Cardiovascular Research 39 (1998) 451–458
Temporal effects of prolonged hypoxaemia and reoxygenation on systemic,
pulmonary and mesenteric perfusions in newborn piglets
a,
1 ,a
b
Po-Yin Cheung *, Keith J. Barrington , David L. Bigam
a
Department of Pediatrics and Perinatal Research Centre, University of Alberta, Edmonton, Alberta, Canada
b
Department of Surgery, University of Alberta, Edmonton, Alberta, Canada
Received 27 May 1997; accepted 11 February 1998
Abstract
Objective: Temporal effects of prolonged hypoxaemia and reoxygenation, on the systemic pulmonary and mesenteric circulations in
newborn piglets, were investigated. Methods: Two groups [control (n55), hypoxaemic (n57)] of 1–3 day old anaesthetised piglets were
instrumented with ultrasound flow probes placed to measure cardiac, hepatic arterial flow and portal venous flow indices, and catheters
inserted for measurements of systemic and pulmonary arterial pressures. Hypoxaemia with arterial oxygen saturation 40–50% was
maintained for 3 h, followed by reoxygenation with 100% inspired oxygen. Results: Cardiac index was transiently elevated at 30–60 min
of hypoxaemia (23% increase from baseline 158639 ml / kg / min), along with increases in stroke volume but not heart rate. A significant
decrease in systemic vascular resistance after 30 min of hypoxaemia was followed by hypotension at 180 min of hypoxaemia. Progressive
pulmonary hypertension with significant vasoconstriction was found after 30 min of hypoxaemia. The hypoxaemic mesenteric
vasoconstriction was transient with a 37% decrease in portal venous flow index at 15 min of hypoxaemia (29612 vs. 46618 ml / kg / min
of baseline, p,0.05). The hepatic arterial to total hepatic oxygen delivery ratio increased significantly during hypoxaemia. In contrast to
the significant increase in systemic oxygen extraction throughout hypoxaemia, elevation in mesenteric oxygen extraction decreased after
30 min of hypoxaemia associated with modest decreases in oxygen consumption. Following reoxygenation, the pulmonary hypertension
was partially reversed. Cardiac index decreased further (130639 ml / kg / min) with reduced stroke volume, persistent systemic
hypotension and decreased systemic oxygen delivery. Conclusions: We demonstrated differential temporal changes in systemic,
pulmonary and mesenteric circulatory responses during prolonged hypoxaemia. Cautions need to be taken upon reoxygenation because the
neonates are at risk of developing myocardial stunning, persistent pulmonary hypertension and necrotising enterocolitis.  1998
Elsevier Science B.V. All rights reserved.
Keywords: Hypoxaemia; Reoxygenation; Flow; Oxygen extraction; Newborn; Piglets
Abbreviations: SAP; mean systemic arterial pressure; PAP; mean
pulmonary arterial pressure; CI; cardiac index; PVFI; portal venous flow
index; HAFI; hepatic arterial flow index; THFI; total hepatic flow index;
SV; stroke volume; SVRI; systemic vascular resistance index; PVRI;
pulmonary vascular resistance index; Sys EO 2 ; systemic oxygen extraction; Sys DO 2 ; systemic oxygen delivery; Sys VO 2 ; systemic oxygen
consumption; Mes EO 2 ; mesenteric oxygen extraction; Mes DO 2 ; mesenteric oxygen delivery; Mes VO 2 ; mesenteric oxygen consumption; HDO 2 ;
total hepatic oxygen delivery; HADO 2 ratio; ratio hepatic arterial to total
hepatic oxygen delivery ratio
*Corresponding author. 462 Heritage Medical Research Centre, University of Alberta, Edmonton, Alberta, Canada, T6G 2S2. Tel.: 11 (403)
492 9652; Fax: 11 (403) 492 9753
1
Dr. K.J. Barrington has moved to Department of Neonatology, University of California at San Diego, UCS.D. Medical Center, 8774, 200 West
Arbor Drive, San Diego, CA 92103-8774, USA.
0008-6363 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved.
PII: S0008-6363( 98 )00080-7
1. Introduction
Acute hypoxaemia causes differential responses in regional and systemic circulations. The pulmonary vasculature constricts [1,2], whereas the reported hypoxaemic
systemic responses have been variable. These variations
may be because of the degree of hypoxaemia used:
moderate hypoxaemia usually causes systemic vasodilatation whereas severe hypoxaemia has been shown to cause
vasoconstriction in some systemic arteries [3]. The discrepancies may also be related to differences in vessel size
Time for primary review 8 days.
452
P.-Y. Cheung et al. / Cardiovascular Research 39 (1998) 451 – 458
and species being studied [4–7], and the use of anaesthetics [2,8]. Leach et al. recently reported responses to
hypoxaemia in their in vitro preparation of pulmonary and
mesenteric arteries derived from adult rats [9]. They
demonstrated a persistent pulmonary vasoconstriction and
a biphasic mesenteric response with initial vasoconstriction
and subsequent vasodilatation. We are unaware of any data
concerning the temporal effects of prolonged hypoxaemia
on regional haemodynamic changes in the newborn.
The regional oxygen metabolism responses in relation to
the prolonged hypoxaemic haemodynamic changes in the
newborn have also not been studied. In particular, information on mesenteric haemodynamic and oxygen metabolic responses would be important for understanding the
pathogenesis of necrotising enterocolitis, which has been
related to hypoxic injury [10]. Furthermore, because of the
unique circulation of the liver, the effect of prolonged
hypoxaemia on hepatic oxygenation is different from other
circulations.
We therefore designed the following experiment to
compare the temporal responses of systemic, pulmonary
and mesenteric vasculatures and their respective oxygen
metabolism to 3 h of hypoxaemia in a newborn animal
model. Animals were then reoxygenated with 100% inspired oxygen. We hypothesised that there would be
differential temporal changes in systemic and regional
circulations in response to systemic hypoxaemia and these
changes would be reversed by reoxygenation.
2. Methods
The study conformed to the regulations of the Canadian
Council of Animal Care (revised 1993) and was approved
by the Health Sciences Animal Welfare Committee, University of Alberta.
Newborn piglets of mixed western breed were obtained,
1–3 days of age, weighing 1.4–2.4 kg (mean 1.84 kg).
Anaesthesia was induced with inhaled halothane 5% and
then decreased to 2%. A double lumen external jugular
catheter was positioned at the right atrium and a common
carotid arterial line was inserted. Following tracheotomy
and commencement of assisted ventilation, halothane was
discontinued after a maximum of 20 min. Subsequently,
anaesthesia was maintained by fentanyl infusion (4 mg / kg /
h) and the piglets were paralysed with 0.1 mg / kg doses of
pancuronium as needed. Two doses of fentanyl (10 mg / kg)
and acepromazine (0.1 mg / kg) were given prior to
thoracotomy and laparotomy. Dextrose–saline solution
was infused at a rate of 15–20 ml / kg / h. Piglets were
ventilated at pressures of 16 / 4 cm H 2 O at a rate of 12 to
18 breaths per min with inspired oxygen concentration of
21–30%. A left thoracotomy was then performed in the
fourth intercostal space. The pericardium was opened and a
20-gauge catheter was inserted into the root of the pulmonary artery for the measurement of pulmonary artery
pressure. A six-millimetre transit time ultrasound flow
probe (Transonic, Ithaca, NY) was placed around the main
pulmonary artery to measure cardiac output. A midline
laparotomy was then performed. A 5-Fr Argyle catheter
was inserted through the umbilical vein into the portal
venous system. A 1-mm and a 2-mm Transonic transit
time ultrasound flow probe were placed around the common hepatic artery and the portal vein. The neck incision,
thoracotomy and laparotomy were closed with sutures after
these procedures finished. Blood gases were drawn and 15
min of recording were done to ensure the animal was
stable. Stability was defined as (1) heart rate and blood
pressure within 10% of the postanaesthetic prethoracotomy
values, (2) right atrial pressure of 3–8 mm Hg, (3) arterial
PaO 2 75–100 mm Hg, PaCO 2 between 35 and 55 mm Hg
and pH between 7.35 and 7.45. The surgical procedure
usually finished within 75 min. The rectal temperature was
maintained between 38.0–38.58C by an electrical heating
blanket and an infrared-heating lamp.
After a baseline monitoring period of at least 15 min,
simultaneous blood samples were drawn for arterial, mixed
venous and portal venous oxygen saturation and haemoglobin concentration determinations by co-oximeter
(OSM2 Hemoximeter, Radiometer, Copenhagen). The
following haemodynamic variables were monitored continuously throughout the study period: mean arterial blood
pressure (SAP), mean pulmonary arterial pressure (PAP),
right atrial pressure (RAP), heart rate, pulse oximetry
(Nellcor, Hayward, CA), cardiac output, portal venous
flow and hepatic arterial flow. Analogue outputs of the
pressure amplifiers and flow monitors were digitised by a
DT 2801-A analogue to digital converter board (Data
Translation, ON) in a Dell 425E personal computer.
Software was custom written using the Asyst programming
environment. All signals were continuously acquired at 24
Hz and saved on hard disk. These variables were monitored continuously for 4 h after stabilisation in the 5
normoxaemic controls. For the 7 piglets in the hypoxaemic
group, hypoxaemia was induced by reducing FiO 2 to
between 0.10 and 0.15, in order to maintain arterial oxygen
saturation between 40–50% (PaO 2 between 30–40 mm
Hg) after stabilisation. The haemodynamic variables at 15,
30, 60, 120 and 180 min of hypoxaemia were averaged
over 5 min for analysis. Cardiac index (CI), portal venous
flow index (PVFI) and hepatic arterial flow index (HAFI)
were calculated by dividing the nonindexed variables by
body weights. At 15, 30, 60, 120 and 180 min of
hypoxaemia, simultaneous blood samples were taken from
common carotid artery, pulmonary artery and portal venous catheters. Oxygen saturations of arterial (SaO 2 ), mixed
venous (SvO 2 ) and portal venous (SpO 2 ) blood were
measured. Arterial blood gases and plasma lactate were
determined at 180 min of hypoxaemia. No bicarbonate
solution was given during the experiment. All measurements were obtained and analysed in all animals.
After 180 min of hypoxaemia, the inspired gas was
P.-Y. Cheung et al. / Cardiovascular Research 39 (1998) 451 – 458
453
Table 1
Systemic and pulmonary haemodynamic effects (mean6SD) of hypoxaemia and reoxygenation (reoxy)
Time of hypoxaemia
0
159
309
609
1209
1809
Reoxy
Heart rate (bpm)
SVRI (mm Hg / ml / kg / min)
PVRI (mm Hg / ml / kg / min)
SAP/ PAP ratio
Sys DO 2 (ml / kg / min)
Sys VO 2 (ml / kg / min)
Sys EO 2 (%)
211626
0.5460.12
0.1760.05
3.360.63
20.464.02
7.161.73
3567.1
210624
0.4760.08
0.1960.06
2.660.75*
10.862.21*
5.560.84
5268.5*
215631
0.4060.07*
0.2160.08*
2.060.52*
11.162.46*
6.362.27
5569.0*
221634
0.4260.11*
0.2360.09*
1.960.44*
11.762.17*
6.361.62
5469.7*
213637
0.4160.12*
0.2660.09*
1.660.23*
10.763.03*
5.361.82
4966.5*
216634
0.3960.14*
0.2960.10*
1.460.28*
9.862.35*
5.161.53
53613.8*
198632
0.4860.14
0.2460.09*
2.160.15*
17.365.19*
5.262.05
31611.9
* p,0.05, compared with baseline (one-way repeated measures ANOVA), SVRI5systemic vascular resistance index, PVRI5pulmonary vascular
resistance index, SAP5mean systemic artery pressure, PAP5mean pulmonary artery pressure, sys DO 2 5systemic oxygen delivery, sys VO 2 5systemic
oxygen consumption, sys EO 2 5systemic oxygen extraction.
switched to a FiO 2 of 1.0 and the animal was monitored
for a further 30 min after which blood sampling was
repeated. The animals were then euthanised with an
intravenous pentobarbital overdose.
We calculated the following variables at each phase of
hypoxaemia and reoxygenation:
1. Stroke volume (SV)5CI4heart rate
2. Total hepatic flow index (THFI)5PVFI1HAFI
3. Systemic vascular resistance index (SVRI)
5(SAP2RAP)4CI
4. Pulmonary vascular resistance index (PVRI)2
5PAP4CI
5. Systemic oxygen extraction (sysEO 2 )
5(SaO 2 2SvO 2 )4SaO 2 3100%
6. Systemic oxygen delivery (sysDO 2 )
5CI3SaO 2 31.343[Hb]
7. Systemic oxygen consumption (sysVO 2 )
5CI3(SaO 2 2SvO 2 )31.343[Hb]
8. Mesenteric oxygen extraction (mesEO 2 )
5(SaO 2 2SpO 2 )4SaO 2 3100%
9. Mesenteric oxygen delivery (mesDO 2 )
5PVFI3SaO 2 31.343[Hb]
10. Mesenteric oxygen consumption (mesVO 2 )
5PVFI3(SaO 2 2SpO 2 )31.343[Hb]
11. Total hepatic oxygen delivery (HDO 2 )
5(HAFI3SaO 2 1PVFI3SpO 2 )31.343[Hb]
12. Ratio of hepatic arterial oxygen delivery to total
hepatic oxygen delivery (HADO 2 ratio)
5HAFI3SaO 2 4(HAFI3SaO 2 1PVFI3SpO 2 )
3100%
In a separate series of experiment, 5 piglets were
instrumented as above and induced with the same degree
of hypoxaemia for 30 min followed by reoxygenation with
100% inspired oxygen. Changes in systemic and mesenteric haemodynamics and oxygen metabolism were
studied.
2
PVRI was an estimation because left atrial pressure was not measured.
3. Statistical analysis
One-way repeated measures analysis of variance and
repeated measures analysis of variance on ranks were used
to analyse the differences at various phases of hypoxaemia
and reoxygenation for parametric and nonparametric variables, respectively (Sigma Stat 1.01 version, Jandel Scientific, San Rafael, CA). Dunnett’s post-hoc test was used
to compare the difference with baseline. A p value of
,0.05 was considered as significant. The results are
expressed as mean6standard deviation.
4. Results
4.1. Normoxaemic control (n55)
The control animals had no significant change in any of
the recorded haemodynamic and oxygen variables over 4 h
of the study.
4.2. Hypoxaemic group (n57)
4.2.1. Systemic haemodynamic changes during hypoxaemia ( Table 1, Figs. 1 and 2)
By 180 min of hypoxaemia with systemic oxygen
saturations being kept between 40–50%, all piglets were
acidotic with pH 7.06–7.27 (mean 7.1760.09), serum
bicarbonate levels 14–23 (mean 1963.8) mmol / l and
plasma lactate levels 7.8–18.9 (mean 11.663.9) mmol / l.
CI was significantly increased at 30–120 min of hypoxaemia, and then started to decline afterwards, being not
significantly different to baseline at 180 min. Maximum
increase in CI, along with SV, was at 30 min of hypoxaemia (CI: 194647 vs. 158639 ml / kg / min at baseline,
SV: 0.5760.19 vs. 0.4860.18 ml / kg at baseline, both
p,0.05). No significant changes in heart rate at different
time intervals were noted. SAP decreased progressively
after hypoxaemia was initiated, and became significantly
lower than baseline at 180 min of hypoxaemia (61616.8
vs. 82612.9 mm Hg, respectively, p,0.05). A persistent
P.-Y. Cheung et al. / Cardiovascular Research 39 (1998) 451 – 458
454
Fig. 1. Effects of prolonged hypoxaemia on cardiac index (CI) and stroke
volume (SV) of 7 newborn piglets. ‘Normox’ refers to normoxaemic
baseline period, ‘reoxy’ refers to data taken 30 min after reoxygenation
with 100% O 2 . (Mean and SD are shown). *5p,0.05 vs. baseline value.
Fig. 3. Effects of prolonged hypoxaemia on portal venous and hepatic
arterial blood flows of 7 newborn piglets (PVFI and HAFI respectively).
‘Normox’ refers to normoxaemic baseline period, ‘reoxy’ refers to data
taken 30 min after reoxygenation with 100% O 2 . (Mean and SD are
shown). *5p,0.05 vs. baseline value.
and significant decrease in SVRI was noted after 30 min of
hypoxaemia.
oconstriction with PAP increasing from 2563.4 mm Hg to
a maximum of 4567.1 mm Hg at 120 min of hypoxaemia,
and then remaining unchanged for the last 60 min of
hypoxaemia. There were progressive increases in PVRI
from 30 min to 180 min of hypoxaemia.
4.2.2. Pulmonary haemodynamic changes during
hypoxaemia ( Table 1 Fig. 2)
The pulmonary vasculature showed progressive vas-
Fig. 2. Effects of prolonged hypoxaemia on mean systemic and pulmonary arterial pressures of 7 newborn piglets (SAP and PAP respectively).
‘Normox’ refers to normoxaemic baseline period, ‘reoxy’ refers to data
taken 30 min after reoxygenation with 100% O 2 . (Mean and SD are
shown). *5p,0.05 vs. baseline value.
4.2.3. Mesenteric haemodynamic changes during
hypoxaemia ( Table 2 Fig. 3)
In the mesenteric circulation, there was a transient
hypoxaemic vasoconstriction with a 37% decrease in PVFI
at 15 min of hypoxaemia (29611.8 vs. 46618.1 ml / kg /
min at baseline, p,0.05). The PVFI returned to values
indifferent from baseline from 30 min of hypoxaemia
onwards. Portal venous flow contributed the major proportion of hepatic blood flow during normoxaemia and
hypoxaemia, corresponding changes in THFI were therefore noted. HAFI was transiently elevated at 120 min of
hypoxaemia (4.763.8 vs. 3.263.5 ml / kg / min at baseline,
p,0.05).
4.2.4. Systemic and mesenteric oxygen metabolism
changes during hypoxaemia ( Tables 1 and 2)
During hypoxaemia there were ¯50% decreases in sys
DO 2 as per protocol design. Mes DO 2 values were reduced
by between 30% and 42% of baseline mes DO 2 . These
Table 2
Effects of hypoxaemia and reoxygenation (reoxy) on hepatic blood flow and oxygen delivery and mesenteric oxygen metabolism (mean6SD)
Time of hypoxaemia
0
159
309
609
1209
1809
Reoxy
THFI (ml/kg/min)
HDO 2 (ml/kg/min)
HADO 2 ratio (%)
Mes DO 2 (ml/kg/min)
Mes VO 2 (ml/kg/min)
Mes EO 2 (%)
49618
4.861.67
9610.1
5.9462.44
1.2960.56
2264.0
33613*
1.160.42*
18612.3**
1.7960.69*
0.8760.46
47614.0*
41614
1.460.55*
1869.4**
2.1760.78*
0.9460.35
4467.5*
44613
1.860.75*
18622.9
2.4860.77*
0.8960.33
40621.0
43618
1.560.59*
18617.0**
2.2360.97*
0.8360.35
39613.1
41620
1.660.71*
16618.3
2.1661.05*
0.7660.43
38616.7
48627
4.762.45
8610.1
6.0263.54
0.8960.40
18612.0
* p,0.05, compared with baseline (one-way repeated measures ANOVA), ** p,0.05, compared with baseline (repeated measures ANOVA on ranks), THFI5total hepatic flow
index, HDO 2 5total hepatic oxygen delivery, HADO 2 ratio5ratio of hepatic arterial oxygen delivery to total hepatic oxygen delivery, mes DO 2 5mesenteric oxygen delivery,
mes VO 2 5mesenteric oxygen consumption, mes EO 2 5mesenteric oxygen extraction. Formulae for calculations see Section 2.
P.-Y. Cheung et al. / Cardiovascular Research 39 (1998) 451 – 458
decreases were accompanied by significant increases in sys
EO 2 and mes EO 2 . The decreases in sys VO 2 and mes VO 2
were not significant ( p50.07 and 0.06 respectively, b 5
0.4). HDO 2 was also decreased corresponding to that of
mes DO 2 . Significant increases in HADO 2 ratio were
found between 15 to 120 min of hypoxaemia when the
total hepatic oxygen delivery was decreased during hypoxaemia (i.e. a greater proportion of hepatic oxygen delivery
was derived from the hepatic arterial supply).
4.2.5. Effect of reoxygenation on haemodynamics and
oxygen metabolism ( Tables 1 and 2 Figs. 1–3)
Upon 30 min of reoxygenation, there was a further fall
in CI that was significantly lower than baseline (130639
ml / kg / min, p,0.05). This was accompanied by a further
decrease in SV (0.4160.14 ml / kg) and persistent systemic
hypotension (59610.0 mm Hg) (both p,0.05 vs. baseline
values). Despite 100% saturated arterial blood, sys DO 2
remained significantly diminished due to the persistently
decreased CI after reoxygenation. While there was a partial
normalisation of PAP (2965.3 vs. 2563.4 mm Hg at
baseline, p.0.05), the PVRI remained significantly above
baseline, with a persistent decrease in SAP/ PAP ratio
despite reoxygenation ( p,0.05). The changes in mesenteric perfusion and oxygen delivery during 180 min of
hypoxaemia were completely reversed after 30 min of
reoxygenation. Although the sys VO 2 and mes VO 2 were
diminished and remained similar to that of 180 min of
hypoxaemia, they were not significantly different from the
respective baseline values. Sys EO 2 and mes EO 2 returned
to baseline values.
4.2.6. Effect of 30 -min hypoxaemia followed by
reoxygenation (n55)
During 30 min of hypoxaemia, piglets were acidotic
with pH 7.25–7.30 (mean 7.2760.02) and similar changes
in systemic and mesenteric haemodynamics and oxygen
metabolism were observed (data not shown). Upon 30 min
of reoxygenation with 100% inspired oxygen, CI gradually
returned to baseline (174650 vs. 146616 ml / kg / min,
respectively, p.0.05). SAP decreased modestly but was
not different from baseline (61619 vs. 7364 mm Hg,
respectively, p.0.05). In regard to oxygen consumption,
both sys VO 2 and mes VO 2 returned to respective baseline
values (9.665.84 and 1.460.69 ml / kg / min at 30 min of
reoxygenation vs. 10.861.44 and 1.460.79 ml / kg / min at
baseline, respectively).
5. Discussion
In this experiment, we maintained severe hypoxaemia
with systemic oxygen saturations being kept between 40–
50% for 3 h. We showed contrasting temporal haemodynamic responses of the systemic, pulmonary and mesen-
455
teric circulations during prolonged alveolar hypoxaemia
and upon reoxygenation with 100% inspired O 2 .
5.1. Systemic and pulmonary haemodynamic responses
The effect of systemic hypoxaemia on cardiac output in
the newborn varies between studies depending on the
degree of hypoxaemia induced [11,12]. No increase in
cardiac output was demonstrated during severe hypoxaemia with oxygen saturation less than 40%, while an
increase in cardiac output has been shown with moderate
hypoxaemia. We showed that cardiac output also varied
with the duration of hypoxaemia. This is in part consistent
with some reports that studied the effect of hypoxaemia on
cardiac output for a maximum duration of 90 min. In our
study, in response to a $50% decrease in the arterial
oxygen content, CI initially increased and peaked at 30–60
min of hypoxaemia. This compensatory increase in CI,
which was attributed to the increase in SV but not heart
rate, was transient and was insufficient to maintain systemic oxygen delivery with this severity of hypoxaemia.
After 1 hour of hypoxaemia, CI decreased gradually and
was then not significantly different from baseline. The
inability to maintain increased CI and SV may be related to
progressive myocardial dysfunction due to acidosis [13],
hyperlactataemia [14], and ATP exhaustion [15], which
occur during sustained hypoxaemia. The systemic vasodilatation, which is the usual response to hypoxaemia that
improves perfusion of hypoxaemic tissues, may be related
to vasodilator metabolite accumulation [16] and increased
nitric oxide release [17,18]. Since SAP reflects the combined effect of CI and SVRI, it was only significantly
decreased at the late stage of prolonged hypoxaemia which
had resulted in a decrease in CI along with marked
vasodilatation.
Hypoxaemic pulmonary vasoconstriction has been extensively investigated. While the response is observed even
in the absence of functional endothelium [19], many
vasoactive agents, including endothelin [20,21], have been
implicated. Inhibition of nitric oxide production may
modulate this response [22–24]. Calcium and potassium
channel activities may also play critical roles in hypoxaemic pulmonary vasoconstriction [25,26]. Some workers
have demonstrated that hypoxaemic pulmonary vasoconstriction is a biphasic phenomenon [9,15,27]. However, we
did not observe that in our in vivo model.
5.2. Mesenteric haemodynamic responses
At the anatomic site of application of the flow probe, the
measurement of portal venous flow includes contributions
from the bowel and also a small contribution from the
splenic vein. In the neonate, the blood flow to the bowel is
approximately 15 to 20% of the cardiac output whereas the
flow to the spleen is only about 2 to 3% of the cardiac
output [28,29]. In a previous study in the newborn lamb of
456
P.-Y. Cheung et al. / Cardiovascular Research 39 (1998) 451 – 458
severe hypoxaemia of 20 min duration, the relative decreases in gastrointestinal and splenic blood flows were
approximately the same [29]. Therefore inferences from
the measurement of portal blood flow at this location can
reasonably be said to apply to the mesenteric circulation.
Furthermore, estimation of hepatic oxygen delivery requires the inclusion of all sources of hepatic blood flow
and placement of the flow probe in the location chosen was
technically the only feasible methodology. Attempts to
cannulate hepatic veins for measurement of hepatic oxygen
consumption had been unsuccessful due to prolonged
surgical time, increased morbidity and mortality.
The hypoxaemic response of the mesenteric circulation
has been variable, with investigators showing either vasodilatation or vasoconstriction [30–32]. This discrepancy
may be related to differences in the severity and duration
of hypoxaemia [2,3,9] vessel size and species being
studied [4–7] and the use of anaesthesia [2,8]. Previous
reports using single arteries in a constant perfusion-pressure preparation demonstrated mesenteric vasodilation to
hypoxaemia [32,33]. In vivo experiments with a short
duration of hypoxaemia have shown an opposite response
[34]. In our in vivo experiment, hypercapnia or hypocapnia, which have been shown to enhance or mask
hypoxaemic vasodilation [31,35], were avoided. We demonstrated a transient hypoxaemic mesenteric vasoconstriction; an initial intense vasoconstriction followed by vasorelaxation to baseline; confirming the results of prolonged
hypoxaemia (1 h) in an in vitro single artery model by
Leach et al. [9]. Corresponding changes in THFI were
observed because PVFI contributes the major proportion of
hepatic blood supply in this newborn model (about 90% at
baseline). We postulate that the transient hypoxaemic
mesenteric response is due to an initial phase of sympathetically mediated vasoconstriction followed by a phase of
vasorelaxation due to progressive accumulation of metabolites during hypoxaemia. But the washout effect of altered
flow on local metabolites may further complicate the
picture.
It is interesting that while the SVRI decreased after 30
min of hypoxaemia that HAFI increased only after 120
min of hypoxaemia. The apparently unchanged HAFI in
the first 60 min of hypoxaemia is due to a decrease in the
arterial pressure despite vasodilatation. However, we cannot exclude a genuine differential response time in the
systemic and hepatic arterial vasculature nor an apparent
difference due to detection limits of our experimental
design.
5.3. Oxygen delivery and metabolism
Many tissues respond to diminished oxygen delivery
with increasing oxygen extraction and decreasing oxygen
consumption [36]. The increasing oxygen extraction in this
study was able to partially compensate for the decrease in
oxygen delivery. Although our experiment did not show a
significant decrease in systemic and mesenteric oxygen
consumption, there was a trend demonstrated ( p50.07 and
0.06, respectively) and failure to show statistical significance may be related to small sample size ( b 50.4). With
the initial mesenteric vasoconstriction, bowel oxygen
extraction increased dramatically. As the transient increase
in MVRI abated, there was a reduction in the oxygen
extraction after 60 min of hypoxaemia. The progressive
fall in mesenteric oxygen extraction was probably a
response to the persistent hypoxaemia with failing oxygen
consumption and extraction in tissue [37]. This indicates
significant cellular dysfunction has resulted from prolonged hypoxaemia. The temporal cellular response to
prolonged hypoxaemia is similar to the response to graded
hypoxaemia [38].
This is the first report to provide data on hepatic oxygen
delivery in newborn animals during prolonged hypoxaemia. Our experiment showed the changes in the relative
contribution of hepatic arterial and portal venous oxygen
delivery in relation to the total hepatic oxygen delivery
during prolonged hypoxaemia. We demonstrated a significant contribution from hepatic arterial flow to the hepatic
oxygen delivery during severe systemic hypoxaemia. By
180 min of hypoxaemia, with the falling mesenteric
oxygen extraction, portal venous oxygen content was
increasing and the percentage portal venous oxygen delivery contribution to total hepatic oxygen delivery had
increased. HADO 2 ratio decreased somewhat to a level not
significantly different from baseline. These changes in the
relative contributions of hepatic oxygen delivery support
the significant role of hepatic arterial supplies in the unique
hepatic circulation in the event of possible hypoxaemic
hepatic injury during systemic hypoxaemia.
5.4. Reoxygenation effect and its clinical relevance
We should be cautious to extrapolate these results to
human pathophysiological conditions. Upon reoxygenation, the mesenteric haemodynamic variables were completely reversed. But that was not the case for the systemic
and pulmonary circulations. Reoxygenation with 100%
inspired oxygen did not improve the myocardial dysfunction (reduced CI and SV) or persistent hypotension which
occurred during the prolonged hypoxaemia. In addition to
lactate acidosis, reoxygenation injury has been widely
investigated and contributes to the development of myocardial stunning [39]. Following reoxygenation, the simultaneous production of nitric oxide and superoxide, hence,
their product—peroxynitrite, may cause further damage to
the already compromised myocardium [40,41]. Thus,
abrupt correction of hypoxaemia with 100% inspired
oxygen may not lead to immediate improvement in cardiac
performance, and may even be followed by further decreases as shown here.
There was only a partial recovery from the severe
hypoxaemic pulmonary vasoconstriction and PAP was still
P.-Y. Cheung et al. / Cardiovascular Research 39 (1998) 451 – 458
elevated although not being significantly different from
baseline. Acidosis may have contributed to the persistent
pulmonary vasoconstriction [42]. The significant decrease
in SAP/ PAP ratio despite reoxygenation can explain in
part, the mechanism of persistent fetal circulation after
perinatal asphyxia.
On the other hand, sys VO 2 and mes VO 2 also did not
improve upon reoxygenation. These findings contrast to
those in 30 min of a similar degree of hypoxaemia, which
showed a normalisation of oxygen consumption. We
speculate that the prolonged ischaemia-hypoxia injury
might have caused irreversible cellular damage. Also, free
oxygen radicals and peroxynitrite generated during reoxygenation might prevent recovery of cellular function.
Peroxynitrite has been shown to inhibit mitochondrial
respiration in vitro [43,44]. Therefore, the prolonged
ischaemia-hypoxia injury may not be reversed or even
aggravated by reoxygenation. Despite a normal PVFI,
modestly decreased mes VO 2 along with the decrease in
elevated mes EO 2 may suggest persistent cellular dysfunction. While necrotising enterocolitis has been associated
with ischaemia-hypoxia injury and enteral feeding, caution
is required for introduction of enteral feeds despite successful reoxygenation after asphyxia.
In this acute newborn animal experiment, we have
demonstrated the contrasting effects of prolonged hypoxaemia on systemic, pulmonary and mesenteric perfusions.
Prolonged hypoxaemia with oxygen saturations between
40–50% over 3 h caused progressive hypoxaemic pulmonary vasoconstriction and transient hypoxaemic mesenteric
vasoconstriction. Following an initial rise, CI decreased
with progressive systemic vasodilatation. In contrast to
significant increases in sys EO 2 throughout hypoxaemia,
elevations in mes EO 2 decreased after 30 min of hypoxaemia along with modest decreases in sys VO 2 and mes
VO 2 . Persistent systemic hypotension with a further decrease in CI was noted on reoxygenation with 100%
inspired oxygen. The hypoxaemic pulmonary and mesenteric responses were partially or wholly reversed with
reoxygenation, respectively. Cautions need to be taken
upon reoxygenation because the neonates are at risk for the
development of myocardial stunning, persistent pulmonary
hypertension and necrotising enterocolitis.
Acknowledgements
This study was supported by the Heart and Stroke
Foundation of Alberta and The Perinatal Research Centre,
University of Alberta, Edmonton, Canada.
References
[1] Fishman AP. Hypoxia on the pulmonary circulation. How and where
it acts . Circ Res 1976;38:221–231.
457
[2] Rodman DM, Yamaguchi T, O’Brien RF, McMurtry IF. Hypoxic
contraction of isolated rat pulmonary artery. J Pharmacol Exp
Therap 1989;248:952–959.
[3] Vanhoutte PM, Luscher TF, Graser T. Endothelium-dependent
contractions. Blood Vessels 1991;28:74–83.
[4] Demiryurek AT, Wadsworth RM, Kane KA. Effects of hypoxia on
isolated intrapulmonary arteries from sheep. Pulmon Pharmacol
1991;4:158–164.
[5] Leach RM, Twort CHC, Cameron IR, Ward JPT. A comparison of
the pharmacological and mechanical properties in vitro of large and
small pulmonary arteries of the rat. Clin Sci 1992;82:55–62.
[6] Madden JA, Dawson CA, Harder DR. Hypoxia-induced activation in
small isolated pulmonary arteries from the cat. J Appl Physiol
1985;59:113–118.
[7] Madden JA, Vadula MS, Kurup VP. Effects of hypoxia and other
vasoactive agents on pulmonary and cerebral artery smooth muscle
cells. Am J Physiol 1992;263:L384–393.
[8] Barrington KJ, Finer NN, Chan WKY. A blinded randomized
comparison of the circulatory effects of dopamine and epinephrine
infusions in the newborn piglet during normoxia and hypoxia. Crit
Care Med 1995;23:740–748.
[9] Leach RM, Robertson TP, Twort CHC, Ward JPT. Hypoxic vasoconstriction in rat pulmonary and mesenteric arteries. Am J Physiol
1994;266:L223–231.
[10] Hsueh W, Caplan MS, Sun X, et al. Platelet-activating factor, tumor
necrosis factor, hypoxia and necrotizing enterocolitis. Acta Paediatr
1994;396(Supplement):11–17.
[11] Ng ML, Levy MN, DeGeest H, Zieske H. Effects of myocardial
hypoxia on left ventricular performance. Am J Physiol 1966;211:43.
[12] O’Laughlin MP, Fisher DJ, Dreyer WJ, O’Brian ES. Augmentation
of cardiac output with intravenous catecholamines in unanesthetized
hypoxemic newborn lambs. Pediatr Res 1987;22:667–674.
[13] Zhou HZ, Malhotra D, Doers J, Shapiro JI. Hypoxia and metabolic
acidosis in the isolated heart: evidence for synergistic injury. Magnet
Resonance Med 1992;29:94–98.
[14] Chiu RC, Bindon W. Why are newborn hearts vulnerable to global
ischemia?The lactate hypothesis. Circulation 1987;76:146–149.
[15] De Scheerder IK, Maas AA, Nieukoop AS, et al. Cardiac ATP
breakdown and mechanical function during recurrent periods of
anoxia. Cardioscience 1992;3:189–195.
[16] Marshall JM, Thomas T, Turner L. A link between adenosine,
ATP-sensitve K1 channels, potassium and muscle vasodilatation in
the rat in systemic hypoxia. J Physiol 1993;472:1–9.
[17] Pohl U, Busse R. Hypoxia stimulates release of endothelium-derived
relaxant factor. Am J Physiol 1989;256:H1595–1600.
[18] Arnet UA, McMillan AB, Lowenstein CJ. Hypoxia induces bovine
aortic endothelial NO synthase expression. Annual Meeting American Heart Association 1994 (Abstract).
[19] Burke-Wolin T, Wolin MS. H 2 O 2 and cGMP may function as an O 2
sensor in the pulmonary artery. J Appl Physiol 1989;66:167–170.
[20] Holden WE, McCall E. Hypoxia-induced contractions of porcine
pulmonary artery strips depend on intact endothelium. Exp Lung
Res 1984;7:101–112.
[21] Rubanyi GM, Vanhoutte PM. Hypoxia releases a vasoconstrictor
substance from the canine vascular endothelium. J Physiol London
1985;364:45–56.
[22] Brashers VL, Peach MJ, Rose Jr. CE. Augmentation of hypoxic
pulmonary vasoconstriction in the isolated perfused rat lung by in
vitro antagonists of endothelium-dependent relaxation. J Clin Invest
1988;82:1495–1502.
[23] Mazmanian GM, Baudet B, Brink C, et al. Methylene blue potentiates vascular reactivity in isolated rat lungs. J Appl Physiol
1989;66:1045–1050.
[24] Shaul PW, Wells LB. Oxygen modulates nitric oxide production
selectively in fetal pulmonary endothelial cells. Am J Resp Cell Mol
Biol 1994;11:432–438.
[25] McMurtry IF, Davidson AB, Reeves JT, Grover RF. Inhibition of
458
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
P.-Y. Cheung et al. / Cardiovascular Research 39 (1998) 451 – 458
hypoxic pulmonary vasoconstriction by calcium antagonists in
isolated rat lungs. Circ Res 1976;38:99–104.
Rembold CM, Murphy RA. Myoplasmic (Ca11) determines
myosin phosphorylation and isometric stress in agonist stimulated
swine arterial smooth muscle. Circ Res 1988;63:593–603.
Bennie RE, Packer CS, Powell DR, Jin N, Rhoades RA. Biphasic
contractile response of the pulmonary artery to hypoxia. Am J
Physiol 1992;261:L156–163.
Cartwright D, Gregory GA, Lou H, Heyman MA. The effects of
hypocarbia on the cardiovascular system of puppies. Pediatr Res
1984;18:685–690.
Fisher DJ. Comparative effects of metabolic acidemia and hypoxemia on cardiac output and regional blood flows in unanesthetized
newborn lambs. Pediatr Res 1986;20:756–760.
Lockhart LK, Lautt WW. Hypoxia-induced vasodilation of the feline
superior mesenteric artery is not adenosine mediated. Am J Physiol
1990;259:G605–610.
Rakugi H, Takuchi Y, Nakamura M, et al. Evidence of endothelin-1
release from resistance vessels of rats in response to hypoxia.
Biochem Biophys Res Comm 1990;169:973–977.
Yuan X-J, Tod ML, Rubin LJ, Blaustein MP. Contrasting effects of
hypoxia on tension in rat pulmonary and mesenteric arteries. Am J
Physiol 1990;259:H281–289.
Shepherd AP. Intestinal oxygen consumption and 86Rb extraction
during arterial hypoxia. Am J Physiol 1978;234:E248–251.
Mathie RT, Blumgart LH. Effect of denervation on the hepatic
haemodynamic response to hypercapnia and hypoxia in the dog.
Pflugers Arch 1983;397:152–157.
Koehler RC, McDonald BW, Krasney JA. Influence of CO 2 on
cardiovascular reponse to hypoxia in conscious dogs. Am J Physiol
1980;239:H545–558.
[36] Suguihara C, Bancalari E, Hehre D, Duara S, Gerhardt T. Changes
in ventilation and oxygen consumption during acute hypoxia in
sedated newborn piglets. Pediatr Res 1994;35:536–540.
[37] Lister G. Metabolic responses to hypoxia. Crit Care Med
1993;21:S340–341.
[38] Torrance SM, Wittnich C. Blood lactate and acid–base balance in
graded neonatal hypoxia: evidence for oxygen-restricted metabolism. J Appl Physiol 1994;77:2318–2324.
[39] Morita K, Ihnken K, Buckberg GD, Sherman MP, Young HH.
Studies of hypoxemia / reoxygenation injury: Without aortic clamping IX. Importance of avoiding perioperative hyperoxemia in the
setting of previous cyanosis. J Thorac Cardiovasc Surg
1995;110:1235–1244.
[40] Matheis G, Sherman MP, Buckberg GD, et al. Role of L-argininenitric oxide pathway in myocardial reoxygenation injury. Am J
Physiol 1992;262:H616–620.
[41] Yasmin W, Strynadka KD, Schulz R. Generation of peroxynitrite
contributes to ischemia-reperfusion injury in isolated rat hearts.
Cardiovasc Res 1997;33:422–432.
[42] Enson Y, Giuntini C, Lewis ML, et al. The influence of hydrogen
ion concetration and hypoxia on the pulmonary circulation. J Clin
Invest 1964;43:1146–1162.
[43] Cheung P-Y, Danial H, Jong J, Schulz R. Thiols protect peroxynitrite-induced inhibition of myocardial aconitase. Arch Biochem
Biophys 1998;350:104–108.
[44] Szabo C, Salzman AL. Endogenous peroxynitrite is involved in the
inhibition of mitochondrial respiration in immuno-stimulated J774.2
macrophages. Biochem Biophys Res Commun 1995;209:739–743.