Pulmonary venous pressure increases during alveolar hypoxia in isolated lungs of newborn pigs CANDICE Department
D. FIKE AND MARK R. KAPLOWITZ of Pediatrics, Baylor College of Medicine,
Houston,
Texas 77030
sponse to hypoxia was confined to pulmonary arteries >60 ,urn diam in lungs of newborn rabbits (5). Hence the role of venous constriction during hypoxia in lungs of The purposeof this study wasto determine whether pulmonary newborns remains uncertain. The purpose of this study venous pressureincreasesduring alveolar hypoxia in lungs of was therefore to determine whether pulmonary venous newborn pigs. We isolated and perfused with blood the lungs from seven newborn pigs, 6-7 days old. We maintained blood pressure increases in response to hypoxia in lungs of flow constant at 50 ml. min. kg-l and continuously moni- newborns of another species. We chose the newborn pig, tored pulmonary arterial and left atria1 pressures.Using the because pigs are commonly used to study the pulmonary micropuncture technique, we measuredpressuresin 10 to 60- circulation and because they have been shown to have an pm-diam venules during inflation with normoxic (21% O,-69- intense pulmonary vasoconstrictor response to hypoxia 74% N,-5-10% CO,) and hypoxic (90-95% N,-5-10% CO,) gas (18, 19). FIKE, CANDICE D., AND MARK R. KAPLOWITZ. Pulmonary
venous pressure lungs of newborn
increases during alveolar hypoxia in isolated pigs. J. Appl. Physiol. 73(Z): 552-556, 1992.-
mixtures. PO, was 142 t 21 Torr during normoxia and 20 t 4 Torr during hypoxia. During micropuncture we inflated the lungs to a constant airway pressureof 5 cmH,O and kept left atria1 pressure greater than airway pressure (zone 3). During hypoxia, pulmonary arterial pressure increased by 69 t 24% and pressure in small venules increased by 40 & 23%. These results are similar to those obtained with newborn lambs and ferrets but differ from results with newborn rabbits. The site of hypoxic vasoconstriction in newborn lungs is speciesdependent.
METHODS Lung isolation, perfusion, and experimental protocols.
We used seven piglets, 6-7 days old and 2.2 t 0.5 kg body wt. Each piglet was preanesthetized with an intramuscular injection of 120 mg ketamine and 2-4 mg xylazine and then anesthetized with 25 mg intravenous pentobarbital sodium. Additional 5 to IO-mg doses of pentobarbital sodium were administered intravenously as necessary to micropuncture technique; microvascular pressures;site of hypmaintain anesthesia during all surgical procedures. oxic vasoconstriction; isolated perfused lungs; pulmonary vasWe cannulated the trachea of each piglet and then cular resistance;papaverine used a Harvard rodent ventilator to inflate the lungs with a gas mixture of 21% O,-5% CO,-74% N, at a tidal volume of 30-45 ml and a frequency of 25-30 breaths/min. ALTHOUGH FOR OVER 40 years alveolar hypoxia has been After placing cannula in an ear vein and femoral artery, known to cause an increase in pulmonary vascular resistance, the type and size of pulmonary vessels that con- we injected heparin (1,000 U/kg iv) and then exanguinated the piglet. Next, we quickly performed a midline strict in response to hypoxia remain controversial. sternotomy and ligated the ductus arteriosus, ascending Whereas results of some studies indicate that hypoxia aorta, descending aorta, and superior and inferior vena causes constriction of pulmonary veins (12, 13, 19) and cavae. Then, we placed cannulas with respective internal capillaries (9), the majority of studies showed that arteries are the predominant site of hypoxia-induced vaso- diameters of 2/16 or l/4 in. into the pulmonary artery and left atrium. We next transected the thoracic cage above constriction in adult lungs (6, 10, 11, 13, 19, 22). and below the level of the lungs, removed the diaphragm, The site of hypoxic vasoconstriction may, however, change with postnatal age. For example, one group of and placed the isolated thoracic cage in a supine position. investigators provided indirect evidence that the pres- The vascular catheters were then connected to the perfusion circuit. sure in vessels downstream from the sites of lung fluid The perfusion circuit was filled with each animal’s hepfiltration increased with hypoxia in lungs of newborn arinized blood plus either heparinized blood from a donor lambs but not in lungs of mature sheep (2). In addition, unlike adult animals of most species, results of some piglet (in 5 studies) or dextran (in 2 studies). We added small amounts of NaHCO, if necessary to maintain the studies in lungs of newborn lambs (8, 15) showed that pH of the perfusate at -7.35. The blood reservoir was pulmonary venous pressure increased during hypoxia. An increase in pulmonary venous pressure with hypoxia suspended in a water bath that maintained the temperain newborn but not in adult lungs would provide one ex- ture of the perfusate between 37 and 39OC. A rotary planation as to why children appear to be more susceptipump (Cole Parmer Masterflex, model 7523-00) continuble than adults to develop edema at high altitude (20). ously circulated blood from the reservoir through a bubHowever, we found that the increase in pressure in re- ble trap and blood filter into the pulmonary artery, 552
0161-7567/92
$2.00
Copyright 0 1992 the American Physiological Society
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PULMONARY
VENOUS
PRESSURES
through the lungs, into the left atrium, and back to the reservoir. measured pulmonary arterial, left We continuously atrial, and airway pressures using strain- #gauge transducers (Statham PD 23) and a chart recorder (Gould TA 2000). The dependent portion of the diaphragmatic surface of the lung, which was the level of micropuncture, served as zero reference for vascular pressure measurements. Airway pressure was measured relative to pleural pressure (atmospheric). After connecting the vascular cannulas to the perfusion circuit, we perfused the lungs for 30-45 min to establish stability of the preparation. During this period, we adjusted the blood flow rate to maintain the pulmonary arterial pressure at 90 mg/dl. The perfusion Hct for these studies was 22 t 7% and the W/D values were 6.1 t 0.2 g/g dry lung weight. These values of W/D were slightly higher than W/D values measured in our laboratory in unperfused lungs of newborn piglets (n = 3, 5.7 t 0.1). During hypoxia for all seven lungs, pulmonary arterial pressure increased by 69 t 24% and lo- to 60-pm-diam venular pressure increased by 40 t 23% (Fig. 1). For those lungs in which we have measurements after hypoxia, pulmonary arterial pressure (n = 5) and lo- to 60pm-diam venular pressure (n = 2) decreased to their previous levels. During normoxia, 46 t 10% of the total pressure drop
0 sol I A 0
H
pulmonary venule left atrium
artery
* I*
I P
I
I
I
+
7
notdoxia
cmH,O
Torr
PC02 Torr
394t48 493k93
4127 41Ik5
l PO,
l minekg
PH
0.24t0.08 0.10t0.03*
7.31t0.07 7.3OkO.05
across the pulmonary circulation occurred upstream and 54 -+ 10% occurred downstream of the lo- to 60-pm-diam venules. This distribution of pressures did not change during hypoxia (53 t 13% of the total pressure drop occurred across vessels upstream and 47 t 13% occurred across vessels downstream of the lo- to 60-pm-diam venules). Thus, although total vascular resistance increased during hypoxia, there was no statistically significant change in the percent contribution of venous resistance to total resistance (54 t 10 vs. 47 t 13%, P = 0.29). Table 2 is a summary of data for the studies in which the perfusate contained papaverine. Whereas pH and PCO, were not different from values obtained in the above set of experiments (Table I), pulmonary vascular resistance before papaverine (Table 2) was slightly less than values obtained during ventilation with the normoxie gas mixture (Table 1). This lower pulmonary vascular resistance may be due to ventilation with 95% 0,. Addition of papaverine decreased total pulmonary vascular resistance by 58% (Table 2). Perfusion Hct (25 t 4%) and W/D (6.3 t 0.5) were not different from values in the studies without papaverine. Vascular pressures before papaverine (Fig. 2) were similar to measurements during normoxia in the first series of studies (Fig. 1). Addition of papaverine caused a significant decrease in both pulmonary arterial and lo- to 60pm-diam venular pressure with no change in left atria1 pressure (Fig. 2). Thus the contribution to total resistance from venous resistance decreased from 52 t 7 to 22 k 8% with the addition of papaverine. This finding indicates that vasomotor tone in veins contributes to baseline nonhypoxic vascular resistance in isolated lungs of newborn piglets. 0 30 ol
a
I
0
E 25
.
pulmonary venule left atrium
artery
I
x*
I
f
Q
*
”
hypbxia
1. Effect of hypoxia on pulmonary vascular pressures in lungs of 7 newborn pigs. Values are means k SD. *Different from value during normoxia bv Student’s naired t test. P < 0.05.
PVR, ml-l
Values are means t SD of 5 newborn pigs. * Different from immediately preceding value by Student’s paired t test, P < 0.05.
u
FIG.
2. Data before and after papaverine
TABLE
Papaverine
l
moxia. We interpreted cal significance.
50 E . 0 - 40 8 I f g 30I 2 e 20I 5
PIGS
and hypoxic gas mixtures
Values are means t SD; n, no. of newborn pigs. PVR, pulmonary vascular resistance. Normoxic gas mixture was 21% O,-5-10% CO,-6974% N,; hypoxic mixture was 90-95% N,-540% CO,. * Different from immediately preceding value by l-way analysis of variance with Scheffe’s F test, P < 0.05.
3
IN NEWBORN
before
pbpaverine
after
pa’paverine
Effect of papaverine on pulmonary vascular pressures in lungs newborn pigs. Values are means rtr SE. *Different from value before nanaverine bv Student’s naired t test. P 0.05. FIG.
2. of 5
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PULMONARY
VENOUS
PRESSURES
IN ‘NEWBORN
PIGS
555
occurs with hypoxia in newborns (8,24). However, most of the other techniques are limited by their inability to Site of hypoxic vasoconstriction. The most important define the size and type of vessels in which pressure finding in this study is that both pulmonary arterial and changes occur. lo- to 60-pm-diam venous pressure increased with hypAs for adult lungs, although the majority of studies oxia in lungs of newborn pigs. Thus, as has been sug- indicate that the major site of hypoxic vasoconstriction gested for newborn lambs (15) and ferrets (17), pulmooccurs in arteries (6, 10, 11, 13, 19, 22), there is some nary venous constriction may play an important role evidence for hypoxia-induced constriction of pulmonary during hypoxia in lungs of newborn pigs. capillaries and veins (9, 12, 13) in adult animals of some species including the pig (19). For example, the occlusion Venous constriction is not, however, the only possible technique divides the pulmonary circulation into upexplanation for the increase in small venular pressure with hypoxia. Consistent with findings in living lambs stream, middle, and downstream compartments. By use (7), sheep (14), and adult humans (3), it is possible that a of the occlusion technique, investigators found that the shift in blood flow away from other regions of the lungs pressure gradients across both the upstream and middle compartments increased during hypoxia in lungs of adult and toward the surface of the lungs used for micropuncpigs (19). The increase in pressure across the middle ture could have elevated small venular pressure during could represent constriction of capillaries. hypoxia. Elevations in blood flow have been shown to compartment increase small venular pressures in isolated lungs of However, the middle segment has been shown to include arteries that are as large as 200-300 pm diam (23) and newborn lambs and rabbits perfused under zone 3 conditions (4, 16). Yet, under zone 3 conditions, as we used in may also include veins. Hence the increase in pressure across the middle segment with hypoxia in adult pig this study, redistribution of blood flow did not occur with lungs could certainly be due to constriction of small arterhypoxia in isolated perfused lamb lungs (15). In addition, in those studies that demonstrated a redistribution in ies and might also reflect constriction of small veins. flow during hypoxia, blood flow shifted toward the upper Because the type and size of vessels included in the nondependent regions and away from the lower depenmiddle segment, as defined by the occlusion technique, dent regions of the lung (3, 7, 14). In this study, we used are not known, it is unclear whether the above findings in adult pig lungs differ from our findings in lungs of newthe lower dependent region of the lung for micropuncture. Therefore we would predict that the result of any born pigs. In other words, there is no current evidence changes as a hypoxia-induced redistribution of blood flow would be to that the site of hypoxic vasoconstriction decrease flow in the region of micropuncture, which, in function of age in pigs, as has been suggested to occur in sheep (2). turn, would decrease small venular pressure. However, because we did not measure the distribution of blood Relevance to lung fluid filtration. Hypoxia-induced inflow, it is not possible to differentiate between hypoxiacreases in pulmonary venous pressure, as found in this induced redistribution of flow and venous constriction as and other studies (15, 17), would elevate hydrostatic the source of the increased venous pressure found in this pressure in upstream fluid filtration sites and thereby study. explain why the rate of lung fluid filtration increases Other investigators using the micropuncture techwith hypoxia in newborn lambs (2,8). Similarly, age-renique have provided evidence that pulmonary veins con- lated differences in the response of pulmonary venous pressure to hypoxia would explain both the finding that strict in response to hypoxia in lungs of newborns. Specifdoes not increase with hypoxia in ically, Raj and co-workers found that the pressure in lung fluid filtration both arteries and veins >20-80 pm diam increased in mature sheep (2, 8) and the observation that children response to alveolar hypoxia in isolated lungs of 5- to have a greater tendency than adults to develop pulmoU-day-old lambs (15) and 3- to 5wk-old ferrets (17). In nary edema at high altitude (20). However, it is not cercontrast, by use of the micropuncture technique, we pre- tain that hypoxia increases the rate of lung fluid filtraviously found that the site of vasoconstriction in re- tion in newborns of all species. In addition, it has not been demonstrated in any species other than sheep that sponse to hypoxia in 7- to Z&day-old newborn rabbits the pulmonary venous response to and/or the change in was limited to arteries >60 pm diam (5). It is of note that rate of lung fluid filtration with hypoxia differs between in our previous study in newborn rabbits the perfusate newborns and adults (2, 8). PO, during hypoxia was 40 Torr, whereas in this study the perfusate PO, during hypoxia was 20 Torr. Hence it is Nonhypoxic vascular pressure profile. During ventilapossible that pulmonary veins of newborn rabbits might tion with 21% O,, we found that approximately one-half constrict at lower levels of perfusate PO,. In addition, it of the total pressure drop occurred downstream of the is possible that younger or older rabbits might show ve- lo- to 60-pm-diam venules. Micropuncture measurenous constriction with hypoxia. Nonetheless, results of ments performed under similar nonhypoxic conditions in studies in which the micropuncture technique has been lungs of both newborns and adults of most other species used suggest that hypoxia-induced venous constriction have shown that a smaller proportion of total vascular may be species dependent. resistance, 5-40%, occurs across the venular segment (1, The site of hypoxic vasoconstriction in newborns has 5, 13, 15, 21). We therefore hypothesized that the presence of venous vasomotor tone was the source of the been evaluated by use of techniques other than the micropuncture technique (8,24). Indeed, results of some of large venous pressure drop that occurred across nonhypthese studies support the notion that venous constriction oxic lungs of newborn pigs. We confirmed this hypotheDISCUSSION
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556
PULMONARY
VENOUS
PRESSURES
sis by showing that the vasodilator papaverine reduced the venous contribution to 22% of the total pressure drop, a level similar to measurements in nonhypoxic lungs of other species. Limitations of methods. One of the problems associated with the use of isolated perfused lung preparations is that pulmonary vascular resistance is often elevated in these preparations (4, 5, 15), as it was in ours. Another concern is whether findings in isolated lungs represent the living animal. We found a similar increase of pulmonary arterial pressure in response to hypoxia, as has been found in living newborn pigs (18). Therefore changes in the pulmonary vasculature during hypoxia in isolated newborn pig lungs might reflect changes in the living newborn pig. To mimic conditions in our previous study in newborn rabbits (5), we used 90-95% N, as the hypoxic stimulus in this study. As has been noted by others, sustained exposure to this severe degree of hypoxia results in an initial increase in pulmonary arterial pressure followed after a variable period of time by a decrease in pulmonary arterial pressure (26). Recent studies have shown that maintaining a glucose concentration greater than -280 mgldl may slow the initial vasoconstriction but reduce the vasodilation during anoxia and that these effects of glucose may be mediated by ATP-sensitive potassium channels (25,26). However, the reason for a multiphasic response with this severe degree of hypoxia remains uncertain. Although all our hypoxic measurements were performed before pulmonary arterial pressure returned to baseline, we were not always able to perform micropuncture during the initial intense phase of arterial constriction. Therefore our measurements probably underestimate the magnitude of increase in pulmonary arterial and venous pressures that can occur with hypoxia in newborn pigs. Our study is also limited by our inability to make measurements in vessels lying deeper within the parenchyma. In addition, we were not able to make measurements in venules >60 ,urn diam or in any size of arterioles, because these vessels were scarce and difficult to see on the surface of the newborn pig lung. Hence the exact size of artery and vein that constrict in response to hypoxia cannot be defined by our results. Nonetheless, our study clearly demonstrates that pressure in lo- to 60-pm-diam subpleural venules increased with hypoxia, and constriction of downstream venules provides one explanation for this. We thank Thomas N. Hansen for helpful review of the manuscript. This work was supported in part by National Heart, Lung, and Blood Institute Grant HL-42883 and a Basil O’Connor Award from the March of Dimes to C. D. Fike. Address for reprint requests: C. D. Fike, Dept. of Pediatrics, University of Utah School of Medicine, 50 North Medical Dr., Salt Lake City, UT 84132. Received 13 May 1991; accepted in final form 6 March 1992. REFERENCES S., M. R. GLUCKSBERG, AND J. BHATTACHARYA. Measurement of lung microvascular pressure in the intact anesthetized rabbit by the micropuncture technique. Circ. Res. 64: 167-172, 1989. 2. BLAND, R. D., M. A. BRESSACK, C. M. HABERKERN, AND T. N. HANSEN. Lung fluid balance in hypoxic, awake newborn lambs and mature sheep. BioZ. Neonate 38: 221-228. 1980. 1. BHATTACHARYA,
IN NEWBORN
PIGS
A. Regional pulmonary blood flow in sitting and supine man during and after acute hypoxia. J. Clin. Inuest. 48: 301-310,
3. DAWSON, 1969. 4. FIKE,
C. D., AND M. R. KAPLOWITZ. Effect of blood flow rate and blood flow history on the newborn pulmonary microcirculation.
Am. J. Physiol. 261 (Heart 5. FIKE, C. D., S. J. LAI-FOOK,
sures during hypoxia
Circ. Physiol. 30): H271-H279, 1991. AND R. D. BLAND. Microvascular presin isolated lungs of newborn rabbits. J. Appl. 1988.
Physiol. 65: 283-287, 6. FISHMAN, A. P. Hypoxia on the pulmonary circulation, how where it acts. Circ. Res. 38: 221-231, 1976. 7. HANSEN, T. N., A. L. LEBLANC, AND A. L. GEST. Hypoxia
angiotensin
II infusion redistribute
Appl. Physiol. 58: 812-818, 8. HAZINSKI, T. A., AND K.
small pulmonary
and
and lung blood flow in lambs. J.
1985.
A. KENNEDY. Alveolar hypoxia increases wedge pressure in awake young lambs. Pediatr.
Res. 22: 679-682, 1987. 9. KAPANCI, Y., A. ASSIMACOPOULOS, GABBIANI. “Contractile interstitial
septa: a possible regulator Biol. 60: 375-392, 10. KATO, M., AND
C. IRLE, A. ZWAHLEN, AND G. cells” in pulmonary alveolar of ventilation/perfusion ratio? J. Cell
1974.
N. C. STAUB. Response of small pulmonary arteries to unilobar hypoxia and hypercapnia. Circ. Res. 19: 426-440, 1966. 11. LINEHAN, J. H., AND C. A. DAWSON. A three-compartment model of the pulmonary vasculature: effects of vasoconstriction. J. Appl. Physiol. 55: 923-928, 1983. 12. MORGAN, B. C., S. C. CHURCH,
constriction
of pulmonary
AND W. G. GUNTHEROTH. Hypoxic artery and vein in intact dogs. J. Appl.
Physiol. 25: 356-361, 1968. 13. NAGASAKA, Y., J. BHATTACHARYA, S. NANJO, M. A. GROPPER, AND N. C. STAUB. Micropuncture measurement of lung microvascular pressure profile during hypoxia in cats. Circ. Res. 54: 90-95, 1984. 14. NEUMANN, P. H., C. M. KIVLEN, A. JOHNSON, F. L. MINNEAR, AND A. B. MALIK. Effect of alveolar hypoxia on regional pulmonary perfusion. J. Appl. Physiol. 56: 338-342, 1984. 15. RAJ, J. U., AND P. CHEN. Micropuncture measurement of microvascular pressures in isolated lamb lungs during hypoxia. Circ. Res. 59: 398-404, 1986. 16. RAJ, J. U., AND P. CHEN. Microvascular pressures measured by micropuncture in isolated perfused lamb lungs. J. Appl. Physiol. 61: 2194-2201, 1986. 17. RAJ, J. U., R. HILLYARD, P. KAAPA, M. GROPPER, AND J. ANDERSON. Pulmonary arterial and venous constriction during hypoxia in 3- to 5-wk-old and adult ferrets. J. Appl. Physiol. 69: 2183-2189, 1990. 18, RENDAS, A., M. BRANTHWAITE, S. LENNOX, AND L. REID. Re-
sponse of the pulmonary circulation to acute hypoxia in the growing pig. J. Appl. Physiol. 52: 811-814, 1982. 19. ROCK, P., G. A. PATTERSON, S. PERMUTT, AND J. T. SYLVESTER. Nature and distribution of vascular resistance in hypoxic pig lungs. J. Appl. Physiol. 59: 1891-1901, 20. SCOGGIN, C. H., T. M. HYERS,
1985.
J. T. REEVES, AND R. F. GROWS. High-altitude pulmonary edema in the children and young adults of Leadville, Colorado. N. Engl. J. Med. 297: 1269-1272, 1977. 21. SHEPARD, J. M., M. A. GROPPER, G. NICOLAYSEN, N. C. STAUB, AND J. BHATTACHARYA. Lung microvascular pressure profile measured by micropuncture in anesthetized dogs. J. Appl. Physiol. 64: 874-879,1988. SHIRAI, M.,
K. SADA, AND I. NINOMIYA. Effects of regional alveolar hypoxia and hypercapnia on small pulmonary vessels in cats. J. Appl. 23. TOD,
Physiol.
61: 440-448,
1986.
M., M. L. MCGEADY, P. ROCK, AND J. T. SYLVESTER. Effects of arterial ligation and embolization on pulmonary vascular pressure distribution. J. Appl. Physiol. 63: 1387-1395, 1987. 24. T’oD, M. L., AND J. T. SYLVESTER. Distribution of pulmonary vascular pressure as a function of perinatal age in lambs. J. Appl. Physiol. 66: 79-87, WEINER, C.
1989.
M., AND J. T. SYLVESTER. Effect of glibenclamide on ;he sustained vasoconstrictor response to anoxia in isolated ferret 1ungs (Abstract). FASEB J. 4: 574A, 1990. 26. WEINER, C. M., AND J. T. SYLVESTER. Effects of glucose on hyplxic vasoconstriction in isolated ferret lungs. J. Appl. Physiol. 70:
25.
139-446,199l.
27. WEST, J. B., C. T. DOLLERY, AND A. NAIMARK. Distribution of llood flow in isolated lung: relation to vascular and alveolar pressures. J. A~wl. PhvsioZ. 19: 713-724. 1964.
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D. FIKE AND MARK R. KAPLOWITZ of Pediatrics, Baylor College of Medicine,
Houston,
Texas 77030
sponse to hypoxia was confined to pulmonary arteries >60 ,urn diam in lungs of newborn rabbits (5). Hence the role of venous constriction during hypoxia in lungs of The purposeof this study wasto determine whether pulmonary newborns remains uncertain. The purpose of this study venous pressureincreasesduring alveolar hypoxia in lungs of was therefore to determine whether pulmonary venous newborn pigs. We isolated and perfused with blood the lungs from seven newborn pigs, 6-7 days old. We maintained blood pressure increases in response to hypoxia in lungs of flow constant at 50 ml. min. kg-l and continuously moni- newborns of another species. We chose the newborn pig, tored pulmonary arterial and left atria1 pressures.Using the because pigs are commonly used to study the pulmonary micropuncture technique, we measuredpressuresin 10 to 60- circulation and because they have been shown to have an pm-diam venules during inflation with normoxic (21% O,-69- intense pulmonary vasoconstrictor response to hypoxia 74% N,-5-10% CO,) and hypoxic (90-95% N,-5-10% CO,) gas (18, 19). FIKE, CANDICE D., AND MARK R. KAPLOWITZ. Pulmonary
venous pressure lungs of newborn
increases during alveolar hypoxia in isolated pigs. J. Appl. Physiol. 73(Z): 552-556, 1992.-
mixtures. PO, was 142 t 21 Torr during normoxia and 20 t 4 Torr during hypoxia. During micropuncture we inflated the lungs to a constant airway pressureof 5 cmH,O and kept left atria1 pressure greater than airway pressure (zone 3). During hypoxia, pulmonary arterial pressure increased by 69 t 24% and pressure in small venules increased by 40 & 23%. These results are similar to those obtained with newborn lambs and ferrets but differ from results with newborn rabbits. The site of hypoxic vasoconstriction in newborn lungs is speciesdependent.
METHODS Lung isolation, perfusion, and experimental protocols.
We used seven piglets, 6-7 days old and 2.2 t 0.5 kg body wt. Each piglet was preanesthetized with an intramuscular injection of 120 mg ketamine and 2-4 mg xylazine and then anesthetized with 25 mg intravenous pentobarbital sodium. Additional 5 to IO-mg doses of pentobarbital sodium were administered intravenously as necessary to micropuncture technique; microvascular pressures;site of hypmaintain anesthesia during all surgical procedures. oxic vasoconstriction; isolated perfused lungs; pulmonary vasWe cannulated the trachea of each piglet and then cular resistance;papaverine used a Harvard rodent ventilator to inflate the lungs with a gas mixture of 21% O,-5% CO,-74% N, at a tidal volume of 30-45 ml and a frequency of 25-30 breaths/min. ALTHOUGH FOR OVER 40 years alveolar hypoxia has been After placing cannula in an ear vein and femoral artery, known to cause an increase in pulmonary vascular resistance, the type and size of pulmonary vessels that con- we injected heparin (1,000 U/kg iv) and then exanguinated the piglet. Next, we quickly performed a midline strict in response to hypoxia remain controversial. sternotomy and ligated the ductus arteriosus, ascending Whereas results of some studies indicate that hypoxia aorta, descending aorta, and superior and inferior vena causes constriction of pulmonary veins (12, 13, 19) and cavae. Then, we placed cannulas with respective internal capillaries (9), the majority of studies showed that arteries are the predominant site of hypoxia-induced vaso- diameters of 2/16 or l/4 in. into the pulmonary artery and left atrium. We next transected the thoracic cage above constriction in adult lungs (6, 10, 11, 13, 19, 22). and below the level of the lungs, removed the diaphragm, The site of hypoxic vasoconstriction may, however, change with postnatal age. For example, one group of and placed the isolated thoracic cage in a supine position. investigators provided indirect evidence that the pres- The vascular catheters were then connected to the perfusion circuit. sure in vessels downstream from the sites of lung fluid The perfusion circuit was filled with each animal’s hepfiltration increased with hypoxia in lungs of newborn arinized blood plus either heparinized blood from a donor lambs but not in lungs of mature sheep (2). In addition, unlike adult animals of most species, results of some piglet (in 5 studies) or dextran (in 2 studies). We added small amounts of NaHCO, if necessary to maintain the studies in lungs of newborn lambs (8, 15) showed that pH of the perfusate at -7.35. The blood reservoir was pulmonary venous pressure increased during hypoxia. An increase in pulmonary venous pressure with hypoxia suspended in a water bath that maintained the temperain newborn but not in adult lungs would provide one ex- ture of the perfusate between 37 and 39OC. A rotary planation as to why children appear to be more susceptipump (Cole Parmer Masterflex, model 7523-00) continuble than adults to develop edema at high altitude (20). ously circulated blood from the reservoir through a bubHowever, we found that the increase in pressure in re- ble trap and blood filter into the pulmonary artery, 552
0161-7567/92
$2.00
Copyright 0 1992 the American Physiological Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 13, 2019.
PULMONARY
VENOUS
PRESSURES
through the lungs, into the left atrium, and back to the reservoir. measured pulmonary arterial, left We continuously atrial, and airway pressures using strain- #gauge transducers (Statham PD 23) and a chart recorder (Gould TA 2000). The dependent portion of the diaphragmatic surface of the lung, which was the level of micropuncture, served as zero reference for vascular pressure measurements. Airway pressure was measured relative to pleural pressure (atmospheric). After connecting the vascular cannulas to the perfusion circuit, we perfused the lungs for 30-45 min to establish stability of the preparation. During this period, we adjusted the blood flow rate to maintain the pulmonary arterial pressure at 90 mg/dl. The perfusion Hct for these studies was 22 t 7% and the W/D values were 6.1 t 0.2 g/g dry lung weight. These values of W/D were slightly higher than W/D values measured in our laboratory in unperfused lungs of newborn piglets (n = 3, 5.7 t 0.1). During hypoxia for all seven lungs, pulmonary arterial pressure increased by 69 t 24% and lo- to 60-pm-diam venular pressure increased by 40 t 23% (Fig. 1). For those lungs in which we have measurements after hypoxia, pulmonary arterial pressure (n = 5) and lo- to 60pm-diam venular pressure (n = 2) decreased to their previous levels. During normoxia, 46 t 10% of the total pressure drop
0 sol I A 0
H
pulmonary venule left atrium
artery
* I*
I P
I
I
I
+
7
notdoxia
cmH,O
Torr
PC02 Torr
394t48 493k93
4127 41Ik5
l PO,
l minekg
PH
0.24t0.08 0.10t0.03*
7.31t0.07 7.3OkO.05
across the pulmonary circulation occurred upstream and 54 -+ 10% occurred downstream of the lo- to 60-pm-diam venules. This distribution of pressures did not change during hypoxia (53 t 13% of the total pressure drop occurred across vessels upstream and 47 t 13% occurred across vessels downstream of the lo- to 60-pm-diam venules). Thus, although total vascular resistance increased during hypoxia, there was no statistically significant change in the percent contribution of venous resistance to total resistance (54 t 10 vs. 47 t 13%, P = 0.29). Table 2 is a summary of data for the studies in which the perfusate contained papaverine. Whereas pH and PCO, were not different from values obtained in the above set of experiments (Table I), pulmonary vascular resistance before papaverine (Table 2) was slightly less than values obtained during ventilation with the normoxie gas mixture (Table 1). This lower pulmonary vascular resistance may be due to ventilation with 95% 0,. Addition of papaverine decreased total pulmonary vascular resistance by 58% (Table 2). Perfusion Hct (25 t 4%) and W/D (6.3 t 0.5) were not different from values in the studies without papaverine. Vascular pressures before papaverine (Fig. 2) were similar to measurements during normoxia in the first series of studies (Fig. 1). Addition of papaverine caused a significant decrease in both pulmonary arterial and lo- to 60pm-diam venular pressure with no change in left atria1 pressure (Fig. 2). Thus the contribution to total resistance from venous resistance decreased from 52 t 7 to 22 k 8% with the addition of papaverine. This finding indicates that vasomotor tone in veins contributes to baseline nonhypoxic vascular resistance in isolated lungs of newborn piglets. 0 30 ol
a
I
0
E 25
.
pulmonary venule left atrium
artery
I
x*
I
f
Q
*
”
hypbxia
1. Effect of hypoxia on pulmonary vascular pressures in lungs of 7 newborn pigs. Values are means k SD. *Different from value during normoxia bv Student’s naired t test. P < 0.05.
PVR, ml-l
Values are means t SD of 5 newborn pigs. * Different from immediately preceding value by Student’s paired t test, P < 0.05.
u
FIG.
2. Data before and after papaverine
TABLE
Papaverine
l
moxia. We interpreted cal significance.
50 E . 0 - 40 8 I f g 30I 2 e 20I 5
PIGS
and hypoxic gas mixtures
Values are means t SD; n, no. of newborn pigs. PVR, pulmonary vascular resistance. Normoxic gas mixture was 21% O,-5-10% CO,-6974% N,; hypoxic mixture was 90-95% N,-540% CO,. * Different from immediately preceding value by l-way analysis of variance with Scheffe’s F test, P < 0.05.
3
IN NEWBORN
before
pbpaverine
after
pa’paverine
Effect of papaverine on pulmonary vascular pressures in lungs newborn pigs. Values are means rtr SE. *Different from value before nanaverine bv Student’s naired t test. P 0.05. FIG.
2. of 5
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PULMONARY
VENOUS
PRESSURES
IN ‘NEWBORN
PIGS
555
occurs with hypoxia in newborns (8,24). However, most of the other techniques are limited by their inability to Site of hypoxic vasoconstriction. The most important define the size and type of vessels in which pressure finding in this study is that both pulmonary arterial and changes occur. lo- to 60-pm-diam venous pressure increased with hypAs for adult lungs, although the majority of studies oxia in lungs of newborn pigs. Thus, as has been sug- indicate that the major site of hypoxic vasoconstriction gested for newborn lambs (15) and ferrets (17), pulmooccurs in arteries (6, 10, 11, 13, 19, 22), there is some nary venous constriction may play an important role evidence for hypoxia-induced constriction of pulmonary during hypoxia in lungs of newborn pigs. capillaries and veins (9, 12, 13) in adult animals of some species including the pig (19). For example, the occlusion Venous constriction is not, however, the only possible technique divides the pulmonary circulation into upexplanation for the increase in small venular pressure with hypoxia. Consistent with findings in living lambs stream, middle, and downstream compartments. By use (7), sheep (14), and adult humans (3), it is possible that a of the occlusion technique, investigators found that the shift in blood flow away from other regions of the lungs pressure gradients across both the upstream and middle compartments increased during hypoxia in lungs of adult and toward the surface of the lungs used for micropuncpigs (19). The increase in pressure across the middle ture could have elevated small venular pressure during could represent constriction of capillaries. hypoxia. Elevations in blood flow have been shown to compartment increase small venular pressures in isolated lungs of However, the middle segment has been shown to include arteries that are as large as 200-300 pm diam (23) and newborn lambs and rabbits perfused under zone 3 conditions (4, 16). Yet, under zone 3 conditions, as we used in may also include veins. Hence the increase in pressure across the middle segment with hypoxia in adult pig this study, redistribution of blood flow did not occur with lungs could certainly be due to constriction of small arterhypoxia in isolated perfused lamb lungs (15). In addition, in those studies that demonstrated a redistribution in ies and might also reflect constriction of small veins. flow during hypoxia, blood flow shifted toward the upper Because the type and size of vessels included in the nondependent regions and away from the lower depenmiddle segment, as defined by the occlusion technique, dent regions of the lung (3, 7, 14). In this study, we used are not known, it is unclear whether the above findings in adult pig lungs differ from our findings in lungs of newthe lower dependent region of the lung for micropuncture. Therefore we would predict that the result of any born pigs. In other words, there is no current evidence changes as a hypoxia-induced redistribution of blood flow would be to that the site of hypoxic vasoconstriction decrease flow in the region of micropuncture, which, in function of age in pigs, as has been suggested to occur in sheep (2). turn, would decrease small venular pressure. However, because we did not measure the distribution of blood Relevance to lung fluid filtration. Hypoxia-induced inflow, it is not possible to differentiate between hypoxiacreases in pulmonary venous pressure, as found in this induced redistribution of flow and venous constriction as and other studies (15, 17), would elevate hydrostatic the source of the increased venous pressure found in this pressure in upstream fluid filtration sites and thereby study. explain why the rate of lung fluid filtration increases Other investigators using the micropuncture techwith hypoxia in newborn lambs (2,8). Similarly, age-renique have provided evidence that pulmonary veins con- lated differences in the response of pulmonary venous pressure to hypoxia would explain both the finding that strict in response to hypoxia in lungs of newborns. Specifdoes not increase with hypoxia in ically, Raj and co-workers found that the pressure in lung fluid filtration both arteries and veins >20-80 pm diam increased in mature sheep (2, 8) and the observation that children response to alveolar hypoxia in isolated lungs of 5- to have a greater tendency than adults to develop pulmoU-day-old lambs (15) and 3- to 5wk-old ferrets (17). In nary edema at high altitude (20). However, it is not cercontrast, by use of the micropuncture technique, we pre- tain that hypoxia increases the rate of lung fluid filtraviously found that the site of vasoconstriction in re- tion in newborns of all species. In addition, it has not been demonstrated in any species other than sheep that sponse to hypoxia in 7- to Z&day-old newborn rabbits the pulmonary venous response to and/or the change in was limited to arteries >60 pm diam (5). It is of note that rate of lung fluid filtration with hypoxia differs between in our previous study in newborn rabbits the perfusate newborns and adults (2, 8). PO, during hypoxia was 40 Torr, whereas in this study the perfusate PO, during hypoxia was 20 Torr. Hence it is Nonhypoxic vascular pressure profile. During ventilapossible that pulmonary veins of newborn rabbits might tion with 21% O,, we found that approximately one-half constrict at lower levels of perfusate PO,. In addition, it of the total pressure drop occurred downstream of the is possible that younger or older rabbits might show ve- lo- to 60-pm-diam venules. Micropuncture measurenous constriction with hypoxia. Nonetheless, results of ments performed under similar nonhypoxic conditions in studies in which the micropuncture technique has been lungs of both newborns and adults of most other species used suggest that hypoxia-induced venous constriction have shown that a smaller proportion of total vascular may be species dependent. resistance, 5-40%, occurs across the venular segment (1, The site of hypoxic vasoconstriction in newborns has 5, 13, 15, 21). We therefore hypothesized that the presence of venous vasomotor tone was the source of the been evaluated by use of techniques other than the micropuncture technique (8,24). Indeed, results of some of large venous pressure drop that occurred across nonhypthese studies support the notion that venous constriction oxic lungs of newborn pigs. We confirmed this hypotheDISCUSSION
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556
PULMONARY
VENOUS
PRESSURES
sis by showing that the vasodilator papaverine reduced the venous contribution to 22% of the total pressure drop, a level similar to measurements in nonhypoxic lungs of other species. Limitations of methods. One of the problems associated with the use of isolated perfused lung preparations is that pulmonary vascular resistance is often elevated in these preparations (4, 5, 15), as it was in ours. Another concern is whether findings in isolated lungs represent the living animal. We found a similar increase of pulmonary arterial pressure in response to hypoxia, as has been found in living newborn pigs (18). Therefore changes in the pulmonary vasculature during hypoxia in isolated newborn pig lungs might reflect changes in the living newborn pig. To mimic conditions in our previous study in newborn rabbits (5), we used 90-95% N, as the hypoxic stimulus in this study. As has been noted by others, sustained exposure to this severe degree of hypoxia results in an initial increase in pulmonary arterial pressure followed after a variable period of time by a decrease in pulmonary arterial pressure (26). Recent studies have shown that maintaining a glucose concentration greater than -280 mgldl may slow the initial vasoconstriction but reduce the vasodilation during anoxia and that these effects of glucose may be mediated by ATP-sensitive potassium channels (25,26). However, the reason for a multiphasic response with this severe degree of hypoxia remains uncertain. Although all our hypoxic measurements were performed before pulmonary arterial pressure returned to baseline, we were not always able to perform micropuncture during the initial intense phase of arterial constriction. Therefore our measurements probably underestimate the magnitude of increase in pulmonary arterial and venous pressures that can occur with hypoxia in newborn pigs. Our study is also limited by our inability to make measurements in vessels lying deeper within the parenchyma. In addition, we were not able to make measurements in venules >60 ,urn diam or in any size of arterioles, because these vessels were scarce and difficult to see on the surface of the newborn pig lung. Hence the exact size of artery and vein that constrict in response to hypoxia cannot be defined by our results. Nonetheless, our study clearly demonstrates that pressure in lo- to 60-pm-diam subpleural venules increased with hypoxia, and constriction of downstream venules provides one explanation for this. We thank Thomas N. Hansen for helpful review of the manuscript. This work was supported in part by National Heart, Lung, and Blood Institute Grant HL-42883 and a Basil O’Connor Award from the March of Dimes to C. D. Fike. Address for reprint requests: C. D. Fike, Dept. of Pediatrics, University of Utah School of Medicine, 50 North Medical Dr., Salt Lake City, UT 84132. Received 13 May 1991; accepted in final form 6 March 1992. REFERENCES S., M. R. GLUCKSBERG, AND J. BHATTACHARYA. Measurement of lung microvascular pressure in the intact anesthetized rabbit by the micropuncture technique. Circ. Res. 64: 167-172, 1989. 2. BLAND, R. D., M. A. BRESSACK, C. M. HABERKERN, AND T. N. HANSEN. Lung fluid balance in hypoxic, awake newborn lambs and mature sheep. BioZ. Neonate 38: 221-228. 1980. 1. BHATTACHARYA,
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