European Journal of Europ. J. Intensive Care Medicine 2, 3-6 (1976) Intensive Care Medicine © by Springer-Verlag 1976 The Effect of Respiration on the Monitoring of Stroke Volume and Cardiac Output by the Electrical Impedance Technique J. Endresen and D. W. Hill Research Department of Anaesthetics, The Royal College of Surgeons of England, London, England Abstract. In six volunteers (5 male, 1 female) it has been shown that normal respiration made no statistical difference to the estimates of the mean stroke volume and the mean cardiac output as determined by the electrical impedance method of Kubicek et al, (1966). The coefficient of variation was usually increased by respiration. The use of those stroke volumes which occur only at end-expiration was not shown to yield a greater reproducibility with 3 other male volunteers. In the female subject it was found that the use of a digital averager triggered from the preceding R-wave of the ECG gave values for the mean stroke volume and cardiac output which were always lower than the conven- tional mean values obtained from a number of strokes. The expense of either of these approaches does not appear to be justified as a means of compensating for the effects of normal respiration on the impedance dZ/dt waveform. Key words: Impedance cardiogram, Cardiac output, Stroke volume, Averaging. Introduction baseline, although the differentiator circuit makes the dZ/dt tracing less affected by respiration than the 2xZ The thoracic impedance technique developed by Kubicek tracing. et al. (1966) for the monitoring of changes in stroke vol- ume and cardiac output has proved to be of value, for The movement of the dZ/dt tracing with respiration is example, in monitoring such changes occurring during an- most evident during automatic ventilation of the lungs aesthesia (Hill and Lowe, 1973; Lenz et al., 1976). How- with a tidal volume of 500 ml or more, and there is also ever, whereas the thoracic electrical impedance at 100 kHz the added complication that the phonocardiogram used may change by approximately 0.2 ohms with each systole for timing the duration of the left ventricular ejection is for an adult with a normal heart, a substantially larger often obliterated by the noise arising from the forceful change occurs with each tidal volume. A typical value inflation of the lungs. Under these circumstances, it is would be 2 ohms per litre (Baker and Hill, 1969). The advisable to stop the ventilator for a period of at least action of breathing can affect the waveform of both the six heart beats in order to obtain a representative length change in thoracic impedance (~Z) and that of its first of dZ/dt tracing. In many patients, the dZ/dt trace has a derivative (dZ/dt). The formula of Kubicek et al. (1966) well defined notch (the X-point) which occurs at the for the calculation of the stroke volume is closure of the aortic valve and then a phonocardiogram SVml = (pL2/Zoz) x T x (dZ/dt)max where "p" is the resis- is not vital for the measurement of T. For moderate auto- tivity of the patient's blood in ohm-cm, L is the distance matic ventilation or for normal spontaneous respiration in cm between the inner pair of the band electrodes it would be convenient not to have to stop the ventilator placed on the neck and lower portion of the chest, Z o is or to ask the subject to hold his breath each time a stroke the basal value of the thoracic impedance in ohms, T is volume estimate was made. Thus the objective of this the left-ventricular ejection time in seconds and study was to ascertain whether there was any statistical (dZ/dt)max is the peak value of the dZ/dt tracing obtain- difference between the mean stroke volume and the mean ing during systole in ohms per second. It is measured as cardiac output values estimated by the electrical impe- the peak height above the dZ/dt = 0 baseline. The action dance technique when the subject was breathing normal- of respiration causes the dZ/dt tracing to vary about this ly or breath-holding. 4 EuropeanJournal of Intensive Care Medicine, Vol. 2, No. 1 (1976) Materials and Methods graph's phonocardiogram amplifier to provide a phono- cardiogram. Finally, an ECG signal is obtained via the The subjects of this study consisted of eight males and outermost pair of the band electrodes. This is a non- one female aged 21 to 46 years. All were in good health standard diagnostic ECG lead, but it is adequate for tim- and had apparently normal hearts. For each subject two ing purposes and for triggering a digital averager. The disposable band electrodes made from self-adhesive Mylar impedance cardiograph provides 0.1 ohm and 1 ohm per strip carrying a 6 mm wide aluminium strip along the second calibration signals for the AZ and dZ/dt tracings. centre-line were placed around the neck spaced approx- The AZ, dZ/dt, ECG and phonocardiogram signals imately 25 mm apart. A second pair of band electrodes were displayed on an ink-jet Mingograf recorder together was placed around the bottom of the thorax with the with the output from a digital averager in the case of the inner electrode at the level of the xiphisternal joint, the female subject. The averager was a 200 point signal recov- electrode spacing again being 25 ram. The four electrodes ery system Type DL 102A (Data Laboratories Ltd.) were connected to a Model 304 Impedance Cardiograph which has 200 words of store. It was fed with the dZ/dt (Instrumentation for Medicine Inc.). A constant current signal and d.c. level triggered from the R-waves of the of approximately 4 mA r.m.s, at 100 kHz was passed ECG. between the outermost pair of the four electrodes (Fig. 1), Each subject lay supine, breathing normally and ade- and the potential changes due to cardiac activity detected quate time was allowed for the subject to relax and for at the inner pair of electrodes. The impedance cardio- the tracings to stabilise. A control sequence of at least graph back-off the potential developed due to the basal six beats was recorded together with the calibration. Im- impedance of the chest Z o and provides a signal output mediately afterwards the subject was requested to refrain corresponding to the change in thoracic impedance AZ. from breathing and a similar set of beats was recorded. The AZ signal is differentiated within the impedance Using the formula of Kubicek et aL (1966), the stroke cardiograph by a low-noise differentiator to provide a volume was calculated for each beat, the value of Z o hav- dZ/dt signal. A crystal microphone placed on the chest ing been obtained from the digital display of the imped- over the heart is connected to the impedance cardio- ance cardiograph. The instantaneous heart rate was cal- ~% Potential electrodes OSC. f=100 kHz d71~t Differentiator Current III / Amplifier J electrodes ][l/ and I "-/J ~ demodulator A Fig. 1. Diagramatic representation of the Impedance Cardiograph L =24.3 cm Zo=32.252 (dZ) T=O.ZJ, sec ~ max : 1.5~2/sec SV = 56.Z, ml HR=60 Q =3.38 [/min Az=O.l~ dZ = l~/sec dt Baseline stabilizer unit output I r I I I sec Fig. 2. The dZ/dt signal "gated" using the IFM Baseline Stabiliser Unit J. Endresen and D. W. Hill: The Effect of Respgation on the Monitoring of Stroke Volume and Cardiac Output Table 1. The cardiac output values are not corrected for haematocrit Name Sex Mean Stroke Volume Mean Heart Rate Mean Cardiac Output BL M (A) 152 ml (8.4%) 67.9 (2.9%) 10.3 L/min (6.2%) (B) 158 ml (14.0%) 65.6 (5.0%) 10.4 L/rain (16.0%) CO M (A) 128 ml (6.5%) 61.5 (2.5%) 7.8 L/min 4.8%) (B) 139 ml (11.7%) 61.3 (4.8%) 8.5 L/rain 14.4%) S-S M CA) 136 ml (5.5%) 54.7 (4.5%) 7.5 L/rain 5.3%) (B) i38 m! (5.0%) 53.9 (2.4%) 7.4 L/rain 7.4%) ME M (A) 105 ml (4.5%) 69.4 (3.6%) 7.4 L/rain (3.4%) (B) 99 ml (5.7%) 70.0 (2.6%) 6.9 L/rain 7.1%) BA M (A) 82 ml (2.2%) 64.0 (1.7%) 5.2 L/rain 3.0%) (B) 82 ml (4.8%) 68.0 (3.7%) 5.6 L/min 6.4%) EE F CA) 94 ml (5.3%) 76.0 (4.7%) 7.1 L/min 5.9%) (B) 84 ml (18.4%) 84.0 (4.6%) 7.0 L/rain (18.1%) (A) Apnoeic, (B) Breathing, (%) Coefficient of variation. Table 2. Name Sex Mean Cardiac Output Averager Cardiac Output Difference Mean (R-(dZ/dt)max) EE F (B) 7.51 L/min 7.04 L/min - 6.3% 96.4 ms (7.4%) EE F (B) 5.70 L/rain 5.68 L/min - 3.2% 96.4 ms (5.7%) EE F (B) 7.48 L/rain 7.08 L/rain - 5.4% 97.5 ms (7.8%) EE F (B) 6.67 L/rain 5.94 L/min -10.9% 93.1 ms (6.4%) EE F (B) 6.78 L/rain 6.31 L/rain - 7.2% 97.4 ms (6.3%) EE F (A) 7.80 L/rain 7.24 L/min - 7.2% 94.1 ms (5.7%) EE F CA) 6.98 L/rain 6.15 L/rain -11.9% 98.4 ms (4.7%) (A) Apnoeic, (B) Breathing, (%) Coefficient of variation.