692 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



Ambient noise in the recording area must, of 

 course, be kept at a minimum. A soundproof room 

 may be necessary or desirable; but before such an 

 expensive investment is made, the investigator should 

 convince himself that the signal-to-noise ratio of his 

 recording equipment is sufficiently favorable that 

 ambient noise, not electronic noise, is the factor 

 limiting clean recording. An advantage of heart sound 

 recording — over the recording of music, for example, 

 in free air — is the considerable attenuation (as much 

 as 20 db) provided merely by the coupling of the 

 microphone to the chest. More often than one might 

 think, a quiet room with attention to avoiding unusual 

 noise suffices. 



The subject should be relaxed, comfortable, and 

 warm to prevent muscle noise. The application of the 

 microphone to the chest can be accomplished by a 

 rubber strap surrounding the chest, by local mooring 

 (with tape or suction cup), or by hand. The strap 

 has the possible disadvantage of variations in pressure 

 on the microphone with varying phase of respiration. 

 The pressure with which the microphone is applied 

 influences the vibrations of the portion of skin under- 

 lying the microphone and therefore the sounds which 

 are recorded. Greater pressure preferentially attenu- 

 ates lower frequencies. Hair on the chest may require 

 shaving or plastering down. 



Standard loci for recording from the surface of the 

 chest in man are /) aortic area (second intercostal 

 space at the right sternal margin) ; 2) pulmonary area 

 (second intercostal space at the left sternal margin); 

 j) LLSB (left lower sternal border; fourth intercostal 

 space at the left sternal border); 4) apex (in the area 

 of the apex beat, on the left border of cardiac dullness 

 in the fifth intercostal space, or the midclavicular line 

 in the fifth interspace). Recordings are made in other 

 areas in which auscultation reveals soimds of interest. 



The value of simultaneous multilocus recording, 

 i.e., parallel recording from at least two precordial 

 locations, has been demonstrated particularly by 

 Leatham. The precordial topography of the several 

 components of the heart sounds and the effects of 

 respiration lend themselves well to study by this 

 method. Especially when studying respiratory effects 

 it is much easier to be certain of results if the changes 

 are observed in the same cardiac cycles in two or 

 more areas. 



STORAGE OF CARDIOVASCULAR SOUND AND 

 CORRELATED INFORMATION 



Storage of cardiovascular sound and other physio- 

 logic data on magnetic tape has increased consider- 



ably the flexibility of methods of study. Data can be 

 preserved for future reference and comparisons, for 

 analysis by newly devised techniques, and for teach- 

 ing purposes. Minimal low-frequency attenuation of 

 cardiovascular sound consistent with placement on 

 tape without overloading is recommended. A tape 

 speed of 71-2 inches per sec has been found satisfactory 

 in our laboratory. 



As an alternative to direct tape recording of the 

 heart sounds, one may consider recording with the 

 use of an FM system. Direct magnetization of the 

 tape by the method normally used for tape recording 

 of voice and music seems adequate; however, there 

 has been no systematic investigation of whether FM 

 recording of the heart sounds might be superior in 

 some applications. At a given tape speed there is, 

 with direct recording, a limit below which distortions 

 on playback become excessive because the recorded 

 wavelengths exceed the length of tape covered by the 

 playback head. With the low-frequenc\' elements of 

 cardiovascular sound such distortion might be serious. 

 Nonuniformity of magnetic material on the tape adds 

 to the distortion. In the FM method a carrier signal 

 saturates the magnetic material, thus minimizing the 

 effects of nonuniformity of tape. Since the physiologic 

 information is, in the FM system, contained in the 

 frequency level of the carrier signal, its faithful repro- 

 duction is dependent on uniformity of tape speed. In 

 direct recording, uniformity of tape speed is a factor 

 of secondary importance, just as uniformity of the 

 magnetizable material on the tape is a second-order 

 factor in frequency modulation. Direct recording 

 probably provides the most favorable signal-to-noise 

 ratio for the investigation of cardio\ascular sound. 



The FM method is used for recording the electro- 

 cardiogram, respiratory phase, pressure pulses, and 

 other low-frequency physiologic phenomena discussed 

 earlier. Details of such recording are presented in 

 Chapter 22. The electronic details of frequency 

 modulation and demodulation of low -frequency 

 piiysiologic data are beyond the scope of this dis- 

 cu.ssion. 



SPEC:IAL TECHNiqUES 



Direct phonocardiography is a term now used to indi- 

 cate the making of recordings directly from the surface 

 of the heart or great vessels (11). (Historically, the 

 same term was used for the method de\ised by Frank.) 

 The technique has difficulties because the surface 

 imderlving the micropiione is almost always moving, 

 possibly introducing vibrations without physiologic 



