Breathing Pattern Analysis

Intensive Care Med (1992) 18:193-201 Intensive Care Medicine Springer-Verlag 1992 Review article Breathing pattern analysis M.J. Tobin Loyola Universityof Chicago Stritch School of Medicine, Program Director, Division of Pulmonary and Critical Care Medicine, Edward Hines Veterans Affairs Hospital, Hines, IL 60141, USA Received: 20 October 1991; accepted: 7 April 1992 Clinical examination of the pattern of breathing has gen- relation between subjective estimations of tidal volume erally consisted of counting the respiratory rate over a 15- made by ICU personnel and objective measurements. Of to 60-s period, noting abnormally deep or shallow respi- concern was the widespread tendency to overestimate tid- rations, and documenting well-known patterns such as al volume, which could be particularly dangerous in pa- Cheyne-Stokes or Kussmaul's breathing. In recent years, tients who have low tidal volumes. it has been recognized increasingly that a more detailed analysis of breathing pattern can provide valuable infor- mation regarding respiratory system performance [1]. At the time that advances were being made in the analytical approach to breathing pattern, improvements were occur- Direct measurement ring simultaneaously in the instrumentation used to re- In the intubated patient, it is easy to directly measure ven- cord ventilation, especially in the non-intubated patient tilation by attaching a spirometer or pneumotachograph [2]. In this article, I will review the methods that can be to the patient's endotracheal tube. The prototype used to record and analyze the pattern of breathing, and pneumotachograph, the Fleisch instrument, presents a the usefulness of this approach as an investigative tool in problem in this setting because condensation of moist the critical care setting. gases and mucus can easily clog the narrow metal tubes within the instrument, and thus, alter resistance and produce inaccurate results. The development of newer vari- able-orifice pneumotachographs will hopefully diminish this problem. In the non-intubated patient, problems Techniques of measurement arise in attempting to measure ventilation with a spirometer or pneumotachograph because critically ill patients Visual inspection have a low tolerance of devices that require a direct con- Clinicians usually measure respiratory frequency by ob- nection to the patient's airway. In addition, several inves- serving chest wall motion over 15 s and multiplying the tigators [5-7] have shown that use of a mouthpiece and resulting value by 4. Using this approach, an error of 4 nose clips produces spurious alterations in breathing pat- or more breaths/min may easily occur - a rather large tern, causing tidal volume to increase and respiratory fre- error for a parameter that is normally 16-20breaths/ quency to decrease. Although most of the studies demon- min. Despite the simplicity of the measurement, bedside strating the alteration in breathing pattern with the use of assessment of respiratory frequency is often inaccurate. a mouthpiece and noseclips have been conducted in In one study, 34% of nurses' recordings of respiratory healthy subjects during resting breathing, this effect has frequencies deviated by more than 20% from the true val- also been shown in exercising subjects with a markedly el- ue [3]. Thus, underappreciation of the clinical impor- evated minute ventilation (75.5 1/min) [8], and in patients tance of respiratory frequency may be due partly to inac- with moderately severe asthma (forced expiratory volume curacies in its measurement. Furthermore, the discon- in 1 s, 43070 of predicted normal) [9]. In addition, Aska- tinuous nature of bedside measurements may miss sud- nazi et al. [6] reported that mouthpiece-induced changes den changes in critically ill patients. Assessment of tidal in breathing pattern were similar in critically ill patients volume is also usually part of bedside examination, and as in healthy subjects, although supporting data were not clinicians commonly comment on the depth or shallow- included in their report. As a result of these problems ness of breathing. However, in a study conducted in criti- with a mouthpiece and nose clips, several devices have cally ill patients, Semmes et al. [4] noted a very poor cor- been developed to measure ventilation indirectly. 194 Impedance pneumography Impedance pneumography is probably the technique f most often used to non-invasively monitor breathing pattern. Two standard electrocardiographic electrodes placed on the chest wall serve both to pass an alternating current through the chest between the electrodes and to return a voltage for calculation of impedance [6]. The resulting waveform is qualitatively similar to that obtained with spirometry [11]. However, the instrument cannot be calibrated to obtain quantitative volume measurements [2], and measurement of respiratory frequency may even be inaccurate [10]. Mercury-in-silastic strain gauges This transducer consists of rubber tubes filled with mercury which are wrapped around the chest [2]. Changes in Fig. 1. Transducer bands of the respiratory inductive plethysmograph the length and width of the mercury columns alter their (RIP) placed around the rib cage and abdomen. From Tobin MJ [23] resistance. These mercury columns form part of a bal- anced Wheatstone bridge. Variation in electrical resistance alters the balance of the bridge, and these changes forms the basis of the various techniques that can be used are amplified and recorded. Provided that there is no to calibrate the RIP - isovolume, least-squares, and mul- change in body posture, this technique can provide a rea- tiple linear regression techniques. sonably accurate measurement of tidal volume. The isovolume calibration technique requires that the subject voluntarily closes his/her glottis and, at a con- Magnetometers stant lung volume, moves air back and forth between the RC and Ab, without pressurizing thoracic gas volume Magnetometers measure changes in the antero-posterior [16]. Since the volume lost by one compartment is gained diameters of the rib cage (RC) and abdomen (Ab) during by the other, the amplitude of their signals must be made respiration [12]. An alternating current in an exciter coil equal by the operator. The RC and Ab signals are then generates a magnetic field in a receiver coil placed on the added to produce a sum signal, and while the subject opposite body surface. The signals from the RC and Ab breathes from a spirometer the operator adjusts the sum are electronically summed, and when calibrated, they signal to match that of the spirometer. The difficulty that provide a very accurate measurement of tidal volume. many subjects have in performing an isovolume maneu- This technique has proven to be an extremely valuable re- ver led to the development of other calibration techniques search tool and has been widely employed in the study of that could be performed during natural breathing. The respiratory muscle function [13-15]. The major limita- least squares calibration technique requires recordings of tion of magnetometers is that relatively small changes in spirometry, RC and Ab signals in two postures, taking ad- body position alter the antero-posterior diameter with re- vantage of the change in compartmental contribution to sulting loss of calibration and inaccurate volumetric mea- tidal volume that occurs with change in body posture [/7, surements. 18]. The RC/spirometer and Ab/spirometer ratios from the breaths in the two postures are measured, and then Respiratory inductive plethysmography (RIP) cast as coordinate pairs on an X-Y plot of RC/spirometer The RIP is the most widely accepted method of measur- against Ab/spirometer, and a least-squares regression line ing ventilation in a non-invasive and quantitative manner. is fitted through the points. The reciprocals of the inter- The transducers consist of two coils of insulated wire cepts of this line are taken as the calibration factors of the sewn onto bands placed around the RC and Ab (Fig. 1). RC and Ab transducers. Although the least-squares tech- The coils are connected to a small oscillator module that nique is probably the most robust method of calibration puts out frequency-modulated signals that are propor- [/7-19], it can be quite tedious to perform, especially in tional to alterations in the self-inductance of the coil. The critically ill patients. The multiple linear regression tech- self-inductance of the coil, and so the frequencies of its nique avoids the need for variation in body posture [20], oscillator, is proportional to the cross-sectional area en- and is based on the normal variation in compartmental closed by the coil. These signals are sent to a demodula- contribution during tidal breathing [21]. During the ini- tor/calibrator unit which converts the signal into a pro- tial part of inspiration, volume change is produced pre- portional voltage that can be amplified and recorded. dominantly by Ab displacement [22]. The contribution of Calibration of the RIP is based on the assumption of the RC progressively increases so that at the end of inspi- Konno and Mead [/6] that the respiratory system can be ration volume change is predominantly due to RC mo- considered as a simple physical system with two moving tion. Using a computer to sample the signals (at 25 Hz) parts, the RC and Ab. Consequently, volume measured at produced by the spirometer, RC transducer and Ab trans- the mouth is equivalent to the sum of volume change of ducer will field a matrix of 30 to 40 equations in two un- the RC and Ab. This concept of two degrees of freedom knowns with the points from a single breath. Using a se- 195 ries of breaths in a single posture, the RC and Ab calibra- tube while he/she breathed room air for one minute. Of tion factors can be computed. Recently, Sackner et al. the patients who had an f/V T value greater than 100 [23] developed a modification of the single posture tech- breaths/min/1, 95~ failed a weaning trial, whereas 80~ nique that permits calibration of the RIP without the use of the patients with lower f/VT values were successfully of an external spirometer or pneumotachograph.

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