Abstract
Hypercapnia (HC) increases systemic oxygen delivery (DO2) and gastric mucosal oxygenation. However, it activates the renin–angiotensin–aldosterone system (RAAS), which conversely reduces mesenteric perfusion. The aims of this study were to evaluate the effect of RAAS inhibition during normocapnia and HC on oral and gastric mucosal oxygenation (μHbO2) and to assess the effect of blood pressure under these circumstances. Five dogs were repeatedly anesthetized to study the effects of ACE inhibition (ACE-I; 5 mg/kg captopril, followed by 0.25 mg/kg per h) on μHbO2 (reflectance spectrophotometry) and hemodynamic variables during normocapnia (end-tidal CO2=35 mmHg) and HC (end-expiratory carbon dioxide (etCO2)=70 mmHg). In the control group, the dogs were subjected to HC alone. To exclude the effects of reduced blood pressure, in one group, blood pressure was maintained at baseline values via titrated phenylephrine (PHE) infusion during HC and additional captopril infusion. ACE-I strongly reduced gastric μHbO2 from 72±2 to 65±2% and mean arterial pressure (MAP) from 64±2 to 48±4 mmHg, while DO2 remained unchanged. This effect was counteracted in the presence of HC, which increased gastric μHbO2 from 73±3 to 79±6% and DO2 from 15±2 to 22±4 ml/kg per min during ACE-I without differences during HC alone. However, MAP decreased similar to that observed during ACE-I alone from 66±3 to 47±5 mmHg, while left ventricular contractility (dPmax) increased from 492±63 to 758±119 mmHg/s. Titrated infusion of PHE had no additional effects on μHbO2. In summary, our data suggest that RAAS inhibition reduces gastric mucosal oxygenation in healthy dogs. HC not only abolishes this effect, but also increases μHbO2, DO2, and dPmax. The increase in μHbO2 during ACE-I under HC is in accordance with our results independent of blood pressure.
Introduction
Hypercapnia (HC) occurs frequently during lung-protective ventilation strategies where it is rather tolerated than intended (‘permissive HC’). However, HC exerts positive effects on systemic perfusion and oxygenation such as an increase in cardiac output (CO) and systemic oxygen delivery (DO2). Additionally, it protects microvascular oxygenation of the gastric mucosa during physiological conditions (Schwartges et al. 2008) and hemorrhagic shock (Schwartges et al. 2010). In this context, we have recently shown that the increase in gastric mucosal oxygenation during otherwise physiological conditions is mediated via vasopressin V1A receptors (Vollmer et al. 2013) and might be due to increased levels of vasopressin during HC (Philbin et al. 1970). However, HC interacts with other hormone systems as well.
In particular, HC enhances renin secretion and thus leads to the activation of the renin–angiotensin–aldosterone system (RAAS; Kurz & Zehr 1978). Conversely, renin via angiotensin reduces especially mesenteric perfusion (McNeill et al. 1977, Bulkley et al. 1985, Bailey et al. 1986, Reilly et al. 1997) due to high affinity and quantity of the angiotensin receptor in the mesenteric region (Gunther et al. 1980) and increased regional angiotensin production (Bailey et al. 1986). The inhibition of the RAAS prior to hemorrhagic shock improves intestinal blood flow and reduces mortality (Aneman et al. 2000). Thus, the positive effects on microcirculatory oxygenation induced by acute HC might be partially counteracted by the activation of the RAAS (Kurz & Zehr 1978).
However, blood flow to the stomach is – in contrast to that to other splanchnic organs – not generally influenced by RAAS inhibition (Cullen et al. 1994). Additionally, RAAS inhibition reduces blood pressure, which might negatively affect gastric microcirculation. Concerning oxygenation of the gastric mucosa, the influence of RAAS inhibition is unclear so far.
The aims of this study were to evaluate the effect of RAAS inhibition with captopril during normocapnia and HC on oral and gastric mucosal oxygenation and assess the role of blood pressure under these circumstances. We expected either an additional increase in gastric oxygenation during RAAS inhibition due to the inhibition of a vasoconstrictor in the splanchnic region or a decrease in gastric oxygenation due to a reduction in blood pressure. Mucosal oxygenation is of particular interest as the gastrointestinal mucosa functions as an effective barrier against bacteria and an insufficient microcirculatory oxygen supply leads to an impaired mucosal barrier function. Subsequently, this enables the translocation of bacteria and bacterial toxins into portal venous and local lymphatic circulation (Deitch et al. 2004) and mediates an inflammatory response syndrome (Chen et al. 2003). Particularly, alterations in the oral microcirculation are an independent predictor of organ failure and associated with death (Maciel et al. 2004, Sakr et al. 2004).
The role of RAAS activation and RAAS inhibition during HC with special regard to blood pressure is of clinical importance for mainly two reasons. First, permissive HC is widely used in intensive care units during lung-protective ventilation, e.g. in patients with acute respiratory distress syndrome. Second, the inhibition of the RAAS with angiotensin-converting enzyme inhibition is very common in patients for therapy of arterial hypertension and heart failure. Thus, it is not only important to know about the effects of HC and the RAAS on gastrointestinal mucosal oxygenation, but also essential to know about the interaction and possible counteracting effects of both therapies with special regard to side effects, e.g. reduction in blood pressure, that again exert effects on mucosal oxygenation.
Taken together, the aim of this study was to assess the influence of RAAS inhibition on gastric and oral mucosal oxygenation during otherwise physiological conditions. Additionally, we evaluated the effect of RAAS inhibition during acute HC on microcirculation and systemic hemodynamic variables. Furthermore, we analyzed the effect of changes in blood pressure during RAAS inhibition.
Materials and methods
Animals
The data were derived from repetitive experiments on five dogs (female foxhounds, weighing 28±1 kg) treated in accordance with the NIH guidelines for animal care. Experiments were performed with the approval of the Local Animal Care and Use Committee (North Rhine-Westphalia State Agency for Nature, Environment and Consumer Protection, Recklinghausen, Germany; ref. 50.05-230-74/05). The same group of five dogs used in this study was used in another study published previously (Vollmer et al. 2013). Each dog underwent each protocol in a randomized order and served as its own control. Based on the assumption that the physiological reaction on a given stimulus is almost identical in the same animal in repetitive experiments, usually observed inter-individual differences are reduced, allowing smaller sample sizes. A higher number of animals would allow us to detect even smallest significant differences; however, smallest differences will not be clinically relevant. The a priori power analysis (G*Power Version 3.1.7, University of Duesseldorf, Duesseldorf, Germany) with five dogs at a given α≤0.05 (two tailed) and an expected difference in μHbO2 of at least 10% (e.g. 70 vs 80%) with an expected s.d. of 5–6% per group (based on previous studies) suggested a power of 81.5%.
Prior to the experiments, access to food was withheld for 12 h with water ad libitum to ensure complete gastric depletion and to avoid changes in perfusion and oxygenation due to digestive activity. Each dog underwent each experimental protocol in a randomized order and served as its own control (except group phenylephrine (PHE), in which was measurements were carried out after completing the measurements in the other groups as described below). The experiments were performed at least 3 weeks apart to prevent carryover effects. The experiments were performed under general anesthesia (induction of anesthesia with 4 mg/kg propofol, maintenance with sevoflurane, end-tidal concentration of 3.0%, and 1.5 minimum alveolar concentration (MAC) in the dogs (Kazama & Ikeda 1988)). The dogs were mechanically ventilated after endotracheal intubation (FiO2=0.3 and VT=12.5 ml/kg, a normal tidal volume for the dogs (Dyson 2012)) with the respiratory frequency adjusted to achieve normocapnia (end-expiratory carbon dioxide (etCO2)=35 mmHg), verified by continuous capnography (Capnomac Ultima, Datex Instrumentarium, Helsinki, Finland). During baseline conditions, the dogs were placed on their right side and covered with warming blankets to maintain body temperature at 37.5 °C (continuous arterial measurement). Throughout the experiments, no additional fluid replacement was carried out to avoid volume effects that could influence tissue perfusion and oxygenation. However, after each blood sample withdrawal, normal saline was infused three times the sampling volume to maintain blood volume.
Measurements
Mucosal oxygenation
Microvascular oxygen saturation (μHbO2) of the oral and gastric mucosa was continuously assessed by tissue reflectance spectrophotometry (O2C, LEA Medizintechnik, Gießen, Germany), as detailed previously (Frank et al. 1989).
White light (450–1000 nm) was transmitted into the tissue of interest via a micro-lightguide and the reflected light was then analyzed. The wavelength-dependent absorption of the applied white light can be used to calculate the percentage of oxygenated hemoglobin (Hb; Kuchenreuther et al. 1996).
Since light is totally absorbed in vessels with a diameter >100 μm (Gandjbakhche et al. 1999), only microvascular oxygenation of nutritive vessels of the mucosa was measured. The highest fraction of blood volume is stored in venous vessels; therefore, mainly postcapillary oxygenation was measured, which represents the critical partial pressure of oxygen (pO2) for ischemia (Siegemund et al. 1999).
The flexible lightguide probe was placed in the mouth facing the buccal side of the oral mucosa, while another one was introduced into the stomach via an orogastric silicone tube and positioned facing the greater curvature (Scheeren et al. 2002). Both sites represent the microcirculation of other gastrointestinal mucosal regions (Temmesfeld-Wollbruck et al. 1998, Verdant et al. 2009). Online evaluation of the signal quality throughout the experiments allowed for the verification of the correct position of the probe tip. The μHbO2 values reported are the means of the last 5 min (150 spectra, 2 s each) of the respective intervention period under steady-state conditions. This method allows for the detection of splanchnic ischemia with a precision similar to that of laser flowmetry (Leung et al. 1987), intravital microscopy (Bellamy et al. 1997), or hydrogen gas clearance (Machens et al. 1995). The non-traumatic instrumentation and in particular non-traumatic access to the gastric mucosa allow for the determination of mucosal oxygenation in the absence of surgical stress. This is particularly desirable with respect to the marked alterations that surgical stress exerts in splanchnic circulation (Mythen et al. 1993). In this situation, reflectance spectrophotometry reliably detects even clinically asymptomatic reductions in mucosal oxygenation (Fournell et al. 2003) and highly correlates with the morphologic severity and extent of gastric mucosal tissue injury (Sato et al. 1986).
Systemic hemodynamics and oxygenation
The aorta was catheterized via the left carotid artery for the continuous measurement of mean arterial pressure (MAP; Gould-Statham pressure transducers P23ID, Elk Grove, IL, USA) and intermittent withdrawal of blood samples for the measurement of carbon dioxide partial pressure (pCO2), pO2, pH, bicarbonate (HCO3 −), Hb, and lactate (Rapidlab 860, Bayer AG). Oxygen saturation (SaO2) was calculated for dog blood from pO2 and adjusted to pH and temperature (Rossing & Cain 1966). Arterial oxygen content (CaO2) and systemic DO2 (DO2=CaO2×CO) were calculated subsequently. CO was determined via transpulmonary thermodilution (PiCCO 4.2 non-US, PULSION Medical Systems, Munich, Germany) and the velocity of pressure increase dPmax=maximum dP/dt as an indicator of left ventricular contractility via pulse contour analysis (PiCCO 4.2) at the end of each intervention, at least every 30 min, as described previously (von Spiegel et al. 1996).
Heart rate was continuously measured by electrocardiography (Powerlab, ADInstruments, Castle Hill, NSW, Australia). All hemodynamic and respiratory variables were recorded on a personal computer after analog to digital conversion (Powerlab, ADInstruments) for later analysis.
Experimental program
The parameters described above were measured in all the experimental groups. Following the instrumentation instructions, 30 min were allowed to establish steady-state conditions and baseline values were recorded before the dogs were randomized to the respective protocol (Fig. 1). Groups that included HC were pretreated with HC for 1 h (end-tidal carbon dioxide was continuously increased over 30 min from 35 to 70 mmHg via reduction of respiratory frequency as described previously (Schwartges et al. 2008)).
Experimental protocol: hypercapnia (HC), HC with ACE inhibition (HC/ACE-I), HC with phenylephrine during ACE-I (HC/ACE-I+PHE), ACE-I, and PHE infusion.
Citation: Journal of Endocrinology 218, 3; 10.1530/JOE-13-0132
ACE inhibition
The effects of ACE inhibition (ACE-I) alone were tested during normocapnia. After baseline conditions, ACE-I was initiated with an additional observation period of 60 min. ACE-I was induced by the administration of 5 mg/kg ACE inhibitor captopril (Sigma–Aldrich), followed by infusion of 0.25 mg/kg per h until the end of the experiments. Complete receptor blockade was confirmed by the administration of 20 ng/kg angiotensin I (Sigma–Aldrich) at the end of each experiment. This dose had no effect on hemodynamic variables in dogs with preceding ACE-I at the end of the observation period, but increased the MAP by about 25 mmHg in dogs without preceding ACE-I.
RAAS inhibition during HC (HC/ACE-I)
To analyze the effects of ACE-I during HC, HC was maintained after hypercapnic pretreatment, while ACE-I was initiated as described above with an additional observation period of 60 min.
RAAS inhibition during HC with PHE (HC/ACE-I+PHE)
To analyze the effects of HC during ACE-I independent of changes in blood pressure, after hypercapnic pretreatment, HC was maintained and ACE-I was initiated as described above. During ACE-I, blood pressure was maintained by continuous infusion of PHE ((R)-(−)-PHE hydrochloride, Sigma–Aldrich). Blood pressure was recorded continuously, and infusion rate was constantly adjusted if necessary.
Hypercapnia
The effects of HC alone on μHbO2 were tested without RAAS inhibition. After pretreatment, HC was maintained for an additional hour. The results of this control group have been published already as those of the control group (Vollmer et al. 2013) because we are forced by law to minimize animal experiments. However, this group was randomized in this series.
Effect of PHE
To distinguish the effects mediated solely by PHE, this group was measured after completing the measurements in the first four groups. After baseline conditions, PHE was infused at the individual rate recorded in the same dog in the preceding protocol HC/ACE-I+PHE.
Blood samples were obtained for blood gas analysis every 30 min.
Statistical analysis
Data for analysis were obtained during the last 5 min of baseline and intervention periods under steady-state conditions. All data are presented as absolute values of mean±s.e.m. for five dogs. Differences within the groups and between the groups were tested using a Wilcoxon's signed-rank test (StatView V4.1, SAS Institute, Inc., Cary, NC, USA); P<0.05 was considered significant.
Results
ACE-I strongly reduced gastric μHbO2, which was completely reversed by HC without differences being observed during HC alone. This effect of HC was independent of blood pressure.
ACE inhibition
In detail, ACE-I alone reduced gastric μHbO2 from 72±2 to 65±2% (Fig. 2), although systemic DO2 remained unchanged (13±1 vs 13±1 ml/kg per min; Fig. 3). However, MAP strongly decreased from 64±2 to 48±4 mmHg (Fig. 4).
Effects of hypercapnia (HC), HC with ACE inhibition (HC/ACE-I), HC with phenylephrine during ACE-I (HC/ACE-I+PHE), ACE-I, and PHE infusion on gastric mucosal microvascular hemoglobin oxygen saturation (μHbO2). Filled symbols, hypercapnic pretreated groups. Data are presented as mean±s.e.m. for n=5 dogs; *P<0.05 vs HC/ACE-I for group ACE-I.
Citation: Journal of Endocrinology 218, 3; 10.1530/JOE-13-0132
Effects of hypercapnia (HC), HC with ACE inhibition (HC/ACE-I), HC with phenylephrine during ACE-I (HC/ACE-I+PHE), ACE-I, and PHE infusion on systemic oxygen delivery (DO2). Filled symbols, hypercapnic pretreated groups. Data are presented as mean±s.e.m. for n=5 dogs; *P<0.05 vs HC/ACE-I for groups HC/ACE-I+PHE and ACE-I.
Citation: Journal of Endocrinology 218, 3; 10.1530/JOE-13-0132
Effects of hypercapnia (HC), HC with ACE inhibition (HC/ACE-I), HC with phenylephrine during ACE-I (HC/ACE-I+PHE), ACE-I, and PHE infusion on mean arterial pressure (MAP). Filled symbols, hypercapnic pretreated groups. Data are presented as mean±s.e.m. for n=5 dogs; *P<0.05 vs HC/ACE-I for groups HC and HC/ACE-I+PHE.
Citation: Journal of Endocrinology 218, 3; 10.1530/JOE-13-0132
HC and HC during ACE-I (HC/ACE-I)
In contrast, during HC and additional ACE-I, DO2 increased from 15±2 to 22±4 ml/kg per min and gastric μHbO2 increased from 73±3 to 79±6%, while oral μHbO2 remained unchanged. This effect was similar to that observed during HC alone, which increased gastric μHbO2 from 70±4 to 80±2% and systemic DO2 from 12±1 to 16±1 ml/kg per min (Fig. 3) without any significant effect on oral μHbO2. The increase in gastric μHbO2 during HC with additional ACE-I was observed despite a severe decrease in MAP from 66±3 to 47±5 mmHg. However, left ventricular contractility (dPmax) increased from 492±63 to 758±119 mmHg/s. In contrast, HC alone increased the MAP from 62±2 to 69±3 mmHg with no effect on dPmax.
Effect of additional PHE (HC/ACE-I+PHE)
When PHE was titrated to maintain blood pressure during HC and additional ACE-I, gastric μHbO2 was increased similar to that observed during HC and HC with additional ACE-I (from 68±3 to 79±2%). However, DO2 was doubled from 13±1 to 26±2 ml/kg per min and was significantly higher than that observed during HC with ACE-I without PHE infusion. This change in systemic oxygen transport did not affect regional oxygenation (Figs 2, 3 and 4). Further hemodynamic variables are given in Table 1.
Hemodynamic variables of the experimental groups. Data are presented as absolute values during baseline conditions and the respective intervention (at the end of the experiment), mean±s.e.m., n=5
| Variable | Group | Baseline | Intervention (1 h) |
|---|---|---|---|
| Gastric μHbO2 (%) | HC | 70±4 | 80±2* |
| HC/ACE-I | 73±3 | 79±6* | |
| HC/ACE-I+PHE | 68±3 | 79±2* | |
| ACE-I | 72±2 | 65±2*, ‡ | |
| PHE | 68±4 | 73±3 | |
| Oral μHbO2 (%) | HK | 72±2 | 81±3 |
| HKACE | 78±2 | 74±4 | |
| HKACEPHE | 77±2 | 75±2† | |
| ACE | 79±2 | 77±4 | |
| PHE | 76±2 | 60±4* | |
| DO2 (ml/kg per min) | HC | 12±1 | 16±1* |
| HC/ACE-I | 15±2 | 22±4* | |
| HC/ACE-I+PHE | 13±1 | 26±2*, ‡ | |
| ACE-I | 13±1 | 13±1‡ | |
| PHE | 14±3 | 17±2* | |
| SVR (mmHg/l per min) | HC | 26±2 | 23±2 |
| HC/ACE-I | 28±3 | 14±1*, † | |
| HC/ACE-I+PHE | 28±3 | 21±3*, ‡ | |
| ACE-I | 29±3 | 21±1*, ‡ | |
| PHE | 30±2 | 54±4* | |
| CO (ml/kg per min) | HC | 86±6 | 111±8* |
| HC/ACE-I | 87±10 | 125±19* | |
| HC/ACE-I+PHE | 84±9 | 126±16* | |
| ACE-I | 83±9 | 83±8‡ | |
| PHE | 80±10 | 75±9 | |
| SV (ml) | HC | 23±2 | 29±3* |
| HC/ACE-I | 22±2 | 31±4* | |
| HC/ACE-I+PHE | 22±2 | 33±4*, † | |
| ACE-I | 21±2 | 21±2‡ | |
| PHE | 20±2 | 23±1* | |
| MAP (mmHg) | HC | 62±2 | 69±3* |
| HC/ACE-I | 66±3 | 47±5*, † | |
| HC/ACE-I+PHE | 62±1 | 70±1*, ‡ | |
| ACE-I | 64±2 | 48±4* | |
| PHE | 65±2 | 111±9* | |
| HR (min) | HC | 104±4 | 109±5 |
| HC/ACE-I | 111±5 | 113±7 | |
| HC/ACE-I+PHE | 106±4 | 105±4 | |
| ACE-I | 110±7 | 111±7 | |
| PHE | 111±5 | 93±6* | |
| dPmax (mmHg/min) | HC | 470±57 | 542±46 |
| HC/ACE-I | 492±63 | 758±119* | |
| HC/ACE-I+PHE | 468±36 | 736±43*, † | |
| ACE-I | 450±53 | 513±45 | |
| PHE | 442±30 | 468±30 |
μHbO2, gastric and oral mucosal hemoglobin oxygenation; DO2, systemic oxygen transport; SVR, systemic vascular resistance; CO, cardiac output; SV, stroke volume; MAP, mean arterial pressure; HR, heart rate; dPmax, left ventricular contractility. *P<0.05 vs baseline, † P<0.05 vs HC for groups HC/ACE-I and HC/ACE-I+PHE, and ‡ P<0.05 vs HC/ACE-I for groups HC/ACE-I+PHE and ACE-I.
Influence of CO and oxygen content
The increase in systemic DO2 during HC is related to not increased arterial oxygen content but rather to CO, which increased from 86±6 to 111±8 ml/kg per min. Increase in CO was similar during HC with ACE-I. CO tended to be higher during additional PHE infusion, though not significantly, but might explain the higher DO2 compared with that during HC alone.
In contrast to the increase in μHbO2 in all the three groups during HC, arterial oxygen partial pressure was reduced during HC from 154±5 to 119±6 mmHg, during HC with additional ACE-I from 161±3 to 127±3 mmHg, and during additional PHE infusion from 158±5 to 132±4 mmHg, while arterial oxygen partial pressure did not change during ACE-I without HC (153±8 vs 155±7 mmHg). Thus, mucosal oxygenation increased despite reduced arterial oxygen content.
Plasma lactate levels
Plasma lactate levels decreased during HC independent of additional ACE-I and independent of blood pressure. In contrast, during ACE-I without HC, plasma lactate levels did not change significantly. Further metabolic and respiratory variables are given in Table 2.
Metabolic and respiratory variables of the experimental groups. Data are presented as absolute values during baseline conditions and the respective intervention (at the end of the experiment), mean±s.e.m., n=5
| Variable | Group | Baseline | Intervention (1 h) |
|---|---|---|---|
| SaO2 (%) | HC | 98±1 | 95±1* |
| HC/ACE-I | 99±1 | 96±1* | |
| HC/ACE-I+PHE | 99±1 | 96±1*,‡ | |
| ACE-I | 98±1 | 98±1‡ | |
| PHE | 99±1 | 98±1 | |
| pCO2 (mmHg) | HC | 39±1 | 67±1* |
| HC/ACE-I | 38±1 | 67±2* | |
| HC/ACE-I+PHE | 38±1 | 64±1* | |
| ACE-I | 38±1 | 38±1‡ | |
| PHE | 38±1 | 36±1 | |
| pO2 (mmHg) | HC | 154±5 | 119±6* |
| HC/ACE-I | 158±5 | 125±5* | |
| HC/ACE-I+PHE | 158±5 | 132±4*,‡ | |
| ACE-I | 153±8 | 155±7‡ | |
| PHE | 149±1 | 147±1 | |
| pH | HC | 7.33±0.01 | 7.15±0.01* |
| HC/ACE-I | 7.37±0.01 | 7.17±0.01* | |
| HC/ACE-I+PHE | 7.36±0.01 | 7.17±0.01* | |
| ACE-I | 7.35±0.01 | 7.35±0.01*,‡ | |
| PHE | 7.38±0.02 | 7.37±0.01 | |
| HCO3 − (mmol/l) | HC | 20±1 | 23±1* |
| HC/ACE-I | 21±1 | 24±1* | |
| HC/ACE-I+PHE | 21±1 | 23±1* | |
| ACE-I | 21±1 | 21±1‡ | |
| PHE | 22±1 | 21±1 | |
| Hb (g/100 ml) | HC | 11±1 | 11±1* |
| HC/ACE-I | 12±1 | 13±1 | |
| HC/ACE-I+PHE | 12±1 | 16±1*,† ,‡ | |
| ACE-I | 12±1 | 12±1 | |
| PHE | 12±1 | 17±1* | |
| Lactate (mmol/l) | HC | 2.1±0.3 | 1.2±0.1 |
| HC/ACE-I | 1.9±0.1 | 1.0±0.1 | |
| HC/ACE-I+PHE | 2.7±0.1 | 1.3±0.1* | |
| ACE-I | 1.9±0.1 | 2.0±0.2 | |
| PHE | 2.1±0.3 | 2.1±0.3 |
SaO2, systemic oxygen saturation; pCO2, carbon dioxide partial pressure; pO2, oxygen partial pressure; HCO3 −, bicarbonate; pH; Hb, hemoglobin; and lactate. *P<0.05 vs baseline, † P<0.05 vs HC for groups HC/ACE-I and HC/ACE-I+PHE, and ‡ P<0.05 vs HC/ACE-I for groups HC/ACE-I+PHE and ACE-I.
Discussion
Interpretation of the results
Our data suggest that ACE-I reduces gastric but not oral mucosal oxygenation in healthy dogs. HC during additional ACE-I seems to counteract the decrease in gastric μHbO2 and even to increase gastric μHbO2, DO2, and left ventricular contractility. The effect of HC on gastric μHbO2 during additional ACE-I is independent of reduced blood pressure, which affects μHbO2 only during normocapnia. The restrictions and implications of these findings will be discussed in detail below.
The results are partially unexpected. First, ACE-I reduced gastric μHbO2. The RAAS is known to reduce mesenteric perfusion (McNeill et al. 1977, Bulkley et al. 1985, Bailey et al. 1986, Reilly et al. 1997), and inhibition of the RAAS during captopril therapy increased blood flow in the superior mesenteric artery (Ray-Chaudhuri et al. 1993). Similar effects are observed during long-term treatment (Rozsa & Sonkodi 1995) and in critically ill trauma patients (Kincaid et al. 1998). Hence, one might expect RAAS inhibition to increase gastric μHbO2. The contradictory reduction in μHbO2 might be related to the severe decrease in MAP reducing blood flow to the splanchnic region. Still, blood pressure was equally reduced during additional HC without negative effects on μHbO2. Hence, our divergent results are not related to blood pressure. However, angiotensin II antagonism does not alter splanchnic perfusion under all circumstances (Strohmenger et al. 1998), and captopril increased blood flow in another study selectively to the small intestine but not to the stomach (Cullen et al. 1994). This might be related to the blood supply in the gastric region that does not depend on the superior mesenteric artery. Thus, our results indicate that gastric oxygenation and perfusion might not benefit from RAAS inhibition. On the contrary, gastric oxygenation even decreased during captopril infusion, which is not attributed to short-term effects such as a decrease in blood pressure. This effect is of major importance as RAAS inhibition with captopril is a widely used therapy for hypertension and heart failure. Especially, the critically ill patients might benefit from increased gastric oxygenation. However, our reported results are based on experiments in five healthy dogs. Whether the results can be transferred into the clinical setting on critically ill patients has to be confirmed in larger clinical trials. Additionally, our results do not indicate the effects of long-term treatment where adaptive mechanisms might occur and where the RAAS is only partially inhibited. However, the effects of captopril seem similar during long-term treatment (Rozsa & Sonkodi 1995).
Still, in critically ill patients, permissive HC could be tested as a preventive option to compensate for the decrease in oxygenation during additional captopril infusion despite the decrease in blood pressure. In healthy dogs, μHbO2 even increased during HC under ACE-I comparable to the increase during HC alone. In addition, CO and accordingly systemic oxygen transport increased. Thus, HC was able to increase μHbO2, despite reduced blood pressure due to captopril. Additionally, HC with additional ACE-I increased left ventricular contractility, while neither HC nor ACE-I alone affected contractility. The increase in dPmax is not related to the reduction in systemic vascular resistance, which is reduced during ACE-I alone as well without an effect on dPmax.
The observation of increased μHbO2 during HC in our previous studies (Schwartges et al. 2008, 2010) was confirmed in the present study. However, as one possibility, we expected HC to activate the RAAS and thus to partially antagonize its positive effects on μHbO2. If this were the case, captopril would further increase μHbO2 during HC. This could not be observed as additional captopril infusion during HC did not change μHbO2 compared with that during HC alone. One explanation might be the above-mentioned effect of angiotensin mainly in the superior mesenteric artery, but angiotensin exerts effects on the gastric mucosa as well. This could be observed during angiotensin I application prior to captopril infusion in pilot experiments. However, another explanation might be the acute reduction in blood pressure during captopril infusion under HC comparable to that during captopril infusion alone, which even reduced μHbO2. However, the maintenance of blood pressure with PHE did not change the effects of HC and captopril on μHbO2, although PHE increased oxygen delivery under physiological as well as under hypercapnic conditions. One further explanation might be that HC alone had already increased μHbO2 to 80%. As μHbO2 represents mainly the postcapillary oxygenation, a further increase might not be possible.
The most likely explanation for our unexpected results of decreased μHbO2 during RAAS inhibition compared with that in other studies (McNeill et al. 1977, Bulkley et al. 1985, Bailey et al. 1986, Reilly et al. 1997) might be related to our approach in assessing oxygenation rather than to perfusion. The measurement of perfusion does not indicate oxygen supply necessary for the gastrointestinal mucosa. Additionally, angiotensin might be necessary for the redistribution of blood in favor of the gastric mucosa during physiological conditions. An explanation for the redistribution of blood might be the unique configuration of parallel and series-coupled circuits. Initially, mesenteric blood flow is divided into two parallel circuits, one serving the muscularis and the other serving the submucosa/mucosa (Ceppa et al. 2003). Angiotensin-mediated vasoconstriction of peripheral arteries might reduce blood flow to the muscularis and thus maintain blood flow to the submucosa and mucosa. Therefore, angiotensin might be necessary for maintaining μHbO2 under physiological conditions, while higher angiotensin levels during pathological conditions reduce splanchnic flow and μHbO2. This theory is supported by the observation that oral μHbO2 does not change during ACE-I or additional HC and thus seems to be independent of angiotensin-mediated redistribution.
We have to critically discuss that enzyme or receptor inhibition might result in an increased activation of other enzymes or receptors with adverse side effects. In this case, ACE-I leads to reduced aldosterone production, which, however, rather exerts a long-term effect on water retention, which did not influence our observation period. Additionally, inactivation of bradykinin is inhibited as well with subsequent endothelium-dependent vasodilation via the B2 receptor, which would partially explain the severe reduction in blood pressure. Without the conversion of angiotensin I to angiotensin II, the substrate angiotensin I might accumulate; however, it is not vasoactive. We did not choose an AT1 receptor antagonist to selectively block the vascular effect of angiotensin II. AT1 receptor antagonism leads to increased binding to the AT2 receptor with possible adverse side effects. Additionally, our main intention was to analyze the effect of the total RAAS on gastric oxygenation during HC, especially since captopril is widely used, particularly in critically ill patients with hypertension or heart failure.
In the repeated-measurements design, treatment cross-over could be a confounding variable. However, the used drugs have a half-life of 1.9 h (captopril) and 2–3 h (PHE), assuring complete elimination after 8–12 h. The experiments were performed at least 3 weeks apart, making the carryover effects unlikely. This is confirmed by the absence of any significant differences concerning macrocirculatory and microcirculatory variables during baseline conditions.
In summary, captopril infusion reduces gastric oxygenation in healthy dogs, despite improved perfusion in the splanchnic region in other studies. We demonstrated that the acute, negative effects of captopril were completely reversed in the presence of HC. During permissive HC, additional therapy with captopril does not seem to affect the increase in gastric oxygenation. However, the combination of HC and captopril increases left ventricular contractility. Nevertheless, the results obtained from dogs have to be tested in the clinical setting in critically ill patients in the future.
Conclusions
Our data suggest that captopril reduces gastric mucosal oxygenation in healthy dogs. In this situation, HC seems to not only abolish this effect but also increase gastric oxygenation, systemic DO2, and left ventricular contractility. According to our data, this effect is independent of blood pressure.
Declaration of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding
This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.
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