A SIMPLIFIED PAIRED NECK CHAMBER
FOR THE DEMONSTRATION OF BAROREFLEX
BLOOD PRESSURE REGULATION
Neil M. Raine1 and N. Timothy Cable2
1School of Health Studies, Edge Hill University College, Fazakerley Hospital, Liverpool L9 7AL; and2Research Institute for Sport and Exercise Science, Liverpool John Moores University,
Liverpool L3 2ET, United Kingdom
I
n this investigation a simplified variable-pressure paired neck chamber was devel-oped as a practical alternative to traditional neck collar designs used to study the arterial baroreceptor reflex in humans. The purpose of this new design was to extend the use of the noninvasive neck chamber method of baroreceptor investigation to teachers of physiology. Performance tests indicate that these new chambers are capable of delineating the stimulus-response relationship for both the blood pressure baroreflex [sensitivity50.42560.13 mmHg mean arterial pressure (MAP)/mmHg neck chamber pressure (NCP); range 5 24.9 6 4.6 mmHg MAP] and the heart rate baroreflex (sensitivity50.27360.12 beats.min21.mmHg NCP21; range516.766.8 beats/min). This was achieved by applying localized positive and negative air pressures to the carotid sinuses throughout the range from 160 to 260 mmHg in steps of 20 mmHg. This simplified neck chamber method offers distinct methodological advantages over tradi-tional neck collars, making it a valuable tool for demonstrating baroreflex regulation of the circulation.AM. J. PHYSIOL. 277 (ADV. PHYSIOL. EDUC. 22): S60–S66, 1999.
Key words:carotid baroreceptors; cardiovascular; neck suction
Experimental techniques in physiology often are not accessible for the purpose of teaching unless the institution conducts specialized research in a given field. This problem can be further complicated by the lack of availability of noninvasive methods that are preferable for the demonstration of key physiological mechanisms in a learning environment. The study of baroreflex blood pressure control is one such mecha-nism that provides students with a valuable insight into regulation of the circulation. This is also an example, however, of a case in which practiced research methods have not made the transition into the teaching laboratory, and that is the problem addressed here.
Study of the carotid baroreflex was greatly expanded after the original development of the noninvasive variable-pressure neck chamber method (9). Because of the accessibility of the carotid sinus, with the use of this approach the carotid baroreceptor afferent nerve activity may be manipulated directly, which was not previously possible in human volunteers. The blood pressure, heart rate, and vascular responses to altered baroreceptor input can then be studied to gain impor-tant information concerning autonomic regulation of the circulation. Since its inception over 40 years ago, however, during which time there have been several advances to the original neck chamber (5, 15, 3, 11, 12), this method has not been widely implemented in
the learning environment. This lack of application appears to be related, at least in part, to difficulties in manufacturing neck chamber devices. Specifically, these chambers are required1) to maintain an airtight seal around the anatomy of the neck and mandible and 2) to be capable of delivering both positive and negative air pressures. The purpose of this paper then is to extend to teachers of physiology the ease and effectiveness of a new variable-pressure paired neck chamber used to study and/or demonstrate baroreflex blood pressure control in humans. The device pre-sented here is an adaptation of the paired neck chamber originally developed by Kelly et al. (12).
METHODS
Theory of neck chamber technique.The applica-tion of pressure to the skin overlying the carotid sinuses influences the frequency of nerve impulses arising from the baroreceptors. With the onset of neck suction there is an increase in afferent nerve impulse frequency that simulates arterial hypertension, the autonomic response to which is a reduction in heart
rate and blood pressure. Conversely, positive neck pressure simulates arterial hypotension by reducing the baroreceptor impulse frequency, resulting in oppo-site autonomic responses. Within the linear range, the measured changes in heart rate and blood pressure are proportional to the degree of neck suction (barorecep-tor stimulation) and pressure (barorecep(barorecep-tor inhibi-tion), allowing the stimulus-response relationship to be calculated. An example of the responses elicited using the paired neck chambers developed in this investigation is shown in Fig. 1.
Construction of neck chambers.Historically, neck chambers have evolved from devices that covered the neck and upper thorax (9, 13) or the entire head (3) to neck collars made from sheet lead (5), glass fiber, silicone rubber (15), and, more recently, molded thermoplas-tic materials (2, 11). The paired neck chamber, again molded from thermoplastic, is the most recent devel-opment that allows access to the suprasternal notch for the simultaneous measurements of aortic blood velocity, blood pressure, and heart rate (12).
FIG. 1.
Repr esentative r ecor ding fr om one subject generated using the simplified pair ed neck chamber device.Left tr ace
The design presented here is an adaptation of the paired neck chambers developed by Kelly et al. (12). The key features of the system are the chambers themselves, which were adapted from commercially available industrial ear protectors (North, Hyde, UK). This new device is ideal for positioning over the carotid sinuses and comes already cushioned with an airtight polyvinyl chloride (PVC) rubber surround to form a dependable seal against the contours of the anterior lateral neck. The simple process of modifica-tion was performed as follows. The ear protecmodifica-tion cups (chambers) were first detached from the head strap of the intact unit by removing a rubber fitting located through a port in the center of each chamber. A tapered suction connector was then introduced through the port of each chamber and coated with a silicone sealant (Dow Corning) to ensure that an airtight seal was maintained. After these modifications have been made, the chambers may then be used for the delivery of neck suction. For the delivery of positive air pressure, a latex rubber membrane was attached to the cushioned PVC rim of two additional chambers using adhesive (Evostik). When inflated with air this membrane distends against the neck tissue, producing a relative compression of the carotid artery. It is possible to use the same pair of chambers for the delivery of both neck suction and positive pressure by fitting a valve mechanism to the latex rubber membrane, as detailed by Kelly et al. (11). However, through our experiences in this laboratory it has proved simpler to use separate pairs of cham-bers. The chambers modified for use in this investiga-tion are shown in Fig. 2.
Once adapted, the neck chambers were connected through suction hosing to a pressure delivery circuit, as detailed in Fig. 3. This circuit was arranged to ensure the accurate and rapid delivery to the neck chambers of forces initiated when a mechanical clamp is released. Throughout the period of measurement, pressure in the chambers was monitored using an electronic pressure transducer (Sensym, SX05DN, Farnell Electronics), which, although not essential, provides a continuous feedback of the stimulus inten-sity. Suction pressure in the reservoir was generated with a standard vacuum cleaner, and the degree of suction pressure was regulated with an adjustable transformer (Variac, Zenith Electric) (a sliding valve mechanism in the vacuum hosing will suffice). For the
data generated in this report, positive air pressure was delivered through a rapid cuff inflator (Hokanson, Bellevue, WA) connected directly to the neck cham-ber tubing. However, a simple positive pressure pump may be used to pressurize the reservoir as described for the delivery of suction forces (Fig. 3).
Measur ement pr otocol. Eight male subjects 26 6 1.1 yr of age (mean6SE), 17861.9 cm in height, and 7163 kg in body mass volunteered to participate in a study to validate the effectiveness of the paired neck chambers. The experiment was formally approved by the Human Ethics Committee of Liverpool John Moores University. These subjects were physically active and presented with arterial blood pressure within the normal range for the population (mean arterial pres-sure580.262.2 mmHg).
To ensure that neck chambers were placed in the correct position, each carotid artery was first palpated and the overlying skin marked. The chambers were then secured onto the neck by using a Velcro strap to maintain a dependable seal (Fig. 4). With the head supported in an extended position, each stimulus was then applied for 20 s at suction levels of220, 240,
FIG. 2.
and260 mmHg and positive pressure at levels of 20, 40 and 60 mmHg. This time interval was chosen to allow for completion of both rapid vagal and slower acting sympathetic nerve responses (14). Each stimu-lus was presented during the early to middle phases of expiration, when pulse interval responses have been shown to be greatest (7, 8), and subjects were asked to maintain a normal respiratory depth and rate throughout. Blood pressure and heart rate were then allowed to return to baseline level between the successive applications of pressure.
Continuous recordings of pulsatile systemic arterial blood pressure were recorded from the digital artery of the finger using the Finapres device (Ohmeda Finapres 2300, Englewood, CO). This instrument uses a photoplethysmographic tec hnique to monitor changes in finger blood volume continuously, with the principle of operation being that the air pressure supplied to the finger cuff to maintain a constant finger blood volume, and thus zero digital artery transmural pressure, equates to digital artery pressure. Continuous records of the electrocardiogram (Life-Trace 12, Albury Instruments) and neck chamber pressure were also recorded on-line throughout the
FIG. 3.
Arrangement of neck chamber pr essur e delivery apparatus. Pr essur e r eservoir may be either evacuated or pr essurized to ensur e accurate and rapid delivery of neck suction and pr essur e, r espectively. 1, Mer cury manometer; 2, pr essur e r esistant r eservoir; 3, suction/ pr essur e port; 4, mechanical clamp; 5, air outlet; 6, pair ed neck chambers; 7, electr onic pr essur e transducer.
FIG. 4.
evaluation of baroreflex sensitivity (Fig. 1). Each evaluation lasted 15–20 min.
Calculation of blood pr essur e and heart rate bar or eflex variables.Baroreflex variables were cal-culated from the relationships between neck chamber pressure (NCP) and the change in mean arterial pressure (NCP-DMAP) and heart rate (NCP-DHR). The DMAP andDHR values were calculated by subtracting the average MAP (5 cycles) and HR (3 cycles) values immediately before the onset of the stimulus from the maximum response during neck suction and neck pres-sure. Maximum baroreflex sensitivity was calculated from each stimulus-response relationship by least-squares linear regression analysis of every three consecutive data points. In addition, total buffering capacity of the baroreflex was taken as the total range of the NCP-DMAP and NCP-DHR stimulus-response relationships.
RESULTS
The mean (6SE) rate of pressure development within the paired neck chambers was 2840 6 22 mmHg/s for suction and 977 6 55 mmHg/s for positive air pressures. This rapid rate of force development, depicted by the neck chamber pressure trace in Fig. 1., is an important consideration for baroreceptor stimulation (6). During the application of neck suction with the paired neck chambers there was a reflex slowing of heart rate followed by a decrease in blood pressure. Conversely, positive neck chamber pressure led to a reflex rise in heart rate associated with an increase in arterial pressure (Fig. 1). Plotting the entire baroreflex functions revealed the sigmoid-shaped stimulus-response relationship for both the blood pressure and heart rate variables (Figs. 5 and 6). The blood pressure baroreflex (n516) displayed a maxi-mum sensitivity of 0.425 6 0.13 mmHg MAP/mmHg NCP and a total range of 24.964.6 mmHg MAP (Fig 5). For the heart rate baroreflex (n 516), maximum sensitivity was 0.27360.12 beats.min21.mmHg NCP21, with a total stimulus-response range of 16.7 6 6.8 beats/min (Fig. 6).
DISCUSSION
The aim of this communication was to extend a useful experimental approach for the study of arterial blood pressure regulation in humans to teachers of
physiol-FIG. 5.
Blood pr essur e arterial bar or eflex . Changes (D) in mean arterial blood pr essur e ar e plotted in r esponse to alter ed car otid sinus transmural pr essur e applied using pair ed neck chambers. Values ar e means6SE of 16 observations measur ed fr om 8 r esting subjects in either supine or seated postur es.
FIG. 6.
ogy. This new design offers major practical advantages over traditional neck chamber arrangements while allowing a meaningful interpretation of the indexes of baroreceptor function. The drawbacks to this modi-fied neck chamber are also discussed here, although from the results gained these drawbacks do not preclude its use as a valuable teaching and research tool.
Advantages of the appr oach.The major advantage of the simplified paired neck chamber is the simplicity with which it can be constructed, requiring only basic modifications to a commercially available product. Other authors have described variable-pressure cham-bers that are constructed by molding material directly onto the skin to achieve an airtight seal (2, 11, 15). With the use of this approach, however, it is difficult to manufacture one chamber to fit all subjects. The paired neck chambers used in this study eliminate the molding process altogether, thereby reducing both the labor and financial costs of production. Other methodological benefits of the device relate to the generation and maintenance of air pressures. Because of the small chamber volume, air is quickly evacuated and introduced, resulting in a rapid presentation of the stimulus to the carotid sinus, an important determi-nant of the baroreflex response (6). Furthermore, these new chambers produce a reliable seal against the skin of all subjects tested while maintaining a stable pressure throughout the period of measure-ment (Fig. 1).
Limitations of the appr oach. From the experi-ments conducted in this laboratory, the main disadvan-tage of the paired neck chamber is its inability to challenge the baroreceptor input in a given propor-tion of volunteers,,30%. Such a problem may relate
to differences in the anatomic position of the carotid sinus because, in all cases, the external pressures applied to the carotid arteries were well maintained. A carotid bifurcation located superior to the border of the mandible would be predicted to escape the stimulus of altered arterial transmural pressure, thereby producing a diminished cardiovascular response. How-ever, it was not possible to confirm this from the present study. Other authors have not reported the failure rate of traditional neck chambers, although it is reasonable that a stimulus applied to the entire
ante-rior aspect of the neck is sufficient to influence the baroreceptor input in most subjects.
With regard to the safety implications of this noninva-sive method, we have observed symptoms of light-headedness in only two volunteers during the applica-tion of neck sucapplica-tion in this laboratory; all other volunteers remained asymptomatic throughout. On one occasion, symptoms of syncope presented when neck suction was commenced immediately after maxi-mal cycling exercise in a subject with marked postex-ercise hypotension. On the other occasion, symptoms were associated with the use of a very high suction pressure beyond that used in this communication. The symptoms just described were quickly reversed in both of these instances on immediate withdrawal of the stimulus. Students of physiology who are super-vised while using this method, under resting condi-tions within the range of pressures reported here, therefore are not considered to be at risk. In support of the suitability of the neck chamber as a teaching and research tool, to our knowledge other authors have reported no adverse effects using this method of baroreceptor investigation.
Per for mance of pair ed neck chamber.Analysis of the blood pressure and heart rate responses derived using the paired neck chambers revealed the pre-dicted sigmoid-shaped stimulus-response relation-ships as shown in Figs. 5 and 6. This is only possible if the arterial baroreflex is forced into both the thresh-old and saturation regions of receptor firing by using
TABLE 1
Comparison of pulse interval and heart rate r esponses to standar dized neck suction between differ ent neck chambers
DR-R, ms
213.9 Lead collar 1
200* Silastic collar 10
23.6* Lead collar 14
107670 Paired-thermoplastic 12
2686166 Lead collar 12
208631 27.861.4 Simplified-paired Present study
an appropriate stimulus range of 120 mmHg. From performance tests conducted at a stimulus intensity of 240 mmHg, the mean heart rate and pulse interval (R-R) responses using the paired neck chambers were within the range reported for conventional neck collar designs (Table 1). This range in the pulse interval response between neck chambers is large (233 ms), however, which may relate to the different subject populations examined in these investigations. Kelly et al. (12) addressed this problem and reported that pulse interval responses were attenuated with the use of a more localized paired neck suction stimulus compared with the use of a lead collar (Table 1), although the explanation for this observation is pres-ently unclear. Additional support for the effectiveness of the device reported here is its ability to detect the predicted changes in both the blood pressure and heart rate baroreflex associated with strenuous dy-namic exercise (4).
The choice of the measurement tool also influences the quality of the results gained, given the speed of the cardiovascular response. Although the best results were obtained using on-line measurements of neck chamber pressure, the electrocardiogram, and pulsa-tile blood pressure (Fig. 1), this does not preclude the use of the paired neck chamber as an effective teaching tool. Assessments of baroreceptor function can be performed using standard auscultatory tech-niques (16), although it would prove difficult to define the entire stimulus-response relationship because of problems in synchronizing the stimulus and the mea-sured response. Alternatively, measurements can be confined to those of heart rate using a simple bipolar electrocardiogram without any loss of interpretation.
In conclusion, the simplified paired neck chamber method offers certain practical advantages over other traditional neck collars used to study human arterial baroreflex function. The cardiovascular response to baroreceptor challenges can be measured using a simple bipolar electrocardiogram. Performance tests indicate this new device is capable of delineating both the blood pressure and heart rate baroreflex relations, making it a valuable teaching and demonstration tool.
We acknowledge the technical assistance of L. Berry and C. Jones throughout this investigation.
Address for reprint requests and other correspondence: N. M. Raine, School of Biological and Earth Sciences, Division of Science
and Engineering, Liverpool John Moores University, James Parsons Bldg., Byrom St., Liverpool L3 3AF, United Kingdom (E-mail: n.raine@livjm.ac.uk).
Received 18 November 1998; accepted in final form 20 July 1999.
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