S. Michelini

Topography of the pleural surface pressure in rabbits and dogs

Abstract A method has been developed that enables the transpulmonary pressure to be measured directly over most of the costal region of animals without introduction of any device into the pleural space. Pleural surface pressure was atmospheric in the lowermost part of the lung of rabbits and dogs in the lateral, supine and prone postures, it was subatmospheric over the rest, and at all levels in the head-up posture, it was positive in the dependent part in the head-down posture. For a given species and posture pleural surface pressure was uniquely related to the percentage of the lung maximal height and no systematic differences were found at a given height. In all points investigated pleural surface pressure was higher than pleural liquid pressure. While pleural liquid pressure at the top of the lung decreases proportionally to the lung height, pleural surface pressure at the top changes less, not only because the overall vertical gradient of transpulmonary pressure is smaller than that of pleural liquid, but also because it decreases with the increase of the size.

Habituation and rate effect in the auditory cortical potentials evoked by trains of stimuli.

SummaryThe effects of the stimulus repetition rate over the habituated auditory cortical evoked responses were studied. The stimulation pattern consisted of trains of pure tone bursts with interstimulus interval (ISI) of 1 s, and intertrain interval (ITI) of 5 s, delivered with constant time and intensity parameters during 93 min. The analysis of the responses was based upon across averaging of the trains, each single response being evaluated in the latency and amplitude parameters. Two time-dependent factors affected the responses in a distinct way: the habituation throughout the whole stimulation and the rate effect within the train. The linear regressions of the time/amplitude functions of the responses were calculated in relation to the duration of ISI and ITI. By introducing a correction factor depending on the repetition rate it was possible to evaluate the relationships between habituation and repetition rate. Changes in the repetition rate do not have any effect on the habituation process. The two phenomena are completely distinct, and they probably have neurophysiologic substrates corresponding to different levels in the central nervous system (CNS).

The Auditory Brainstem Response to Binaural Delayed Stimuli in Man

The brainstem responses (BER) evoked by binaural clicks (0.1 ms) with interaural time differences (delta t) from 0 to 3.5 ms were studied in 6 normal subjects. The responses analysis was carried out via computer in two different ways: (a) Comparison between binaural BER and templates obtained with the addition of two monaural BERs. (b) Extraction from the binaural BER of the second delayed pattern. The results suggest that an algebraic addition of the responses from the two stimulated sides is the main mechanism of the binaural BER generation. In comparison with the templates, the binaural response shows some differences which are evident at delta t = 0--1.5 ms and are limited in the region of VI and VII waves. Such differences are caused essentially to an amplitude decrease of the second pattern--that is response to the delayed stimulus--mainly at delta t 1.5 ms. However these results do not allow us to say if the phenomenon recorded at the surface is due to central adaptation or to specific mechanism of binaural interaction.

Partition of factors contributing to the vertical gradient of transpulmonary pressure

Abstract Pleural surface pressure at various heights was measured after evisceration and diaphragm removal with closed airways and after change of the lung weight obtained by exsanguination (with intrathoracic extrapulmonary blood restored) or by placing tungsten beads in the airways. After diaphragm removal at constant lung weight the vertical gradient decreased by about 35 and 25% of the normal in supine and head-up rabbits, respectively. This gradient fraction, unrelated to gravityindependent shape effects, should be due to the gravity effect on the diaphragm-rib cage interaction. The gradient left over after diaphragm removal (25 and 15%, respectively) should be entirely due to the lung weight in the supine posture and nearly so in the head-up posture. The contribution of the lung weight to the vertical gradient computed in the supine posture by changing the lung weight agrees with that obtained through diaphragm removal experiments. The abdomen contribution to the vertical gradient was about 40 and 60% in supine and head-up rabbits, respectively, in line with previous findings.

Local motion of the chest wall during passive and active expansion

Abstract Relative motion of the various parts of the rib cage of rabbits and dogs during passive and active inflation was related to corresponding pressure across the rib cage (Prc) under the following conditions: (a) supine and head-up postures, (b) normal and eviscerated, (c) with and without pneumothorax. Local specific distensibility of the rib cage was nearly uniform in relaxed supine animals, whereas in the head-up posture it was larger in the caudal parts, particularly in the normal animals. Local distensibility was similar in animals with or without pneumothorax, being essentially related to the gravitational effect on the abdomen and the rib cage. In spontaneously breathing supine animals local distensibility was still uniform and larger than in relaxed animals; in the head-up posture that of the caudal parts was smaller than that of relaxed animals whereas that of the cranial parts was larger. The features of rib cage motion were in line with the different behaviour of the vertical gradient of transpulmonary pressure during active and passive expansion of the respiratory system.

Evaluation of the Monaurally Evoked Brainstem Response in the Diagnosis of Sensorineural Hearing Loss

The auditory brainstem responses (ABRs) obtained in 47 subjects with asymmetric hearing loss (12 with surgically confirmed cerebello-pontine angle tumours, 35 without otoneurologic and/or neuroradiologic evidence of tumour) were evaluated by means of an index named delta V. The calculation of this index was based upon the patients wave V absolute latency obtained at a fixed intensity of 90 dB HL and the value of latency predictable by means of the normative data. The index clearly separates retrocochlear from cochlear sites of lesion. Moreover delta V values obtained in defined cochlear lesions show a linear relation with the patients pure tone hearing loss at 2 and 4 kHz; this behaviour is probably due to a reduction of the auditory dynamic range in the recruiting ears. delta V appears to have clinical usefulness because of two main points: first it is based upon an evaluation of the monaurally evoked ABR; second, it improves the diagnostic specificity of the responses. The rate of false positive results can be further reduced by combining delta V and IT5 values.

Electrocochleography and brainstem potentials in the diagnosis of the deaf child.

The deaf child must receive sound amplification before he reaches the age of two years. At this age the hearing threshold is best measured objectively by electrocochleography (ECochG) and auditory brainstem response (ABR) audiometry. When used correctly, both methods allow an exact threshold estimation which is informative enough for adequate hearing aid prescription. Both methods have advantages and disadvantages when used in children. The advantages of ECochG are: (a) a more exact threshold estimation and (b) strictly monaural evaluation. The advantages of ABR are: (a) ease of performance; it is not invasive and does not require general anaesthesia and (b) allows for exploration of higher levels in the auditory pathway up to the midbrain. We believe that ECochG and ABR are compatible and complementary in the diagnosis of childhood deafness. ABR could be used in first instance, while ECochG could be reserved for doubtful cases and for those who cannot be adequately sedated. Extra-audiological factors such as the availability of anaesthetists and varying hospital facilities, play a further role in determining the choice of electric response technique.

Logarithmic display of auditory evoked potentials

Auditory evoked potentials (AEP) can be simultaneously recorded on-line as a succession of 11 waves, through a single input channel of a mini-computer. Since the response waves differ widely in frequency, a computing routine has been developed to display the whole response pattern in a single picture. Based upon a non-linear samples reduction of the digitized response, this routine allows a logarithmic transformation of the time axis. The method improves the identification of the AEP components and provides an objective estimate of the central auditory pathway for both neurophysiological and neuroclinical studies.

Displacements of the lung hilum, pleural surface pressure and alveolar morphology☆

Abstract When supine rabbits and dogs at the resting volume of the respiratory system were placed in the head-up posture the lung hilum moved caudadrelatively to the suprastenal notch by 0.79 and 1.66 cm, respectively. When in the head-up posture the hilum was pulled craniad to its supine position pleural surface pressure increased in the upper regions and decreased in the lower ones. In the supine posture displacements of the hilum by the same amount in the cranio-caudal or lateral direction changed significantly pleural surface pressure in some parts. Both in the head-up and supine postures the variance of the alveolar volume to surface ratio (V/S) after hilum displacement was greater than normal, i.e. there was a wider distribution of alveolar size. The variance of the ratio (Q) between V/S in one direction and that in the perpendicular one was higher than normal, i.e. there was an alveolar deformation. This deformation was mostly not oriented since Q did not differ significantly from one. Alveolar deformation was nil for hilum displacements 1 3 the above ones.

Simultaneous recording of auditory evoked potentials. Relationships among the fast, middle and long latency components

The whole pattern of the fast, middle and long latency auditory evoked potentials (AEP) was recorded simultaneously from the scalp surface of 13 normal-hearing adults. The individual responses were displayed on a nonlinear time axis in order to improve identification of the components. Stimulation consisted of 2048 unfiltered clicks, delivered monaurally at 80, 60, 40 dB HL with an ISI of 750 ms. Changes in mean latency and amplitude of each AEP component were statistically evaluated in relation to intensity and electrode montage (vertex-mastoid ipsi- and contralateral to the stimulated ear). The latencies of fast components I-VI and the slow P1 increase significantly with declining stimulus intensity. The amplitudes of the fast, I, II, III, V and the slow P1-N1, P2-N2 decrease significantly with intensity. As regards differences due to the electrode montage the contralateral recording causes significant changes in latency of the fast potentials up to wave IV, and changes in amplitude of the fast up to wave V, and of the slow P1-N1 and P2-N2. Therefore, as their latency and amplitude seem to be less closely related to the stimulus and electrode placement, the middle components behave differently, compared with the preceding and following components. Based on parametric comparisons of potentials ranging widely in latency, but each one evoked by an equal sensory input, this kind of AEP evaluation may be useful both for neurophysiological and clinical studies of the whole auditory pathway function.