During various neurosurgical procedures on the head and neck, the patient is frequently placed in the sitting position in order to improve surgical exposure and drainage of blood away from the operative site. Despite its advantages, the sitting position introduces an unavoidable complication of air emboli, or intravascular air bubbles, as air enters open veins of the head and neck to be carried by the blood to the heart and lung. In sufficient amounts, air emboli can cause cardiac failure, hypotension, hypoxia and pulmonary edema.
It has been estimated that approximately 10,000 patients per year are exposed to the risk of air embolism as a result of undergoing neurosurgical operations while in the sitting position. The total incidence of air embolism in this position is about 20-30%. In cases where air embolism does occur, the incidence of severe embolism resulting in hypotension and hypoxia may be as high as 78%. In addition, it has been noted that there is a significant incidence of air embolism during any operation in which there is a major hydrostatic gradient between the operative site and the heart, i.e., the operative site is above the heart, whether the patient is in the sitting, supine, prone or lateral positions.
Treatment of air emboli may be effected: by aspirating, or withdrawing, the air through a catheter which has been pre-operatively introduced through a peripheral vein and advanced so that its tip is in the right atrium of the heart; by compression of the patient's jugular veins; by continuously flushing the wound with a saline solution; by detecting and blocking any open venous channels in the surgical wound; by application of 100% oxygen to the patient; or, by the administration of a vasopressor or an antiarrhythmic drug to the patient. These prophylactic methods are most effective when a diagnosis of air emboli is made at a time when only small amounts of air have entered the circulatory system.
The ability of the physician to make an early diagnosis of air emboli has been greatly facilitated by the use of an ultrasound instrument incorporating a precordial Doppler probe. Typically, such a probe includes a housing having a circular disk through which a transmitting tranducer and a receiving transducer located within the housing can respectively transmit and receive ultrasonic energy. The transducers are coupled with a circuit which applies a high frequency electrical signal (typically in the mHz range) to the transmitting transducer to cause the transmission of ultrasonic energy thereby in a fairly broad beam. Electrical output signals from the receiving transducer, resulting from returns of the transmitted ultrasonic energy from objects within the beam, are compared with the high-frequency electrical signal applied to the transmitting transducer to develop a Doppler signal representative of any frequency shift caused by relative movement between an object and the probe. The Doppler signal is then amplified and applied to a loudspeaker for audible reproduction.
In the use of the precordial Doppler probe, the disk of the probe is coated with a conductive jelly, and then placed on the patient's precordium and positioned until characteristic Doppler sounds produced by blood flow and vessel wall movement of the patient's heart are heard. Once the probe is positioned, the passage of even minute amounts of air through the patient's heart is accompanied by a characteristic Doppler sound that is higher in frequency and amplitude than the normal heart sounds, so diagnosis of air emboli can be made.
Although the use of ultrasound instruments incorporating a precordial Doppler probe has greatly improved the ability of the physician to diagnose air emboli, the technique suffers from several difficulties. Since the precise region of the patient's body being ensonified with ultrasonic energy is not known to the physician, the physician can not be sure that only a portion of the right side of the heart is being monitored. As a result, air emboli may pass through the right side of the heart without being detected. The problem of proper location of the transducer probe is intensified in obese patients and in heavy-breasted women, where returning ultrasonic energy is significantly attenuated by subcutaneous tissue.
Once the probe is positioned, it is then taped or strapped to the precordium. It is very difficult in many instances to secure the position of the probe on heavy-breasted women and obese patients, or when the patient is in the lateral or prone positions. Occasionally during pre-operative and operative procedures, the probe may be dislodged or may slip from position, especially when the patient is in the lateral or prone positions. A change of probe position may not be detectable, since characteristic heart Doppler sounds may continue to be produced, but may result in the loss of the physician's ability to detect the Doppler sounds characteristic of air emboli.
In addition to the aforementioned problems of properly positioning and securing the position of the precordial Doppler probe, which may result in the inability of the physician to diagnose air emboli, the use of a precordial Doppler probe is disadvantageous in that the Doppler sounds produced thereby which are characteristic of air emboli cannot be quantified, that is, there is no way for the physician to determine the amount of air that is passing into the right side of the heart over a given period of time. Because of difficulties in positioning the precordial Doppler probe, the ultrasonic beam therefrom is necessarily fairly broad so that air throughout the right side of the heart may be detected. Quite frequently, air emboli become entrapped in the right ventricle before passing on to the lungs. If an attempt is made to quantify the Doppler sounds that are characteristic of air emboli, the resultant determinination will include not only a component related to the amount of air entering the right side of the heart, but also a component related to the amount of air entrapped in the right ventricle. Therefore, since the physician is unable to determine how much air is entering the heart over a given period of time, he is likely to apply the various prophylactic methods discussed even in situations where such methods are not called for due to the small amount of air entering the venous system.
As previously discussed, one of these prophylactic methods is aspiration through an intravascular catheter. In order to be effective for aspiration, the tip of the catheter preferably should be located in the right atrium. The catheter tip is frequently located by X-ray equipment, or by using the catheter as an ECG lead and monitoring the patient's ECG for a P-wave change characteristic of the atrial position. These methods are time consuming, involve extra equipment, and typically are used only before an operation. There are many occasions when the catheter tip migrates away from the right atrium, either pre-operatively or operatively, when the patient is moved.
It is therefore an object of this invention to provide an improved method for the detection of air emboli in blood vessels.
It is a further object of this invention to provide such a method and apparatus which utilize ultrasonic energy.
It is another object of this invention to provide such a method which utilizes a catheter on which is located an ultrasonic transducer, which catheter is positioned within a body in proximity to a blood vessel.
It is yet another object of this invention to provide such a method which can be used to reliably position the catheter within the body by monitoring electrical signals from the transducer on the catheter.
It is still another object of this invention to provide a method which can be used to properly position an intravascular catheter in a blood vessel for aspiration of air emboli therefrom.
It is also an object of this invention to provide a method which provides for the early and reliable diagnosis of air emboli in blood vessels, notwithstanding the pre-operative or operative position of the patient or the patient's bodily characteristics.