1. Field of the Invention
The present invention generally relates to a method of and an apparatus for detecting atrial fibrillation by monitoring and analyzing pulse beats using an algorithm that can exclude many rhythm abnormalities that are not atrial fibrillation.
2. Discussion of the Related Art
The heart is the major muscle that functions as the primary pump for blood flow throughout the body. The heart contains two upper chambers called atria and two lower chambers called ventricles. The right atrium receives oxygen-depleted blood while the left atrium receives blood enriched with oxygen from the lungs. When the atria are full, the outlet valves within the heart open and the atria squeeze blood into the ventricles. The right ventricle then pumps oxygen-depleted blood to the lungs while the left ventricle pumps oxygen-enriched blood to all parts of the body. In this fashion, the heart functions primarily as a double sided pump.
The heart's internal pacemaker, known as the sinus node, signals the start of each heart beat. This signal originates in the right atrium in the sinoatrial node and travels simultaneously to the left atrium and down to the interatrial septum to the atrioventricular node. The cycle of electrical stimulation that normally occurs is referred to as normal sinus rhythm. The contraction of the ventricles will be referred to as the heart beats.
Many rhythm abnormalities may be present. Atrial fibrillation is one rhythm abnormality in which the atria do not contract normally. Instead, there is a continuously varying pattern of electrical activation of the atria resulting in a rapid highly irregular pattern of impulses reaching the atrioventricular node. The atrioventricular node acts as a filter and allows a reduced number of these impulses to reach the ventricles which results in a highly irregular heartbeat pattern. This irregular pattern has been shown in previous studies to be a random pattern (Bootsma et al: Analysis of R-R Intervals in Patients with Atrial Fibrillation at Rest and During Exercise. Circulation 41: 783, 1970). Whenever the term “irregular’ is used in this application it refers to this random pattern of beats found almost exclusively in atrial fibrillation.
Atrial fibrillation is one of the most common arrhythmias requiring medical attention. Atrial fibrillation may be caused by a number of heart conditions, such as angina, myocardial infarction, heart valve abnormalities, and high blood pressure. These conditions may stretch or scar the atria, thereby causing irregularities in the heart system. Atrial fibrillation may also accompany lung problems or thyroid gland disorders and is also associated with significant morbidity and possible mortality. All persons, young and old, female or male, including the visually and/or sight impaired, may experience atrial fibrillation.
Atrial fibrillation may occur intermittently or chronically. The most serious complication of atrial fibrillation is formation of a blood clot in the left atrium which may result in a stroke. Many people who develop atrial fibrillation, however, are unaware of their abnormal rhythm. Some in the medical profession have, therefore, advocated self screening of the pulse to detect for the possible occurrence of atrial fibrillation. The literature, however, is generally limited to disclosing instructions for manually taking one's pulse accompanied with additional descriptive information.
The reason for using the pulse to detect atrial fibrillation is that the pulse usually corresponds to the heartbeat. The contraction of the left ventricle ejects blood from the heart into the aorta and the resulting pressure wave is detected as a pulse in the arterial system. However, when atrial fibrillation is present, the amount of time between beats varies irregularly.
With a longer time interval between beats, there is more time to fill the ventricles with blood and more blood is ejected by the ventricle in the beat following this long interval. This larger volume of blood in the aorta results in a higher systolic pressure for that beat.
Conversely, when the time interval between beats is short, there is less time for ventricular filling and the volume of blood ejected in the beat following the short time interval is less. This results in a lower systolic pressure for that beat. In some cases, the time interval between beats is so short that the systolic pressure of the following beat is so low that it cannot be palpated as a pulse. A ventricular contraction that cannot be palpated as a pulse in the arterial system results in what is called a “pulse deficit.” This is very common in atrial fibrillation. This pulse deficit means that an irregular pattern of heartbeats in the ventricle may result in a less irregular pulse beat pattern since the shortest intervals between heartbeats may not be detected in the pulse. Therefore, any method used to determine the presence of atrial fibrillation by analyzing the time intervals between beats in the ventricles may not be valid when applied to the pulse beats.
An article by Bert K. Bootsma, Adriann J. Hoelen, Jan Strackee and Frits L. Meijler, entitled Analysis of R-R Intervals in Patients with Atrial Fibrillation at Rest and During Exercise, Circulation, Volume XLI, May 1970 describes an analysis of the time intervals between ventricular contractions using the electrocardiogram. The article evaluates the standard deviation divided by the mean of the time intervals between ventricular beats in normal subjects and in those with atrial fibrillation. The article finds that atrial fibrillation can be accurately differentiated from normal sinus rhythm using this formula. However, this was based on ventricular contractions determined by the electrocardiogram and was not applied to the pulse beat intervals.
Due to the presence of a pulse deficit in atrial fibrillation, results based on ventricular contractions determined by the electrocardiogram may not apply to time intervals determined from analyzing the pulse. Furthermore, the extent of the pulse deficit depends on the method used to determine the pulse beats. A method which detects only pulse beats with high systolic pressures will detect fewer pulse beats compared to a more sensitive method. The more sensitive techniques may be better for detecting more pulse beats but they may also give more false positive readings.
For example, with a photoplethysmograph using a finger probe with a light source and a photoelectric detector, when the sensitivity of the device is increased, the slightest finger movement is detected as a pulse beat. This device at the highest sensitivity setting detects an irregular pulse in those with normal sinus rhythm due to random noise from finger movement. At the highest sensitivity setting, this device would not be useful to detect atrial fibrillation in the home setting. At the lowest sensitivity setting, very few pulse beats would even be detected. Therefore, any device and algorithm which uses the pulse to detect atrial fibrillation must be designed specifically for the purpose of detecting atrial fibrillation.
Other rhythm abnormalities that are not associated with an increased risk of stroke are found very commonly. For example, premature atrial contractions or premature ventricular contractions are found in many people, even those with no heart conditions. These rhythm abnormalities are intermittent but they are found much more commonly than atrial fibrillation and generally do not require treatment. Therefore, a device that is designed to detect atrial fibrillation should include an algorithm that would not detect premature beats as an irregular rhythm. If a device designed to detect atrial fibrillation does not include an algorithm to reduce the detection of premature beats, the number of false positive readings would be very high and the device would be of very limited value. Those who might use such a device on a regular basis to detect asymptomatic atrial fibrillation would quickly stop using this device if the vast majority of abnormal readings turn out to be due to premature beats and not atrial fibrillation.
U.S. Pat. No. 6,485,429 describes a method and apparatus that can detect arrhythmia using an oscillometric blood pressure monitor. This patent discloses a method for detecting multiple rhythm abnormalities including supraventricular premature contractions, ventricular premature contractions, atrial fibrillation, paroxysmal supraventricular tachycardia, sinus tachycardia, and ventricular bradycardia. A study performed by the inventor of the patent using a commercially manufactured embodiment of the patent found that the device only detected 66.6% of patients with atrial fibrillation while detecting 85.7% of premature ventricular contractions (Forstner K W, American Society of Hypertension 16th Annual Meeting 2003, page 25). Premature ventricular contractions are found much more commonly than atrial fibrillation. Therefore a device that has a very high rate of detecting premature ventricular contractions and a significantly lower rate of detecting atrial fibrillation would not be a very useful device if the goal is to detect atrial fibrillation exclusively.
Other patents that can detect rhythm abnormalities would also be subject to very high false positive rates if they are used to detect atrial fibrillation. U.S. Pat. No. 6,095,984 describes an embodiment (section 1-2-1) where a pulse variation over 0.5% would be detected as abnormal. The most common rhythm abnormality, sinus arrhythmia, by definition (Braunwald, E. Heart Disease A textbook of Cardiovascular Medicine 1992, p 674) has a heart rate that varies by more than 10%. U.S. Pat. No. 6,095,984 did not describe methods and apparatus to look for atrial fibrillation and would not be useful for home monitoring of atrial fibrillation, because the algorithm can result in multiple sources of false readings and the apparatus was not specifically set for optimal detection of pulse beats in atrial fibrillation.
An algorithm designed to detect atrial fibrillation by the irregularity of the pulse beat intervals should be designed to reduce the effects of premature beats. One method of reducing the effect of premature beats is to limit the number of beats used in determining the irregularity of the pulse intervals. For example, if premature beats occur on average every twenty beats, then limiting the analysis to only ten beats would reduce the likelihood of a premature beat occurring during the period being analyzed. This method was used in a trial and published in Wiesel et al, PACE, 27:639-643 (2004). In this study, the rhythm of 450 outpatients of which 54 were documented to have atrial fibrillation was analyzed. The study used an algorithm that calculated the irregularity index, defined as the standard deviation of the time intervals between beats divided by the mean of the time intervals between beats, and analyzed only the last ten beats recorded by an automatic oscillometric blood pressure monitor. All recordings with an irregularity index greater than 0.06 were considered to be irregular. This study found that 100% of patients with atrial fibrillation were detected while 16% of patients without atrial fibrillation were also found to be irregular (sensitivity for detecting atrial fibrillation 100%, specificity 84%). This is a marked improvement in the rate of detecting atrial fibrillation over the method used by Forstner.
Though the sensitivity for detecting atrial fibrillation using the irregularity index applied to the last ten beats recorded by an automatic blood pressure monitor is excellent, there is room for improvement in specificity. Premature beats are common rhythm abnormalities that can reduce the specificity of the irregularity index. For example, a ventricular premature beat occurs earlier than the normal beats but results in a pause following the beat. A premature beat usually results in a lower pulse pressure than the normal beats. This premature beat with its lower pulse pressure would often not be recorded by the blood pressure monitor at all. As a result of this premature beat, there would be a pause equal to the time interval between two normal beats. For a patient with a heart rate of 60 beats per minute, the pulse interval would be one second between normal beats. A premature ventricular beat would result in a pulse interval of two seconds as measured by a blood pressure cuff. The irregularity index for nine time intervals between beats of one second and one interval of two seconds due to one premature ventricular beat would be equal to 0.29, well above the threshold value of 0.06. If the premature beat occurs 0.5 seconds following a normal beat and it is detected by the blood pressure monitor, then there will be a short interval followed by a long interval. With eight intervals of one second, one interval of 0.5 seconds and one interval of 1.5 seconds, the irregularity index would be 0.24, still very abnormal.
U.S. Pat. No. 6,519,490 issued to Joseph Wiesel discloses a method to reduce the impact of premature beats by deleting only those beats that occur at shorter intervals than the mean beat to beat interval. This method was found to improve the specificity of an automatic blood monitor. An algorithm that can recognize when a premature beat occurs and eliminates both the short intervals and the long intervals associated with these beats could further reduce the false positive rate and improve the specificity for the blood pressure monitors used to detect atrial fibrillation. Some patients have very frequent premature beats occurring every other beat. In that case, all the time intervals are either shorter or longer than the mean of the time intervals. If all the beats are eliminated because they fall outside the upper and lower threshold values around the mean time interval then that pattern is not typical of atrial fibrillation and will be considered a regular rhythm. Atrial fibrillation has continuously varying time intervals around the mean time interval such that there are usually at least a few time intervals near the mean time interval that will not be eliminated by this algorithm.
Occasionally, premature beats will occur frequently with varying intervals. For example, if the normal beats occur at time intervals of one second, there may be premature beats occurring at 0.5 second, 0.75 second and 0.85 second all in the same patient during the period of time recorded by the blood pressure monitor. If we eliminate some of these intervals because they exceed a threshold value, other intervals may still be present that can cause a high irregularity index. Another algorithm can be used to reduce the false positive rate for those patients with frequent premature beats. In atrial fibrillation there are no normal beats and having more than half of the time intervals almost exactly equal in length during the 10 to 40 seconds of a typical blood pressure monitor reading is unlikely. On the other hand, even with frequent premature beats, more than half the beats are usually normal beats. Thus, if we find that most of the time intervals are almost equal during one blood pressure reading, it is much more likely that this is not atrial fibrillation.
What is needed is a home monitoring method and apparatus to detect the possible presence of atrial fibrillation and communicate this condition to the user so that the user is alerted to consult a medical practitioner for further testing and/or treatment.
What is also needed is a method that can differentiate atrial fibrillation from a normal pulse pattern and from common heart rhythm abnormalities that are not of significant risk such as sinus arrhythmia, atrial premature beats and ventricular premature beats.
What is further needed is a method of and an apparatus for detecting irregular pulse rhythms during a time period and storing this information so that comparisons may be made with the pulse rate rhythms at later times.
What is further needed is a noninvasive and relatively simple method and apparatus that monitors pulse rate irregularities to detect atrial fibrillation, and that is suitable for use of all ages, and by the hearing and/or visually impaired and that is relatively easy to use.
What is still further needed is a monitoring method and apparatus that detects the presence of irregular pulse beats and then displays and stores: i) the number of irregular pulse beats during a pre-selected time interval; and ii) the duration of time between beats during selected intervals.
Yet another need is for a monitoring method and apparatus that determine whether or not a pulse beat pattern is irregular based on algorithmic or heuristic operations performed on selected pulse beat data.
What is needed is a method and apparatus for detecting the presence of atrial fibrillation by detecting an irregular pattern of pulses using a sphygmomanometer.
What is needed is a method and apparatus for detecting the presence of atrial fibrillation by detecting an irregular pattern of pulses using a plethysmograph such as finger probe with a light source and photodetector.