Impedance cardiography in mitral valve disease.
Impedance cardiography basically senses blood volume changes with each cardiac ejection, with the help of high frequency alternating current used as a carrier. It has been used for assessing left ventricular function, stroke volume and cardiac output.,,, Its utility for the assessment of functions of various cardiac valves is currently being investigated. It has been reported to be useful in diagnosing aortic regurgitation (AR).In this paper we are presenting our observations on 15 cases of mitral stenosis (MS) and ten cases of mitral regurgitation (MR). These cases underwent routine clinical examination, and investigations; and the diagnosis was further confirmed by echocardiography (all cases), cardiac catheterisation and angiocardiography (14 cases) and at surgery (all cases).
Impedance cardiograph unit used for this investigation was developed at the electronics division of Bhabha Atomic Research Centre, Bombay. It employs an isolation transformer to pass a 3 mA, r.m.s. constant current at 100 kHz through the body with the help of the current electrodes (prepared from braided wire in the form of loop around the body segment). The voltage developed across any two points along the current path is sensed with the help of voltage electrodes (similar to current electrodes). This signal is transformer coupled to a precision rectifier and an amplifier so as to display the basal impedance (Zo) in ohms. The small changes in the impedance caused by the blood flow, respiration etc. are further amplified (16-160 Hz, 3 dB points). The output is fed to a strip chart recorder to display the time derivative of impedance, referred to as dz/dt wave-form or impedance cardiographic wave-form.
Fifteen male healthy volunteers, 15 male patients with mitral stenosis and ten male patients with mitral regurgitation were subjected to this study. The patients were proven cases of isolated mitral stenosis and pure mitral regurgitation as mentioned earlier.
The subjects were asked to lie in a supine position. Current electrodes were connected around the forehead and around the feet of the subject, voltage electrodes were placed around the base of the neck and around the thorax at the level of xiphisternal joint. Stroke volume (SV) and cardiac output (CO) were computed from the above data using Kubicek's formula.,,
Blood flow through the extremity (right arm) was also assessed, using the same equipment. For the assessment of blood flow through the arm, the current electrodes were positioned as far apart as possible along the arm. The distance between the voltage electrodes were kept constant (5 cm) on either side of the elbow.
[Fig. 1a]shows a typical dz/dt waveform recorded from thorax of a normal subject.[Fig. 1b]shows dz/dt waveform recorded from the right arm of the same subject (10 mm deflection on Y axis corresponds to 1 ohm /sec for recording from thorax and 0.25 ohm/sec for arm tracing. The recording speed of 25 mm/sec corresponds to 400 msec/10 mm on X axis). It is seen that in normal individuals, the dz/dt waveform (recorded from thorax) has two major components corresponding to each cardiac cycle. They are conventionally labelled as "C" wave and "O" wave.,"C" wave-is usually 3-4 times larger than the "O" wave and is triangular in shape with rise time faster than the fall time. The "O" wave looks like gaussian curve skewed towards left. Normally "O" wave immediately follows "C" wave.
[Fig. 2a]shows a typical pre-operative dz/dt tracing recorded from thorax of a patient suffering from mitral stenosis. It can be seen that the skewed gaussian type "O" wave of [Fig. 1a]assumes an `M shape' pattern in [Fig. 2a]due to restricted atrioventricular transport in MS cases.
[Fig. 2 (b]shows dz/dt tracing from the arm of the same patient, which does not show any significant change in comparison to similar tracing in a normal individual [Fig. 1 b].
[Fig. 3a] and [Fig. 3b] show postoperative dz/dt tracings recorded from thorax and arm respectively of the same patient with mitral stenosis. The thorax impedance waveform as seen in [Fig. 3a] illustrates the return of "O" wave as skewed gaussian type from the earlier `M shape' pattern within one week after closed mitral valvotomy whereas the arm tracing in [Fig. 3b] shows no significant change from [Fig. 2b] except slight rise in the amplitude of "C" wave. Similar changes in dz/dt waveforms were seen in all patients with mitral stenosis before and after surgery.
[Fig. 4a] shows atypical dz/dt thoracic tracing of a patient suffering from mitral regurgitation. Its comparison with [Fig. 1a] apparently gives a feeling that "C" wave gets distorted during its fall time, "O" wave gets diminished and a new wave appears just before the next ejection. But correlation with electrocardiogram and phonocardiogram indicates that the larger wave is actually the "O" wave and the smaller wave preceding that is the "C" wave. The larger amplitude of the "O" wave indicates more atrioventricular transport than the left ventricular ejection which is of course true in case of MR patients. [Fig. 4b] shows similar dz/dt tracing from the arm of the same patients. Gross abnormalities in this waveform are very apparent. Such features were not observed in cases of mitral stenosis.
[Fig. 5a] and [Fig. 5b] show dz/dt tracing (thoracic and arm respectively) recorded from the same case of mitral regurgitation one week after mitral valve replacement. Gross abnormalities seen preoperatively in this case are found to be eliminated qualitatively after corrective surgery.
Apart from changes observed in dz/dt tracings as described earlier, other changes were also observed in pre- and post-operative Zo, SV and CO values in cases of mitral stenosis.
[Table 1] gives data on 15 normal subjects. Average ± 1 standard deviation (SD) values of Zo, SV and CO are 25.15 ± 1.74 ohms, 80.04 ± 11.39 ml/beat and 5.45 ± 0.73 1/'min respectively.
[Table 2] gives similar pre- and postsurgical observations on 15 MS cases. Pre-operative group average ± 1 standard deviation values of Zo, SV and CO are 30.5 ± 3.2 ohms, 32.7 ± 8.4 ml/beat and 2.6 ± 0.7 1/min respectively. Student `t' test applied to these observations of Tables 1 and 2 show significant difference at 5% level of confidence. Post-operative group average ± 1 standard deviation values for Zo, SV and CO are 26.4 ± 3.5 ohms, 44.8 ± 18.1 ml/beat and 4.3± 1.6 1/min respectively. These values show a tendency to record normal values after corrective surgery.
The Zo values recorded in cases of mitral regurgitation show no significant deviation from normals. Calculation of SV and CO in such cases was not possible because of gross abnormalities in the waveform.
The present impedance cardiography study on 15 normals, 15 cases of mitral stenosis and 10 cases of regurgitation shows that
(1) In cases of mitral stenosis the "O" wave of dz/dt waveform (which is normally a skewed gaussian type) records `M' shape, which reverts back to its normal shape after corrective surgery.
(2) In cases of mitral regurgitation the amplitude of `O' wave of dz/dt waveform is larger than the 'C' wave indicating a rich atrioventricular transport. The arm dz/dt tracing is also similarly distorted. These changes revert back to normal after replacement of mitral valve.
(3) Pre-operative Zo values in cases of mitral stenosis are significantly higher as compared to those in normal individuals whereas in cases of mitral regurgitation no significant difference is observed. Preoperative stroke volume and cardiac output values in cases of mitral stenosis are significantly lower when compared to normals. After corrective surgery for mitral stenosis these values are restored to normal.
The authors are thankful to Shri C. Ambasankaran, Director, E. & I. Group of B.A.R.C. and Dr. C. K. Deshpande, Dean, Seth G.S. Medical College and K.E.M. Hospital for permission to collaborate during this study and to communicate this paper; to Shri V. A. Pethe, Head, Electronics Division and Shri K. R. Gopalkrishnan, Head, Medical Instruments Section for providing necessary facilities and having encouraged us right through the studies.