| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Rapid Report |
1 Department of Circulation and Medical Imaging, Norwegian University of Science and Technology, Trondheim, Norway
2 Department of Cardiology, University of Split School of Medicine, Split, Croatia
3Department of Physiology and Biophysics, University of Split School of Medicine, Split, Croatia
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
(Received 4 May 2005;
accepted after revision 15 June 2005;
first published online 16 June 2005)
Corresponding author A. O. Brubakk: ISB, Medical Technology Center, Olav Kyrres gt. 3, 7489 Trondheim, Norway. Email: alfb{at}ntnu.no
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
This present study was designed to determine if a dive would induce a reduction in arterial endothelial function in divers and if any changes were related to venous bubble formation in the pulmonary artery.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Nine male divers aged 31 ± 5 years participated in the diving study. Their mean body mass index (BMI) was 26 ± 3 kg m–2. The subjects were all experienced divers with a mean of 726 h hours of diving (range 50–2000). None had experienced DCS in the past. At the time of the study all had a valid medical certificate for diving. An additional group of five divers aged 31 ± 6 years with a BMI of 28 ± 2 kg m–2 represented the oxygen breathing group, while a group of seven healthy subjects who never had dived served as controls. Their mean age was 31 ± 4 years and their body mass index was 25 ± 3 kg m–2, not significantly different from the two other groups. All experimental procedures were conducted in accordance with the Declaration of Helsinki, and were approved by Research Ethics Committee of the University of Split School of Medicine. Potential risks were explained to all subjects in detail and they gave written informed consent before the experiment. None of the participants of the study used any drugs, and food intake, alcohol and smoking were prohibited for 6 h prior to the test. All protocols were performed in the morning.
The nine divers were compressed in a hyperbaric chamber (Brodosplit, Split, Croatia) to 280 kPa at a rate of 100 kPa min–1 breathing air; they remained at pressure for 80 min. They were then decompressed at a rate of 90 kPa min–1 to 130 kPa, where they remained at rest for 7 min before they were decompressed to the surface pressure (100 kPa) at the same rate (United States Navy air decompression procedure). This shallow, long air dive has been shown to reproducibly produce a significant number of bubbles (Dujic et al. 2004). Following the dive, subjects were placed in the left supine position and an echocardiographic investigation with a phase array probe (1.5–3.3 MHz) using a Vivid 3 Expert ultrasonic scanner (GE, Milwaukee, USA) was performed by an experienced cardiologist (DD). High quality images were obtained in all subjects and gas bubbles were seen as high intensity echoes in the right heart and the pulmonary artery. Monitoring was performed every 20 min for 80 min after reaching surface pressure. Images were graded as previously described (Eftedal & Brubakk, 1997). This grading system has been used extensively in several animal species as well as in man. It has also been demonstrated that the grading system for Doppler (Spencer, 1976) coincides with that used for images (Brubakk & Eftedal, 2001). The grading system is non-linear when compared to the actual number of bubbles in the pulmonary artery. The bubble grades were converted into number of bubbles per square centimetre as previously described (Nishi et al. 2003). The number of bubbles were determined at each of the measurement times and then integrated to give an average bubble number for the whole observation period. The oxygen breathing group breathed a 60% oxygen–nitrogen mixture for 80 min at surface pressure (100 kPa).
Endothelial function
Endothelial function was determined according to the method of Raitakari & Celermajer, 2000) in all subjects participating in the study. This method determines the arterial response to reactive hyperaemia, flow-mediated dilation (FMD) (Corretti et al. 2002).
The subjects were placed in a quiet room with temperature about 20°C and were rested for 15 min on the bench in a supine position before measurement. Measurements were then performed with a 5.7–13.3 MHz linear transducer using a Vivid 3 Expert ultrasonic scanner (GE). Brachial artery diameter was measured from longitudinal images with lumen–intima interface visualized on both (anterior and posterior) walls. Images were acquired using ECG gating during acquisition, using the onset of R wave to identify end diastole. When the images were chosen for analysis, the boundaries for diameter measurement were identified manually with an electronic caliper. Pulsed Doppler measurements for measuring blood flow velocity were performed with the sample volume placed in mid-artery. The position of the transducer was marked to ensure the same position of the transducer for all measurements. Once the basal measurements were obtained, arterial occlusion was created by inflating a cuff placed on the forearm to 240 mmHg for 5 min. After 5 min inflation the cuff was deflated producing a brief high-flow state resulting in artery dilatation due to increased shear stress. Flow and diameter were measured during the first 15 s and then at the end of the first, second, third, fourth and fifth minute. Subjects were then rested for 10 min to get back to baseline diameter. We did not use nitroglycerine to assess endothelial-independent dilatation in the divers as we have previously shown that endogenous nitric oxide may influence the degree of bubble formation (Wisloff et al. 2003, 2004). The data were saved on tape (TDK SVHS Xp PRO) on a video recorder (Sony SVHS Hi-Fi SVO 9500 MDP). Measurements were performed before the dive and approximately 30 min after surfacing from the dive as well as before and after oxygen breathing.
The accuracy of the method was tested by having the investigator perform multiple measurements of diameter and FMD in five healthy non-divers on two separate days.
Flow mediated dilatation (FMD) was calculated as the percentage increase in brachial artery diameter from the resting state to maximal dilatation. Blood flow was calculated from the mean velocity measurements and the vessel diameter, assuming that the vessel was circular.
The oxygen breathing group underwent the same investigation for FMD before and after exposure. Furthermore, in this group, the endothelium-independent dilatation was determined by giving 1 mg nitroglycerine sublingually. The seven individuals in the control group were studied once using the same procedure as above.
Differences in diameter and FMD before and after the dive and oxygen breathing, respectively, were calculated as percentage changes.
Statistical analysis
Data are given as the mean ± standard deviation (S.D.) or as mean and 95% confidence intervals. Differences in arterial diameter and response to hyperaemia were determined using a one-sided Student's t test for paired samples. Differences between the diver and the control groups were compared using an unpaired t test. The limit of significance was set at P = 0.05.
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
|
|
The changes in brachial artery diameter and FMD after oxygen breathing can be seen in Fig. 2. The basal diameter increased from 4.4 ± 0.3 to 4.7 ± 0.3 mm (P = 0.005); this increase is not significantly different from what was observed after the dive (7.9 versus 7.9%). FMD decreased from 6.4 ± 2.7 to 4.7 ± 4.2%; this decrease was not significant, and smaller than the decrease observed following the dive (32 versus 63%). The maximum dilatation following nitroglycerine decreased slightly from 11.9 ± 4.5 to 8.9 ± 2.2% after oxygen breathing; this difference was not significant.
|
When measurement of diameter and FMD was performed repeatedly on separate days in five healthy individuals, the mean percentage intraobserver variability in diameter was 0.3 (95% CI 0.3) and 8.0 (95% CI 4.6) in FMD.
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
In the divers breathing 160 kPa oxygen for 80 min, there was a significant increase in arterial diameter with a concomitant non-significant reduction in FMD. As a non-significant decrease in dilatation following nitroglycerine was also seen, it is tempting to suggest that increased oxygen tensions influence smooth muscle function and that this effect is present even after oxygen breathing is stopped.
The diameter increases observed in this study are considerable. It is possible that the percentage reduction in FMD is partly caused by the increase in diameter. The two are probably linked, and both reduced dilatation and increased diameter are associated with increased risk of cardiovascular disease (Celermajer et al. 1994). In a large study, Kuvin et al. (2001) found FMD of 10.5% in normal controls and of 6.3% in patients with significant atherosclerosis. These are values similar to what was observed in the divers before and after the dive (9 and 5%, respectively). The divers in the present study had a brachial diameter and FMD before the dive similar to that seen in normal individuals and in the present control group.
FMD is most probably mediated by nitric oxide (NO) produced by the endothelial cells, as the response can be nearly completely abolished by L-NMMA (Mullen et al. 2001). This is also supported by other studies (Meredith et al. 1996). Following cigarette smoking, an intervention that is known to impair NO function, the diameter decreased by approximately 10% both at rest and following ischaemia (Stoner et al. 2004). We do not know the mechanism for the increase in basal diameter seen in this study. However, it is known that hyperoxia leads to vasoconstriction and that this may act as a trigger for an increased NO production (Demchenko et al. 2000). Increased NO production has been seen following hyperbaric oxygen breathing, but the increase was small when 100 kPa oxygen was used (Thom et al. 2003). Furthermore, oxygen breathing induces NO production in pulmonary endothelial cells (Cucchiaro et al. 1999), but seems to have little effect on endothelial cells from the systematic circulation (Whorton et al. 1997). We do, however, have preliminary data indicating that NO production is increased at 650 kPa breathing air, corresponding to a partial pressure of oxygen of 103 kPa (authors' unpublished observation, 2005).
As a degree of bubble formation occurred following this dive, the question remains if this will have an effect on arterial endothelial function. There was a considerably larger reduction in FMD in the divers with bubbles than in those without (7.8 versus 1.4%), although this difference was not significant (P = 0.09). We have shown in the rabbit that few bubbles will lead to endothelial dysfunction in the pulmonary artery between 1 and 6 h after exposure (Nossum et al. 2002). If we assume that endothelium in the venous circulation is activated following a dive, studies have shown that activated endothelium will produce endo-thelial microparticles and those particles can initiate endo-thelial dysfunction at remote sites (Brodsky et al. 2004). Endothelial microparticles have a size of a few micrometres and could possibly pass the lung filter and enter the arterial system. Thus, it is conceivable that changes in arterial endothelial function can occur without direct contact with the bubbles.
Even if bubble formation and its effect on the endothelium may be a likely candidate for causing the reduction in FMD, other mechanisms may also be involved. Following sympathetic stimulation, there is significant reduction in FMD (Hijmering et al. 2002). Many factors related to the dive may cause sympathetic stimulation; the present study demonstrates that hyperoxia also may play a role.
This study was not designed to elucidate the possible mechanism of the increase in basal diameter and the reduction in FMD observed. In normal individuals the maximum diameter is usually found 1 min after the release of the occluding cuff (Celermajer et al. 1994; Mullen et al. 2001). This was also found in our study in the control subjects. In the divers, even before the dive, the increase was much more gradual, with maximum vasodilatation seen at the end of the observation period of 5 min. We do not know the importance of this, but note that some of the divers clearly had reduced endothelial function even before the dive and some of them even showed a reduction in diameter following release of the cuff (divers 1 and 5).
The dive procedure followed was similar to the one used in a previous study (Dujic et al. 2004) and there was no significant difference in age and body size between the two diver groups. In that previous study considerable bubble formation was seen, with a median bubble grade of 3 and an integrated bubble load over the observation period of 0.98 bubbles cm–2. Compared to the mean bubble load of 0.0125 bubbles cm–2 seen in the present study, this demonstrates a significant individual variability causing a difference in bubble formation by a factor of 100 between the two studies.
In order to distinguish between endothelium-mediated vasodilatation and direct smooth muscle reaction, it is usual to give nitroglycerine, which will lead to vasodilatation even in the absence of endothelium (Corretti et al. 2002). This was not done in the individuals performing the dive in this study, as we have shown in several studies that NO may influence bubble formation (Wisloff et al. 2004; Wisloff et al. 2003). Based on the changes seen in the divers breathing oxygen in this study, it cannot be excluded that some of the reduction in FMD was caused by a reduction in smooth muscle function. However, a pilot study in five divers showed normal response to administration of nitroglycerine, indicating that smooth muscle function remains intact following a dive and that the reduced FMD following a dive is mainly endothelium dependent.
This is a study in a few individuals and the results therefore have to be viewed with caution. There has for many years been considerable controversy about the long-term effect of diving on the organism and some authors have claimed that diving will lead to permanent CNS changes (Todnem et al. 1990). However, while it is well acknowledged that decompression accidents can lead to CNS changes, no changes have been conclusively documented in divers who have not had any such accidents (Dutka, 2003). The present study shows that diving, even with minimum bubble formation, may impair endothelial function. However, regeneration of injured endothelial cells occurs and may prevent permanent injury (Dimmeler & Zeiher, 2004). Further studies are needed to determine the importance of these findings as a possible mechanism for long-term changes.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Brodsky
SV, Zhang
F, Nasjletti
A
&
Goligorsky
MS (2004). Endothelium-derived microparticles impair endothelial function in vitro. Am J Physiol Heart Circ Physiol
286, H1910–H1915.
Brubakk AO & Eftedal O (2001). Comparison of three different ultrasonic methods for quantification of intravascular gas bubbles. Undersea Hyperb Med 28, 131–136.[Medline]
Celermajer DS, Sorensen KE, Bull C, Robinson J & Deanfield JE (1994). Endothelium-dependent dilation in the systemic arteries of asymptomatic subjects relates to coronary risk factors and their interaction. J Am Coll Cardiol 24, 1468–1474.[Abstract]
Corretti
MC, Anderson
TJ, Benjamin
EJ, Celermajer
D, Charbonneau
F, Creager
MA, Deanfield
J, Drexler
H, Gerhard-Herman
M, Herrington
D, Vallance
P, Vita
J
&
Vogel
R (2002). Guidelines for the ultrasound assessment of endothelial-dependent flow-mediated vasodilation of the brachial artery: a report of the International Brachial Artery Reactivity Task Force. J Am Coll Cardiol
39, 257–265.
Cucchiaro G, Tatum AH, Brown MC, Camporesi EM, Daucher JW & Hakim TS (1999). Inducible nitric oxide synthase in the lung and exhaled nitric oxide after hyperoxia. Am J Physiol 277, L636–L644.[Medline]
Demchenko
IT, Boso
AE, O'Neill
TJ, Bennett
PB
&
Piantadosi
CA (2000). Nitric oxide and cerebral blood flow responses to hyperbaric oxygen. J Appl Physiol
88, 1381–1389.
Dimmeler S & Zeiher AM (2004). Vascular repair by circulating endothelial progenitor cells: the missing link in atherosclerosis? J Mol Med 82, 671–677.[CrossRef][Medline]
Dujic
Z, Duplancic
D, Marinovic-Terzic
I, Bakovic
D, Ivancev
V, Valic
Z, Eterovic
D, Petri
NM, Wisloff
U
&
Brubakk
AO (2004). Aerobic exercise before diving reduces venous gas bubble formation in humans. J Physiol
555, 637–642.
Dutka AJ (2003). Long term effects on the central nervous system. In Bennett and Elliott's Physiology and Medicine of Diving, ed. Brubakk AO & Neuman TS, pp. 680–699. Saunders, London.
Eckenhoff
RG, Olstad
CS
&
Carrod
G (1990). Human dose–response relationship for decompression and endogenous bubble formation. J Appl Physiol
69, 914–918.
Eftedal O & Brubakk AO (1997). Agreement between trained and untrained observers in grading intravascular bubble signals in ultrasonic images. Undersea Hyperbaric Med 24, 293–299.
Eldridge
MW, Dempsey
JA, Haverkamp
HC, Lovering
AT
&
Hokanson
JS (2004). Exercise-induced intrapulmonary arteriovenous shunting in healthy humans. J Appl Physiol
97, 797–805.
Gersh I, Hawkinson GE & Rathburn EN (1944). Tissue and vascular bubbles after decompression from high pressure atmospheres – correlation of specific gravity with morphological changes. J Cell Comp Phy 24, 35–70.[CrossRef]
Grulke DC, Marsh PL & Hills BA (1973). Experimental air embolism: Measurement of microbubbles using the coulter counter. Br J Exp Path 54, 684–691.
Hijmering
ML, Stroes
ES, Olijhoek
J, Hutten
BA, Blankestijn
PJ
&
Rabelink
T (2002). Sympathetic activation markedly reduces endothelium-dependent, flow-mediated vasodilation. J Am Coll Cardiol
39, 683–688.
Kuvin
JT, Patel
AR, Sliney
KA, Pandian
NG, Rand
WM, Udelson
JE
&
Karas
RH (2001). Peripheral vascular endothelial function testing as a noninvasive indicator of coronary artery disease. J Am Coll Cardiol
38, 1843–1849.
Meredith IT, Currie KE, Anderson TJ, Roddy MA, Ganz P & Creager MA (1996). Postischemic vasodilation in human forearm is dependent on endothelium-derived nitric oxide. Am J Physiol 270, H1435–H1440.[Medline]
Moon RE, Camporesi EM & Kisslo JA (1989). Patent foramen ovale and decompression sickness in divers. Lancet i, 513–514.
Mullen
MJ, Kharbanda
RK, Cross
J, Donald
AE, Taylor
M, Vallance
P, Deanfield
JE
&
MacAllister
RJ (2001). Heterogenous nature of flow-mediated dilatation in human conduit arteries in vivo: relevance to endothelial dysfunction in hypercholesterolemia. Circ Res
88, 145–151.
Nishi RY, Brubakk AO & Eftedal O (2003). Bubble detection. In Bennet and Elliott's the Physiology and Medicine of Diving, ed. Brubakk AO & Neuman TS, pp. 501–529. Saunders, London.
Nossum V & Brubakk AO (1999). Endothelial damage by bubbles in the pulmonary artery of the pig. Undersea Hyperbaric Med 26, 1–8.
Nossum V, Hjelde A & Brubakk AO (2002). Small amounts of venous gas embolism cause delayed impairment of endothelial function and increase polymorphonuclear neutrophil infiltration. Eur J Appl Physiol 86, 209–214.[Medline]
Raitakari OT & Celermajer DS (2000). Testing for endothelial dysfunction. Ann Med 32, 293–304.[Medline]
Spencer
MP (1976). Decompression limits for compressed air determined by ultrasonically detected blood bubbles. J Appl Physiol
40, 229–235.
Stoner
L, Sabatier
M, Edge
K
&
McCully
K (2004). Relationship between blood velocity and conduit artery diameter and the effects of smoking on vascular responsiveness. J Appl Physiol
96, 2139–2145.
Thom
SR, Fisher
D, Zhang
J, Bhopale
VM, Ohnishi
ST, Kotake
Y, Ohnishi
T
&
Buerk
DG (2003). Stimulation of perivascular nitric oxide synthesis by oxygen. Am J Physiol Heart Circ Physiol
284, H1230–H1239.
Todnem K, Nyland H, Kambestad BK & Aarli JA (1990). Influence of occupational diving upon the nervous system. Br J Ind Med 47, 708–714.[Medline]
Whorton AR, Simonds DB & Piantadosi CA (1997). Regulation of nitric oxide synthesis by oxygen in vascular endothelial cells. Am J Physiol 272, L1161–L1166.[Medline]
Wilmshurst PT, Byrne JC & Webb-Peploe MM (1989). Relation between interarterial shunts and decompression sickness in divers. Lancet ii, 1302–1306.
Wisloff
U, Richardson
RS
&
Brubakk
AO (2003). NOS inhibition increases bubble formation and reduces survival in sedentary but not exercised rats. J Physiol
546, 577–582.
Wisloff
U, Richardson
RS
&
Brubakk
AO (2004). Exercise and nitric oxide prevent bubble formation: a novel approach to the prevention of decompression sickness?
J Physiol
555, 825–829.
|
Acknowledgements |
|---|
This article has been cited by other articles:
|
|
 |
|
  A. Obad, I. Palada, Z. Valic, V. Ivancev, D. Bakovic, U. Wisloff, A. O. Brubakk, and Z. Dujic The effects of acute oral antioxidants on diving-induced alterations in human cardiovascular function J. Physiol., February 1, 2007; 578(3): 859 - 870. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
