SEARCH · PubMed Health

Results for “COMPRESSION”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24Linked to original sources

PubMed1996

Wavelet compression of medical imagery.

Wavelet compression is a transform-based compression technique recently shown to provide diagnostic-quality images at compression ratios as great as 30:1. Based on a recently developed field of applied mathematics, wavelet compression has found success in compression applications from digital fingerprints to seismic data. The underlying strength of the method is attributable in large part to the efficient representation of image data by the wavelet transform. This efficient or sparse representation forms the basis for high-quality image compression by providing subsequent steps of the compression scheme with data likely to result in long runs of zero. These long runs of zero in turn compress very efficiently, allowing wavelet compression to deliver substantially better performance than existing Fourier-based methods. Although the lack of standardization has historically been an impediment to widespread adoption of wavelet compression, this situation may begin to change as the operational benefits of the technology become better known.

Diagnostic Imaging↗

PubMed1979

Compressibility of skinfolds and the measurement of subcutaneous fatness.

Compressibility of subcutaneous fat thickness when measured with skinfold calipers was investigated in 65 white American youths. Compression of skinfolds was determined relative to measurements of subcutaneous fat thickness from radiographs at each of seven sites. There is statistically significant heterogeneity among sites in skinfold compression, with skinfolds on the medial and lateral calf being the least compressible of those measured. There is little statistically significant sex difference in skinfold compression in the present sample and, within the range concerned, there were no significant correlations between skinfold compressibility and age. When compressibilities of the seven skinfolds were intercorrelated within individuals, statistically significant average correlations were obtained, indicating that individuals tend toward similar degrees of skinfold compressibility among sites. This communality of skinfold compression within individuals is such that, at least in male youth, there are significant differences among individuals in the average compressibility of the seven skinfolds.

Adipose Tissue↗

PubMed1996

Compression? Yes, but for low or high frequencies, for low or high intensities, and with what response times?

Several rationales for using compression in hearing aids are outlined. These rationales comprise discomfort avoidance, loudness normalization, noise reduction, short term signal dynamic range reduction, empirically determined compression, and long-term signal dynamic range reduction. The compression systems needed to implement each of these differ greatly, and these differences can be viewed as differences in the frequency range undergoing most compression, the intensity range undergoing most compression, and the speed at which the compressor(s) operate. A classification system along these lines is introduced and examples of currently available hearing aids falling into each category are given. The effects of each type of compression on speech intelligibility is investigated via a review of published research. The results of this indicate that, for speech in quiet at a comfortable level, no compression scheme yet tested offers better intelligibility than individually selected linear amplification. If input level is then decreased and the aid wearer is prevented from adjusting the volume control, many types of compression provide intelligibility superior to that available from linear amplification. In broadband noise, only one system, containing wideband compression followed by fast acting high-frequency compression, has so far been shown to provide significant intelligibility advantages.

Equipment Design↗

PubMed2002

Effects of multi-channel compression time constants on subjectively perceived sound quality and speech intelligibility.

OBJECTIVE: The purpose of this study was to determine the influence of the compression time constants in a multi-channel compression hearing aid on both subjectively assessed speech intelligibility and sound quality in realistic binaural acoustical situations for normal-hearing and hearing-impaired listeners. DESIGN: A nonlinear hearing aid with 15 independent compression channels of approximated critical bandwidth was simulated on a personal computer. Various everyday life situations containing different sounds such as speech and speech in noise were recorded binaurally through original hearing aid microphones placed in BTE hearing aid cases. Two experiments were run with normal hearing and hearing-impaired subjects. For each subject, hearing thresholds were established using in situ audiometry. The static I/O-curve parameters in all channels of the hearing aid were then adjusted so that normal speech received an insertion gain corresponding to the NAL-R formula (Byrne & Dillon, 1986). The compression ratio was kept constant at 2.1:1. In the first experiment with six normal-hearing and six hearing-impaired subjects, the hearing aid was programmed to four different settings by changing only the compression time constants while all the parameters describing the static nonlinear Input/Output-curve were kept constant. The compression threshold was set to a very low value. In the second experiment with seven normal-hearing and eight hearing-impaired subjects, the hearing aid was programmed to four settings by changing the release time constants and the compression threshold while all other remaining parameters were kept constant. Using a complete A/B pair comparison procedure, subjects were presented binaurally with the amplified sounds and asked to subjectively assess their preference for each hearing aid setting with regards to speech intelligibility and sound quality. RESULTS AND CONCLUSIONS: In Experiment 1, all subjects showed a significant preference for the longest release time (4 sec) over the two shorter release times (400 msec and 40 msec), pertaining to quality and intelligibility. In combination with the long release time, the attack time (1 msec or 100 msec) was of less importance. Larger inter-individual differences were observed among all subjects with regard to the perceived sound quality of musical and nonspeech signals. In Experiment 2 the hearing-impaired subjects showed a significant preference for the hearing aid setting with a long release time (4 sec) and a low compression threshold (20 dB SPL), both with regard to sound quality and speech intelligibility. With a short release time (40 msec), a lower compression threshold (20 dB SPL) was preferred over a higher threshold (50 dB SPL). Among normal-hearing subjects, the same settings were also preferred for speech intelligibility. However, no significant differences were observed in these subjects regarding the assessment of sound quality. Again, more individual variability was found during the assessment of music and nonspeech signals. In sum, the preference of a rather long release time raises the question of whether a compression system with a very short time constant is the optimal means to compensate for the sensory recruitment phenomenon.

Acoustic Stimulation↗

PubMed1990

Quadtree-based data compression of abdominal CT images.

An ROC study was performed to evaluate the effect of quadtree-based data compression on the diagnostic yield of CT images. Seventy images were selected from a series of consecutive abdominal/pelvic CT scans. Following the application of quadtree-based compression, all images were reviewed independently by three radiologists. The images were analyzed at six decreasing irreversible compression ratios (30.6:1 to 7.4:1), as well as after reversible compression (2.9:1). ROC curves reveal a gradual decrease in clinical accuracy with increasing compression ratios. At a compression ratio of 7.4:1, sensitivity for all major abnormalities was 99% with a specificity of 93%. As the compression ratio was increased to 30.6:1, sensitivity and specificity dropped to 75% and 83% respectively. Execution times for compression and decompression of the CT images with a PC-AT based digital radiography system varied from 24.7 to 18.5 seconds and from 16.2 to 5.1 seconds respectively, decreasing with higher levels of compression. We conclude that quadtree-based compression of abdominal CT images may have practical applications for a PC based digital radiography system.

Algorithms↗

PubMed1993

A new method for computed tomography image compression using adjacent slice data.

RATIONALE AND OBJECTIVES: The authors developed and subjectively evaluated an interslice compression algorithm that explores the redundancy among adjacent slices of an x-ray computed tomography (CT) scan. This algorithm has been compared to an intraslice compression algorithm based on the two-dimensional discrete cosine transform. METHODS: Nine x-ray CT head images from three patients were compressed with this interslice method at compression ratios of 5:1, 10:1, and 15:1. The same images were also compressed with the intraslice method at the same ratios. Six radiologists judged quality of randomly selected compressed and decompressed images compared to that of the originals. The evaluation data were analyzed statistically with the analysis of variance and Tukey's multiple comparison. Kappa-like statistics (Williams index and O'Connell and Dobson indexes) were also calculated to measure the agreement among readers beyond the amount expected by chance. RESULTS: The interslice coding algorithm showed significantly better quality than the intraslice method at significance level 0.05, even though there was no difference in the objective distortion measure (signal-to-noise ratio). Also, the quality of 10:1 compressed images with the interslice coding algorithm was not significantly different from that of the originals at level 0.05. While large variations in agreement occurred among readers, the overall agreement was statistically significant. CONCLUSIONS: By using adjacent slice information in compressing x-ray CT images, significantly better quality in compressed and decompressed images was achieved. While 10:1 compressed images with the interslice algorithm were not significantly different from the originals in quality at level 0.05, effect on diagnostic accuracy remains to be investigated.

Algorithms↗

PubMed1992

The direct effects of graded axonal compression on axoplasm and fast axoplasmic transport.

The direct effects of mechanical compression on axoplasm and fast axoplasmic transport were studied by video-enhanced differential interference microscopy. Single axons, isolated from the squid, were compressed with 0.5, 5, 20, or 100 gram (g) weights placed over a 1 millimeter (mm) length of axon. Brief compressions (10 seconds) at low pressures (0.5 g/mm) momentarily deformed the axon, but the axoplasm and axon returned to their normal shape and position after the pressure was removed, and no residual changes in axoplasmic structures, fast axoplasmic transport or membrane function were seen. Compressing the axon with 5-20 g/mm, however, broke the axoplasm at the site of the crush and squeezed the axoplasm out from under the compression site. Though the axoplasm usually returned to the crush site after the weight was removed and organelles continued to move in the axoplasm under the crush, the organelles failed to cross a dense line that marked the site of the rejoined axoplasm, instead they accumulated over time at the crush site. This results suggests that the blockage of fast transport at moderate compressions was due to a mechanical breakage of the axoplasm at the compression site. The plasma membrane was apparently not transected after moderate compressions (5-20 g/mm) since the resting membrane potential returned to nearly control levels after the weight was removed. Compressions with 100 g/mm, however, did break the plasma membrane as evidenced by the rapid and irreversible loss of the action potential and resting potential and the ion-dependent liquefaction of axoplasm and loss of all organelle transport at the 100 g/mm compression site. Thus, small mechanical pressure elastically deformed the axoplasm, moderate pressures mechanically broke the axoplasm, and high pressures broke the axoplasm and the plasma membrane.

Action Potentials↗

PubMed2006

Influence of dynamic compression on directional hearing in the horizontal plane.

OBJECTIVE: The purpose of this study was to assess the influence of dynamic compression on directional hearing in the frontal horizontal plane. DESIGN: Compression schemes with various compression ratios and attack times were created by using a digital signal processor. The influence of compression on isolated interaural level differences (ILDs) and interaural time differences (ITDs) was examined in discrimination experiments. The combination of ILDs and ITDs was investigated with measurements of localization based on head-related transfer functions. Five normal listeners and seven hearing-impaired subjects participated in the study. RESULTS: Dynamic compression revealed substantial effects on discrimination of ILDs but not on ITDs. With higher compression ratios and shorter attack times, just-noticeable differences for ILDs increased. Lateralization via head-related transfer functions was predominantly affected by compression in the higher frequency range because for lower frequencies, interaural time cues dominate over interaural level cues. The effect of compression was similar for normal and hearing-impaired listeners, but the latter performed worse on almost all tasks, especially with ITD discrimination at 4000 Hz. CONCLUSIONS: The influence of compression ratio and attack time could be shown for ILDs, but the effect was rather small, at least for the low compression ratios commonly used in hearing aids. The influence further decreased when attack time was prolonged. The dramatic impairment of the discrimination of ITDs with the hearing-impaired subjects in the high-frequency range suggests that they primarily rely on ILD cues.

Acoustic Stimulation↗

PubMed1983

External pressure--blood flow relations during limb compression in man.

The effect of increased tissue pressure on blood flow in subcutaneous and skeletal muscle tissue was studied in 8 healthy humans resting in horizontal position. Blood flow was measured by the local 133Xe washout technique in the mid-calf region. Tissue pressure in the depot area was increased by inflating a compression cuff, 50 cm wide (knee to ankle). Blood flow rates were obtained from compressed tissues with normal vasomotor tone, at papaverine-induced vasoparalysis and during distension of the compressed vessels. The latter was achieved by inflating a proximal (extra) compression cuff on the thigh 10 or 20 mmHg above the pressure level in the more distally placed compression cuff. Increased tissue pressure was a potent stimulus for arteriolar dilatation (autoregulation) in both tissues. The autoregulatory response was to some extent counteracted by an increase in local vascular resistance in the postcapillary section as evidenced by the results of simultaneous venous stasis. Blood flow ceased in vasoparalysed tissues as well as in tissues with normal vasomotor tone, when the compression cuff was inflated to the level of the local diastolic blood pressure. Maintaining external compression at the diastolic blood pressure level, blood flow reappeared in both tissues, when the compressed vessels were distended by adding the proximal (extra) compression. It is concluded, that blood flow cessation in compressed tissues is caused by a widespread arterial-arteriolar collapse in diastole, as the volume of blood injected during the systolic peak is too small to expand also the distal sections of the precapillary vessels.

Adult↗

PubMed1982

Bioengineering studies of periodic external compression as prophylaxis against deep vein thrombosis-part I: numerical studies.

This paper presents the results of a numerical study of the technique of periodic external compression for the prevention of deep vein thrombosis. In the model the veins of the lower leg are portrayed as a continuous system rather than as discrete elements as in previous models. Consequently, we are able to explore the detailed effects of different modes of compression including (i) uniform compression, the simultaneous application of uniform pressure over the entire lower leg, (ii) graded compression, the application of nonuniform pressure, maximum at the ankle and minimum at the knee, and (iii) wavelike compression, a wave of compression proceeding from the ankle toward the knee. These numerical results indicate that the effectiveness of uniform compression is severely compromised by the formation of a flow-limiting throat at the proximal end of the compression cuff that reduces both the rate at which blood is discharged from the lower leg and the total blood volume removed. Both of these detrimental effects can be avoided by the use of either wavelike of graded compression. Both alternated methods are shown to produce more uniform augmentation of volume flow rate, flow velocity and shear stress, throughout the entire lower leg. In the companion paper, Part II [18] (see following article), these same compression modes are tested using a simple hydraulic model consisting of a single latex tube inside a foam cylinder as a highly simplified representation of a human leg.

Blood Flow Velocity↗

PubMed1997

Oncologic image compression using both wavelet and masking techniques.

A new algorithm has been developed to compress oncologic images using both wavelet transform and field masking methods. A compactly supported wavelet transform is used to decompose the original image into high- and low-frequency subband images. The region-of-interest (ROI) inside an image, such as an irradiated field in an electronic portal image, is identified using an image segmentation technique and is then used to generate a mask. The wavelet transform coefficients outside the mask region are then ignored so that these coefficients can be efficiently coded to minimize the image redundancy. In this study, an adaptive uniform scalar quantization method and Huffman coding with a fixed code book are employed in subsequent compression procedures. Three types of typical oncologic images are tested for compression using this new algorithm: CT, MRI, and electronic portal images with 256 x 256 matrix size and 8-bit gray levels. Peak signal-to-noise ratio (PSNR) is used to evaluate the quality of reconstructed image. Effects of masking and image quality on compression ratio are illustrated. Compression ratios obtained using wavelet transform with and without masking for the same PSNR are compared for all types of images. The addition of masking shows an increase of compression ratio by a factor of greater than 1.5. The effect of masking on the compression ratio depends on image type and anatomical site. A compression ratio of greater than 5 can be achieved for a lossless compression of various oncologic images with respect to the region inside the mask. Examples of reconstructed images with compression ratio greater than 50 are shown.

Algorithms↗

PubMed2003

Cochlear compression estimates from measurements of distortion-product otoacoustic emissions.

Evidence of the compressive growth of basilar-membrane displacement can be seen in distortion-product otoacoustic emission (DPOAE) levels measured as a function of stimulus level. When the levels of the two stimulus tones (f1 and f2) are related by the formula L1 = 39 dB + 0.4 x L2 [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] the shape of the function relating DPOAE level to L2 is similar (up to an L2 of 70 dB SPL) to the classic Fletcher and Munson [J. Acoust. Soc. Am. 9, 1-10 (1933)] loudness function when plotted on a logarithmic scale. Explicit estimates of compression have been derived based on recent DPOAE measurements from the laboratory. If DPOAE growth rate is defined as the slope of the DPOAE I/O function (in dB/dB), then a cogent definition of compression is the reciprocal of the growth rate. In humans with normal hearing, compression varies from about 1 at threshold to about 4 at 70 dB SPL. With hearing loss, compression is still about 1 at threshold, but grows more slowly above threshold. Median DPOAE I/O data from ears with normal hearing, mild loss, and moderate loss are each well fit by log functions. When the I/O function is logarithmic, then the corresponding compression is a linear function of stimulus level. Evidence of cochlear compression also exists in DPOAE suppression tuning curves, which indicate the level of a third stimulus tone (f3) that reduces DPOAE level by 3 dB. All three stimulus tones generate compressive growth within the cochlea; however, only the relative compression (RC) of the primary and suppressor responses is observable in DPOAE suppression data. An RC value of 1 indicates that the cochlear responses to the primary and suppressor components grow at the same rate. In normal ears, RC rises to 4, when f3 is an octave below f2. The similarities between DPOAE and loudness compression estimates suggest the possibility of predicting loudness growth from DPOAEs; however, intersubject variability makes such predictions difficult at this time.

Auditory Threshold↗

PubMed2004

Side effects of fast-acting dynamic range compression that affect intelligibility in a competing speech task.

Using a cochlear implant simulator, Stone and Moore [J. Acoust. Soc. Am. 114, 1023-1034 (2003)] reported that wideband fast-acting compression led to poorer intelligibility than slow-acting compression in a competing speech task. Compression speed was varied by using different pairs of attack and release times. In the first experiment reported here, it is shown that attack times less than about 2 ms in a wideband compressor are deleterious to intelligibility. In experiment 2, fast wideband compression was applied to the target and background either before or after mixing. The former reduced the modulation depth of each signal but maintained the independence between the two signals, while the latter introduced "comodulation." Using simulations with 6 and 11 channels, intelligibility was higher when compression was applied before mixing. In experiment 3, wideband compression was compared with multichannel compression; the latter led to reduced comodulation effects. For 6 channels, the position of the compressor, either wideband or within each channel, had no effect on intelligibility. For 11 channels, channel compression severely degraded intelligibility compared to wideband compression, presumably because of the greater reduction of across-channel contrasts. Overall, caution appears necessary in the use of fast-acting compression in cochlear implants, so as to preserve intelligibility.

Adult↗

PubMed1991

Evaluation of orthogonal polynomial compression.

In orthogonal polynomial compression, the short-term speech spectrum is first approximated by a family of orthogonal polynomials. The coefficients of each polynomial, which vary over time, are then adjusted in terms of their average value and range of variation. These adjustments can be used to compress (or expand) temporal variations in the average level, slope, and various forms of curvature of the short-term speech spectrum. The analysis and reconstruction of the short-term speech spectrum using orthogonal polynomials was implemented using a digital master hearing aid. This method of compression was evaluated on eight sensorineurally hearing-impaired listeners. Speech recognition scores were obtained for a range of compression conditions and input levels with and without frequency shaping. The results showed significant advantages over conventional linear amplification when temporal variations in the average level of the short-term spectrum were compressed, a result comparable to that obtained with conventional amplitude compression. A subset of the subjects showed further improvement when temporal variations in spectrum slope were compressed, but these subjects also showed similar improvements when frequency shaping was combined with level-only compression. None of the subjects showed improved speech recognition scores when variations in quadratic curvature were compressed in addition to level and slope compression.

Adult↗

PubMed2003

Wavelet compression of low-dose chest CT data: effect on lung nodule detection.

PURPOSE: To assess the effect of using a lossy Joint Photographic Experts Group standard for wavelet image compression, JPEG2000, on pulmonary nodule detection at low-dose computed tomography (CT). MATERIALS AND METHODS: One hundred sets of lung CT data ("cases") were compressed to 30:1, 20:1, and 10:1 levels by using a wavelet-based JPEG2000 method, resulting in 400 test cases. Each case consisted of nine 1.25-mm sections that had been obtained with 20-40 mAs. Four thoracic radiologists independently interpreted the test case images. Performance was measured by using area under the receiver operating characteristic (ROC) curve (Az) and conventional sensitivity and specificity analyses. RESULTS: There were 51 cases with and 49 without lung nodules. Az values were 0.984, 0.988, 0.972, 0.921, respectively, for original and 10:1, 20:1, and 30:1 compressed images. Az values decreased significantly at 30:1 (P =.014) but not at 10:1 compression, with a trend toward significant decrease at 20:1 (P =.051). Specificity values were unaffected by compression (>98.0% at all compression levels). Sensitivity values were 86.3% (176 of 204 test cases with nodules), 77.9% (159 of 204 cases), 76.5% (156 of 204 cases), and 70.1% (143 of 204 cases), respectively, for original and 10:1, 20:1, and 30:1 compressed images. Results of logistic regression model analysis confirmed the significant effects of compression rate and nodule attenuation, size, and location on sensitivity (P <.05). CONCLUSION: While no reduction in nodule detection at 10:1 compression levels was demonstrated by using ROC analysis, a significant decrease in sensitivity was identified. Further investigation is needed before widespread use of image compression technology in low-dose chest CT can be recommended.

Humans↗

PubMed1991

Data compression: effect on diagnostic accuracy in digital chest radiography.

High-resolution digital images make up very large data sets that are relatively slow to transmit and expensive to store. Data compression techniques are being developed to address this problem, but significant image deterioration can occur at high compression ratios. In this study, the authors evaluated a form of adaptive block cosine transform coding, a new compression technique that allows considerable compression of digital radiographs with minimal degradation of image quality. To determine the effect of data compression on diagnostic accuracy, observer tests were performed with 60 digitized chest radiographs (2,048 x 2,048 matrix, 1,024 shades of gray) containing subtle examples of pneumothorax, interstitial infiltrate, nodules, and bone lesions. Radiographs with no compression, with 25:1 compression, and with 50:1 compression ratios were presented in randomized order to 12 radiologists. The results suggest that, with this compression scheme, compression ratios as high as 25:1 may be acceptable for primary diagnosis in chest radiology.

Humans↗

PubMed1988

Influence of compression rate on initial success of resuscitation and 24 hour survival after prolonged manual cardiopulmonary resuscitation in dogs.

The influence of chest compression rate on initial resuscitation success and 24 hr survival after prolonged manual cardiopulmonary resuscitation (CPR) was investigated in 26 morphine-anesthetized dogs (17 to 30 kg). After placement of aortic and right atrial micromanometers and induction of ventricular fibrillation, manual CPR was commenced immediately and continued for 30 min. One group of 13 dogs underwent manual CPR at a compression rate of 60/min, and the other group at a rate of 120/min. The compression durations in the two groups were not significantly different (51.7 +/- 1.8% at 60/min vs 51.6 +/- 1.9% at 120/min). No drugs other than sodium bicarbonate were administered during CPR. A maximum of three attempts was permitted to defibrillate the heart. Successfully defibrillated animals were followed for 24 hr, during which time no treatment, other than naloxone, was given to reverse the effects of morphine. Arterial blood pH, PCO2, and PO2 were not significantly different in the two groups throughout the CPR period. When compared with the compression rate of 60/min, the compression rate of 120/min produced more successfully defibrillated animals (12/13 at 120/min vs 2/13 at 60/min, p less than .002) and more 24 hr survivors (8/13 at 120/min vs 2/13 at 60/min, p less than .03). All 24 hr survivors were conscious and able to sit, stand, and drink normally. One 24 hr survivor in each group had difficulty walking. Improved survival with the high-rate compression technique was consistent with the significantly higher mean aortic (systolic and diastolic) and coronary perfusion pressures attained with high-rate compressions (all p less than .002). Although the clinical applicability of these findings has yet to be demonstrated, they provide empirical support for the recent decision to increase the chest compression rate for manual CPR recommended by the American Heart Association, and indicate that the hemodynamic and survival benefits of faster compression rates in this experimental preparation were not dependent on covariant alterations in compression duration.

Animals↗

PubMed1996

Assessment of cerebral autoregulation using carotid artery compression.

BACKGROUND AND PURPOSE: A simple method of testing cerebral autoregulation by observing transcranial Doppler changes in middle cerebral artery flow velocity (FV) during a brief ipsilateral carotid artery compression (the transient hyperemic response test) was studied in 11 normal healthy volunteers. The aim of this study was to assess the reliability of the method and to compare derived autoregulatory indices with those of a standard noninvasive test of autoregulation, Aaslid's leg-cuff test. METHODS: Volunteers were subjected to repeated carotid compressions and leg-cuff tests at different levels of CO2. Hypercapnia was induced using inhalation of a mixture of 5% CO2 in air. Hypocapnia was induced by moderate hyperventilation. To assess the influence of the duration of carotid compression, a series of carotid compressions lasting 3, 4, 5, 7, and 9 seconds were performed in random sequence. Monitored parameters included ipsilateral FV, end-tidal CO2, and arterial blood pressure. The transient hyperemic response ratio (THRR), calculated as the maximum increase of FV divided by baseline values after release of the carotid compression, was taken as the autoregulation index. This index was compared with the rate of autoregulation index derived from the leg-cuff test. RESULTS: Both tests were significantly associated with end-tidal CO2 (ANOVA, P < .000001 for both carotid compression and cuff test). There was a linear correlation between THRR and autoregulation index (r = .86). However, the reproducibility of the THRR was more consistent than for the autoregulation index from single tests (13% versus 46%, respectively; P < .0001). Although the influence of the duration of carotid compression on THRR values was significant for carotid compressions lasting up to 5 seconds, there was no relation to the relative magnitude of FV drop during the compression. CONCLUSIONS: Brief (> 5 seconds) carotid artery compression provides an index of cerebral autoregulation that is reproducible and is affected by CO2 tension in a fashion similar to autoregulatory indices derived from a standard leg-cuff test. The simplicity of the method provides a potentially useful addition to other noninvasive autoregulation tests for clinical assessments, particularly when repeated measurements are required.

Adaptation, Physiological↗