Instant of vascular occlusion defined with laser-Doppler flowmetry TAWFIC S. HAKIM Department of Surgery,
State University
of New
HAKIM, TAWFIC S. Instant of vascular occlusion defined lvith J. Appl. Physiol. 73( 1): 284-289, floun-netry. 1992.-Derivation of capillary pressure from tracings postarterial (AO) or -venous (VO) occlusion requires back extrapolation to an instant near the time of occlusion. This instant is difficult to identify because of pressure artifacts created by the occlusion maneuver. Theoretically, when the flow in the main artery (or veins) is stopped instantaneously, the flow in the arterioles (or venule) will stop after a short time delay (perhaps ~100 ms). When flow had stopped in the main artery and in the arteriole, the pressure in the main artery at that instant would equal the pressure in the arterioles. We sought to identify the instant when flow stops in the arterioles and venules after A0 and VO, respectively. In an isolated perfused dog left lower lobe preparation flow in the main vessels were monitored with inline flow probes, whereas flow in the microcirculation was monitored with laser-Doppler flow (LDF) probe placed on the lung surface. A sudden decline in arterial flow was detected by the LDF probe after 54 ms, while a sudden decline in venous flow was detected in the venules after 35 ms. These time delays were used as wave transmission time across the arterial and venous trees. Consequently, it was concluded that after AO, flow in the arterioles would stop 54 ms after it had become zero in the main artery, while after VO flow in the venules would stop 35 ms after it had become zero in the main vein. The pressure postA0 and post-V0 was read at these instants (54 and 35 ms after flow in the main vessel reached zero). The postocclusion pressures analyzed in this manner presumably represent the pressure in the arterioles and venules. These pressures were found to be nearly equal to the double occlusion capillary pressure. Thus derivation of the arteriolar and venular pressure from the A0 and VO would best be accomplished by using the pressure at ~54 and 35 ms after the flow had reached zero. laser-Doppler
arterial and venous occlusion; dog; subpleural nin; wave speed velocity
region; seroto-
OR VENOUS OCCLUSION is associated with a rapid change in either arterial or venous pressure followed by a slow change (9, 10). The initial rapid change represents the pressure gradient across the arterial or venous segments while the slow change represents the emptying or filling of the microvasculature. Theoretically, there is a break point between the two phases of pressure change. However, this break point is difficult to identify because the pressure signal is always distorted by artifacts generated during and immediately after the occlusion maneuver (7). Therefore, investigators have resorted to different analysis techniques that presuppose a specific model for the vasculature and therefore use indirect ways of guessing at what instant after the start ARTERIAL
284
0161-7567192
$2.00 Copyright
York Health
Science Center, Syracuse, New York 13210
of occlusion does this point of inflection occur. Waves travel with a finite velocity across the arteries and veins (2,15,17), and consequently when blood flow is stopped at the inlet artery, the pulmonary arterial pressure would equilibrate after a time delay with the pressure in the precapillary arterioles. Once this equilibration between the two pressures is complete, the pressure in the main artery represents the pressure in the arterioles. In a preliminary study we have utilized video microscopy to visualize blood flow in the subpleural vessels during arterial and venous occlusion (l&16). It was unequivocally clear that after arterial occlusion, the first event that occurs is arrest of blood flow in the arterioles. Likewise, after venous occlusion, the first detectable change in the microcirculation is the arrest of blood flow in the venules. Thus, when flow is arrested in the main artery (arterial occlusion) or vein (venous occlusion), a fraction of a second later blood flow stops in the arterioles or venules, respectively. Thereafter blood continues to flow out of or into the capillaries for a few seconds until the pressure throughout the vasculature equilibrates. In this study laser-Doppler flowmetry (LDF) was used to monitor blood flow in the subpleural region (Qs). We measured the instant when Qs begins to change after the occlusion. Because the area being monitored is relatively large (m 1 mm3) and contains a number of arterioles, capillaries, and venules, we considered that the first sign of rapid decline in LDF signal after arterial occlusion would represent the instant when flow in the subpleural arterioles began to decelerate. Similarly we considered that the first sign of a rapid decline in LDF signal after venous occlusion would represent the instant when blood flow in the subpleural venules had begun to decelerate. The linearity, stability, and adequacy of the response time of LDF in a variety of tissues (3,18,19), including the lung surface (8), have been discussed previously. MATERIALS
AND METHODS
Seven mongrel dogs of either sex (16-19 kg) were anesthetized with pentobarbital sodium (25 mg/kg body wt), heparinized (700 U/kg), and exsanguinated through a catheter in the carotid artery. The animals were placed in the right oblique decubitus position and ventilated mechanically (Harvard pump). Through a left thoracotomy the left upper and middle lobes were tied off and excised. The left lower lobe (LLL) was rapidly exposed to prepare it for perfusion in situ as described previously (7). Can-
0 1992 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 18, 2019.
PULMONARY Flow
arterial P836PUO
VASCULAR
OCCLUSIONS
285
tleter
iP ~
Laser
Doppler
Flowmeter
FIG. 1. In situ left lower lobe preparation. A, bypass kept closed during constant pressure perfusion; &a, arterial blood flow; Qs, subpleural blood flow; Pa, arterial pulmonary pressure; Pv, venous pulmonary pressure; Palv, alveolar pressure.
Flow
water
uenow
Probe
bath
Pressure Transducers
reaeruoir
nulas were placed in the main artery and vein of the LLL. The arterial and venous cannulas were connected to a perfusion system as shown in Fig. 1. The perfusion system was filled with autologous blood (hematocrit 40%) and consisted of two reservoirs, two in-line probes (8 mm ID, Transonic Systems, Ithaca, NY), and a pump. The lobe was perfused with blood from the arterial reservoir, and the blood was allowed to drain into the venous reservoir. The arterial reservoir was set at a level above the lobe to provide a nonpulsatile perfusion pressure of 1015 mmHg. The venous reservoir was adju sted to provide a venous plressure of ~1 mmHg and was immersed in a water bath at 40°C to maintain the blood temperature normal. The blood was pumped from the venous reservoir into the arterial reservoir to maintain level of blood constant . The pulmonary arte rial and venous pressures (Pa and Pv, respectively) were measured from side ports (15 gauge) in the cannulas using stiff Tygon tubing (30 cm long) and two identical pressure transducers (PdI23B). The vascular pressures were references to the top most region of the lobe. The in-line flow probes (8 mm ID, Transonic Systems) were interposed between the arterial reservoir and the arterial cannula and between the venous reservoir and venous cannula. The two probes were connected to a two-channel blood flowmeter (T201, Transonic Systems) with the frequency response set at 30 Hz on both channels. The outputs from the two flow channels were fed into the polygraph recorder for continuous recording. The lobe was ventilated with a gas mixture of 6% CO,-35% O,-59% N,. End-expiratory pressure was set at l-2 Torr, and inspiratory pressure was kept to
State University
of New
HAKIM, TAWFIC S. Instant of vascular occlusion defined lvith J. Appl. Physiol. 73( 1): 284-289, floun-netry. 1992.-Derivation of capillary pressure from tracings postarterial (AO) or -venous (VO) occlusion requires back extrapolation to an instant near the time of occlusion. This instant is difficult to identify because of pressure artifacts created by the occlusion maneuver. Theoretically, when the flow in the main artery (or veins) is stopped instantaneously, the flow in the arterioles (or venule) will stop after a short time delay (perhaps ~100 ms). When flow had stopped in the main artery and in the arteriole, the pressure in the main artery at that instant would equal the pressure in the arterioles. We sought to identify the instant when flow stops in the arterioles and venules after A0 and VO, respectively. In an isolated perfused dog left lower lobe preparation flow in the main vessels were monitored with inline flow probes, whereas flow in the microcirculation was monitored with laser-Doppler flow (LDF) probe placed on the lung surface. A sudden decline in arterial flow was detected by the LDF probe after 54 ms, while a sudden decline in venous flow was detected in the venules after 35 ms. These time delays were used as wave transmission time across the arterial and venous trees. Consequently, it was concluded that after AO, flow in the arterioles would stop 54 ms after it had become zero in the main artery, while after VO flow in the venules would stop 35 ms after it had become zero in the main vein. The pressure postA0 and post-V0 was read at these instants (54 and 35 ms after flow in the main vessel reached zero). The postocclusion pressures analyzed in this manner presumably represent the pressure in the arterioles and venules. These pressures were found to be nearly equal to the double occlusion capillary pressure. Thus derivation of the arteriolar and venular pressure from the A0 and VO would best be accomplished by using the pressure at ~54 and 35 ms after the flow had reached zero. laser-Doppler
arterial and venous occlusion; dog; subpleural nin; wave speed velocity
region; seroto-
OR VENOUS OCCLUSION is associated with a rapid change in either arterial or venous pressure followed by a slow change (9, 10). The initial rapid change represents the pressure gradient across the arterial or venous segments while the slow change represents the emptying or filling of the microvasculature. Theoretically, there is a break point between the two phases of pressure change. However, this break point is difficult to identify because the pressure signal is always distorted by artifacts generated during and immediately after the occlusion maneuver (7). Therefore, investigators have resorted to different analysis techniques that presuppose a specific model for the vasculature and therefore use indirect ways of guessing at what instant after the start ARTERIAL
284
0161-7567192
$2.00 Copyright
York Health
Science Center, Syracuse, New York 13210
of occlusion does this point of inflection occur. Waves travel with a finite velocity across the arteries and veins (2,15,17), and consequently when blood flow is stopped at the inlet artery, the pulmonary arterial pressure would equilibrate after a time delay with the pressure in the precapillary arterioles. Once this equilibration between the two pressures is complete, the pressure in the main artery represents the pressure in the arterioles. In a preliminary study we have utilized video microscopy to visualize blood flow in the subpleural vessels during arterial and venous occlusion (l&16). It was unequivocally clear that after arterial occlusion, the first event that occurs is arrest of blood flow in the arterioles. Likewise, after venous occlusion, the first detectable change in the microcirculation is the arrest of blood flow in the venules. Thus, when flow is arrested in the main artery (arterial occlusion) or vein (venous occlusion), a fraction of a second later blood flow stops in the arterioles or venules, respectively. Thereafter blood continues to flow out of or into the capillaries for a few seconds until the pressure throughout the vasculature equilibrates. In this study laser-Doppler flowmetry (LDF) was used to monitor blood flow in the subpleural region (Qs). We measured the instant when Qs begins to change after the occlusion. Because the area being monitored is relatively large (m 1 mm3) and contains a number of arterioles, capillaries, and venules, we considered that the first sign of rapid decline in LDF signal after arterial occlusion would represent the instant when flow in the subpleural arterioles began to decelerate. Similarly we considered that the first sign of a rapid decline in LDF signal after venous occlusion would represent the instant when blood flow in the subpleural venules had begun to decelerate. The linearity, stability, and adequacy of the response time of LDF in a variety of tissues (3,18,19), including the lung surface (8), have been discussed previously. MATERIALS
AND METHODS
Seven mongrel dogs of either sex (16-19 kg) were anesthetized with pentobarbital sodium (25 mg/kg body wt), heparinized (700 U/kg), and exsanguinated through a catheter in the carotid artery. The animals were placed in the right oblique decubitus position and ventilated mechanically (Harvard pump). Through a left thoracotomy the left upper and middle lobes were tied off and excised. The left lower lobe (LLL) was rapidly exposed to prepare it for perfusion in situ as described previously (7). Can-
0 1992 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 18, 2019.
PULMONARY Flow
arterial P836PUO
VASCULAR
OCCLUSIONS
285
tleter
iP ~
Laser
Doppler
Flowmeter
FIG. 1. In situ left lower lobe preparation. A, bypass kept closed during constant pressure perfusion; &a, arterial blood flow; Qs, subpleural blood flow; Pa, arterial pulmonary pressure; Pv, venous pulmonary pressure; Palv, alveolar pressure.
Flow
water
uenow
Probe
bath
Pressure Transducers
reaeruoir
nulas were placed in the main artery and vein of the LLL. The arterial and venous cannulas were connected to a perfusion system as shown in Fig. 1. The perfusion system was filled with autologous blood (hematocrit 40%) and consisted of two reservoirs, two in-line probes (8 mm ID, Transonic Systems, Ithaca, NY), and a pump. The lobe was perfused with blood from the arterial reservoir, and the blood was allowed to drain into the venous reservoir. The arterial reservoir was set at a level above the lobe to provide a nonpulsatile perfusion pressure of 1015 mmHg. The venous reservoir was adju sted to provide a venous plressure of ~1 mmHg and was immersed in a water bath at 40°C to maintain the blood temperature normal. The blood was pumped from the venous reservoir into the arterial reservoir to maintain level of blood constant . The pulmonary arte rial and venous pressures (Pa and Pv, respectively) were measured from side ports (15 gauge) in the cannulas using stiff Tygon tubing (30 cm long) and two identical pressure transducers (PdI23B). The vascular pressures were references to the top most region of the lobe. The in-line flow probes (8 mm ID, Transonic Systems) were interposed between the arterial reservoir and the arterial cannula and between the venous reservoir and venous cannula. The two probes were connected to a two-channel blood flowmeter (T201, Transonic Systems) with the frequency response set at 30 Hz on both channels. The outputs from the two flow channels were fed into the polygraph recorder for continuous recording. The lobe was ventilated with a gas mixture of 6% CO,-35% O,-59% N,. End-expiratory pressure was set at l-2 Torr, and inspiratory pressure was kept to
