displacement at the multiple points along the depth range are weighted by their signal intensities before the averaged displacement is calculated. Note that in this situation the displacement is directly related to the phase and hence the phase difference can be considered as analogous to a displacement difference. The measured phases are monitored over time in order to identify the difference in phase with time between at least the first and second echo signal at different points on the depth range. The first and second echo signals may be consecutive signals or they may be spaced apart by other echo signals. The difference in phase with time may be determined over more than two echo signals. The multiple points along the depth range include at least two points providing an echo signal of sufficient strength, for example a strength over a given threshold. Three or more points may be used. It should be understood that the term “point” could in certain circumstances be construed as a short distance or range along the depth range. The method may comprise using interferometry to determine a phase of a returned ultrasound echo and cumulatively summing the phase shift between ultrasound measurements at different depths to determine a displacement of the structure.

The method may comprise determining a phase shift caused by motion at two or more locations along the depth range within the tissue and determining an average phase shift to determine the motion. Doing so will determine the motion with improved accuracy. The multiple locations may be independently measured using two or more transducers, or may be measured at different depths of an ultrasound beam from a single transducer. The average is preferably an intensity-weighted average of the multiple measurements as explained above. Solid tissue, such as the liver, has structural irregularities that scatter ultrasound and the echo received from the tissue will be a sum of individual contributions from such scattering elements. Since the sum is made up from individual vectors each with a phase and amplitude, the sum might occasionally become close to zero, creating a singularity characterised by loss of echo signal. When measuring tissue motion by analysis of phase variations, this becomes a problem. When the signal becomes close to zero, quite unpredictable phase variations might occur, causing lasting errors in the estimate of tissue position. This can be overcome by making multiple observations of phase variations from a range of locations, and computing an average phase difference weighted by the intensity of the signal.

The ultrasound beam from the transducer should be at a non-perpendicular angle (α) to the motion vector of the internal structure, i.e. the cranio-caudal direction of the patient. Preferably the angle is below 60° and more preferably below 45°.

The method may further comprise determining one or more derived respiration properties from the motion, which may include a respiratory pattern, a breathing rate, a tidal volume, or a motion amplitude.

The technique above may be particularly applicable to patients receiving support from a mechanical ventilator. Thus, in various embodiments, the patient may be receiving support from a mechanical ventilator or is undergoing a spontaneous breathing trial for removal of support from a mechanical ventilator.

The method may comprise setting initial or on-going operational parameters of the mechanical ventilator, based on the monitored respiration.

The method may comprise synchronising the operation of the mechanical ventilator with the breathing of the patient, for example a frequency or phase of pressure assistance or a general pressure assistance provided by the mechanical ventilator, based on the monitored respiration. In one embodiment, the operation of the mechanical ventilator may be controlled so as to provide pressure assistance responsive to detecting patient contribution.

Where the patient is undergoing a spontaneous breathing trial, the method may comprise reducing or removing mechanical ventilation support for a period of time, such as 5 to 30 minutes or 0.5-30 minutes, and monitoring respiration during this time to determine a likelihood of a successful spontaneous breathing trial. The method may further comprise determining, in preferably less than the duration of the trial, e.g. less than 25 minutes, that a likelihood of the patient successfully completing the spontaneous breathing trial is below a predetermined threshold and returning mechanical ventilation support before unnecessary complication of the patient's condition.

In further embodiments, the patient may be a trauma patient, a cardiac arrest patient, a spinal cord injury patient, a pulmonary patient, such as a COPD patient, a post-operative patient, or a patient under the influence of analgetic drugs known to negatively affect respiration.

The present invention also provides an ultrasound apparatus for non-invasively monitoring respiration of a patient, the apparatus comprising: at least one ultrasound transducer element for placing on the patient to aim at an internal structure of the patient's body; and a controller for controlling the ultrasound transducer element and processing ultrasound signals; wherein the controller is arranged to transmit ultrasound into the body toward the internal structure of the patient's body; measure the phase of ultrasound echo signals received from the internal structure at multiple points along a depth range selectable by a user; the phase being measured for at least a first and a second echo signal, the first and second echo signals being received at different times; detect the motion of the internal structure within the patient's abdomen by reference to differences in the measured phase between the first and the second echo signals; and thereby monitor the respiration of the patient.

The apparatus is for use with internal structures including the liver, spleen and kidneys and thus is arranged to process ultrasound echo signals from those internal organs in order to monitor the respiration of the patient by associating movement of those organs with movement resulting from respiration. There may be a single ultrasound transducer element acting to both transmit and receive the ultrasound, or alternatively multiple ultrasound transducer elements may be used. The apparatus can include an input device allowing a user to select a depth range.

The controller may be arranged to carry out method steps as discussed above. For example, the motion of the internal structure may be calculated based on the measured phase at the multiple points along the depth range and the differences in phase for at least two echo signals received at different times by calculating the displacement of the tissue as an average along the ultrasound sound beam, where the observations of displacement at the multiple points along the depth range are weighted by their signal intensities before the averaged displacement is calculated.

The apparatus may comprise a contact layer to be positioned between the transducer and the skin. The contact layer might be made from an adhesive material. The contact layer comprise an ultrasound contact gel, a glue, or an adhesive tape material that allows transmission of ultrasound, such as the sonolucent silicone tape described in WO2011/135288. Gel, glue and tape might also be used in various combinations. The contact layer may comprise removing protective covers that may be removed to expose the adhesive before the surface is brought in contact with the body.

The surfaces of the apparatus adapted to be in contact with the patient's body are configured to provide conformal contact with the surface of the body.

The apparatus may include at least one power source for powering the parts of the apparatus.

The transducer may be connected either by wire or by (short-range) digital or analogue radio communication to the processing circuitry, which may provide signals from the target tissue. The processing circuitry may be partly or fully digital.

The apparatus may comprise a monitor that provides information about the measured motion of the tissue (or a respiratory property derived from the motion) to an observer (e.g. a physician or the patient). The processing circuitry and/or the monitor may provide for control of the measurements performed. Such control may include calculation of parameters related to measurements.

Preferably, the apparatus is a portable device adapted such that a patient can wear the device during normal life. This allows continuous monitoring outside of a hospital environment and with a non-invasive and safe technology.

As above, the apparatus (and preferably the processing circuitry of the apparatus) may determine a displacement of the internal structure obtained by any one of: integrating a velocity of the internal structure detected using the Doppler Effect; interferometry analysis of the phase of the reflected wave; mapping of ultrasound speckles within the internal structure; and determining displacement of one or more anatomical landmark.

The apparatus can advantageously be used in a mechanical ventilation system along with a mechanical ventilator, and thus the invention extends to such a system, wherein the mechanical ventilator is for providing support to a patient and the apparatus is for non-invasively monitoring the respiration of the patient during support via the mechanical ventilator.

The mechanical ventilation system may be arranged to synchronise the operation of the mechanical ventilator with the breathing of the patient based on the monitored respiration and/or to control the operation of the mechanical ventilator so as to provide pressure assistance responsive to detecting patient contribution. The system may be arranged to carry out a spontaneous breathing trial by reducing or removing mechanical ventilation support for a period of time and monitoring respiration during this time to determine a likelihood of a successful spontaneous breathing trial.

