physiology of cardiac auscultation (pcna)

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  • 1.Pediatr Clin N Am 51 (2004) 1515 1535The physiology of cardiac auscultationAndrew N. Pelech, MD Division of Pediatric Cardiology, Medical College of Wisconsin,8701 Watertown Plank Road Milwaukee, WI 53226, USA There has been a reduction in the skills and abilities of medical students andresidents to diagnose and interpret heart sounds and murmurs. This loss hasoccurred despite new and sophisticated teaching modalities including graphicinstruction, digital recordings, and computer-enhanced simulation [13]. Al-though the reason for reduction in these skill sets is not entirely clear, the in-creased availability and promotion of newer diagnostic modalities, a reduction inemphasis on auscultation instruction [4], a lack of physician confidence, andincreased concern about litigation may be involved in the increased frequency ofordering of additional imaging modalities. Auscultation by a skilled practitionerenables diagnosis in the majority of pediatric cases referred for evaluation of acardiac murmur. In childhood the great majority of cardiac murmurs are innocent [5,6].Although the ECG may rule out significant anatomic structural heart disease, theonly definitive way of diagnosing an innocent heart murmur is by auscultation.Thus, it is important that pediatricians be capable of auscultation both as a time-and cost-saving modality and to provide accurate diagnosis, engender patientconfidence, provide an explanation of presumed cause, and to provide ap-propriate instruction concerning lifestyle changes and endocarditis prophylaxis. Frequently, individuals are inappropriately referred for diagnostic studiesalone, most commonly ECG. The procedure is not cost ineffective as a pre-liminary diagnostic study [7], and parents may not be provided with appropriatefeedback. Although they are often informed that no abnormality was detected,parents are often left wondering about the significance of the murmur with re- E-mail address: apelech@mew.edu0031-3955/04/$ see front matter D 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.pcl.2004.08.004

2. 1516 A.N. Pelech / Pediatr Clin N Am 51 (2004) 15151535gards to exercise, participation in competitive activities, and endocarditis pro-phylaxis. In addition, patients referred for echocardiographic evaluation ofcardiac murmurs are often in the age range in which conscious sedation, withthe attendant risks, is necessary to obtain quality diagnostic studies [8]. Ifsedation is not employed in the uncooperative patient, and the imaging studiesare incomplete or technically poor (as can happen in ECG laboratories that in-frequently perform pediatric studies), mistaken interpretations may occur, andsignificant disease may be missed or dismissed.Primary care physicians are in the best position to provide accurate diagnosisand reassurance. Not all cardiac murmurs warrant referral. Improved physiologicunderstanding of the physics of sound and murmur creation should provide thefoundation for improved skills and confidence in the description and interpre-tation of pediatric heart sounds and murmurs.What is sound? Sound is the sensation of hearing resulting from stimulation of the auditorynerves by vibrations transmitted in a medium. Vibrations are caused by therhythmic compression and rarefaction of molecules within a medium (Fig. 1).The transmission characteristics, speed, and fidelity of these rhythmic vibrationsor waves vary both within and at interfaces between different media, generally airor water. The eardrum receives and responds to these pressure waveforms byconverting them to minute kinetic movements conducted by the ossicular chainwithin the middle ear. The ossicular chain serves to amplify or attenuate thevibratory signal before it is conducted to the oval window of the cochlea of theinner ear. Within the inner ear, movement of lymphatic fluids on either side ofReissners membrane stimulates fine hair cells of the organ of Corti, resulting in Fig. 1. The dimensions of sound. Sound represents compression and rarefaction of air molecules. 3. A.N. Pelech / Pediatr Clin N Am 51 (2004) 151515351517Fig. 2. Waves are measured by the distance between peaks. This distance is termed the cycle length.Variations in cycle length result in variation in frequency or pitch. The force or distance of thecompression is termed amplitude and is measured in decibels. Variations in wave amplitude resultin a change in the intensity or loudness of a sound. Variations in the character and component parts ofthe waveform or variations in waveform complexity result in a change in quality or timbre.ionic fluxes and neurotransmission of signals to the cerebral cortex. Sound waveshave three dimensions: intensity, frequency, and timbre or quality (Fig. 2).Intensity or loudness The crest of a pressure wave has a given height or amplitude (Fig. 2). Thisamplitude determines the perception of loudness. There is a large range ofamplitudes over which sounds can be heard. The loudest sounds cause pain andhave amplitudes more than 1 million times greater than the quietest audiblesounds. To accommodate this enormous range, sound amplitudes are usuallymeasured on a logarithmic scale. The unit of measurement is the decibel (dB).Sound intensity or the subjective perception of loudness varies with the squareof the sound amplitude. Sound intensity is measured by comparing it to somereference value (usually the softest sound intensity that humans can hear). Ona decibel scale, sound measured at 10 decibels has 10 times the intensity ofthe reference value. At 20 decibels the intensity is 100 times greater, and at30 decibels, it is 1000 times greater than the reference value. 4. 1518A.N. Pelech / Pediatr Clin N Am 51 (2004) 15151535Frequency or pitchThe distance from the crest of one pressure wave to the next is the wavelength(Fig. 2). This wavelength depends on the speed at which sound can travel througha given medium and the temperature. The number of pressure waves per secondis the frequency and is measured in Hertz (Hz). With shorter wavelengths thereare more pressure waves per second, and the frequencies are higher. Conversely,with longer wavelengths the frequencies are lower. The frequency of a pressurewave determines the perception of pitch; low frequencies are heard as a low pitchand high frequencies as a high pitch. The range of frequencies over whichhumans can perceive sound is the audible range. It extends from a minimum of20 to a maximum of 20,000 Hz. The human ear is most sensitive to soundsbetween 500 and 4000 Hertz. This range contains most frequencies of normalspeech. The majority of heart sounds and murmurs range between 5 and 800 Hz.Low frequencies are not heard as well and need larger amplitudes to soundequally loud. Note that many sounds emanating from the heart are below thethreshold of audibility and consequently are best perceived by palpation.Quality or timbreBesides frequency and intensity, other aspects influence the perception of asound or murmur. A guitar string, for example, vibrates not only along its wholelength but also in segments of halves, thirds, fourths, and so forth. Each segmentthen vibrates twice, three times, four times, and so forth, as fast as the wholestring. The vibration of the whole string produces the fundamental or primaryfrequency; the weaker amplitudes of the fundamental frequency or secondaryfrequencies make up the harmonics. The same musical note played on differentinstruments sounds distinctly different. The timbre or quality of a sound refers tothe component parts of a complex waveform. These multiple accessory wave-forms or overtones differentiate the character of musical instruments playing amusical note of a similar cycle length. Thus, it is overtones that differentiate aviolin or piano playing the same note (eg, middle C at a cycle length of 256 Hz).Although a variety of qualitative terms have been employed to describe heartmurmurs (eg, rumbling, machinery, or blowing) there are only twofundamental qualities of sound: music and noise.Harmony or music The individual characteristics of the sound depend on the number and relativeamplitude of the overtones. When the overtones are multiples of the primaryfrequency, they create a pleasing sound termed harmony or music. Forexample, if the primary note is middle C (256 Hz), music is produced byovertones at cycle lengths of 512, 1024, and 2028 Hz. An awareness ofharmonics and overtones will help in understanding the musical quality of somecardiac murmurs. 5. A.N. Pelech / Pediatr Clin N Am 51 (2004) 15151535 1519Dissonance or noise When a sound wave is composed of many different and unrelated frequencies,a musical quality can no longer be perceived. Instead the sound wave seemsdissonant and rough and is described as noise. Noise can originate fromvariable amounts of turbulent flows of gasses and fluids. For example, waterrushing through pipes, air escaping from a jet, or blood being ejected from theheart across a stenotic valve causes noise.What are murmurs? Cardiac murmurs are audible turbulent sound waves in the range of 20 to20,000 cycles/second emanating from the heart and vascular system. They arecommon in pediatric patients and represent the most common reason for referralto pediatric cardiologists [9]. Whereas only 0.8% to 1% of the population hasstructural congenital cardiac disease, as many as 50% to 85% of the populationhas an innocent heart murmur during childhood [10], and these murmurs must bedistinguished from murmurs associated with significant congenital or acquiredheart disease. The nature of cardiac murmurs is often influenced by the age of the patient atpresentation, and a thorough understanding of the sequence of cardiac events andthe evolution of the fetal, transitional, and postnatal circulations is required toidentify and diagnose murmurs correctly.The cardiac cycleCardiac sounds and murmurs that arise from turbulence or vibrations withinthe heart and vasc