Pulmonary vasoconstriction in response to alveolar hypoxia [hypoxic pulmonary vasoconstriction (HPV)] diverts blood flow from nonventilated to ventilated lung areas, thereby reducing venous admixture and maintaining arterial oxygen . The exact mechanism of HPV is not fully understood, but it appears to involve direct effects on both the endothelium and vascular smooth muscle cells . There is growing evidence to suggest that hypoxia mediates vasoconstriction, at least in part, through various types of potassium channels [3,4].
Endotoxic shock or sepsis is a common problem in patients with endotoxaemia that often resists intensive medical treatment. It is characterized by profound systemic hypotension, progressive metabolic acidosis and dysfunction of multiple organs. HPV has been shown to be markedly impaired during sepsis or the acute respiratory distress syndrome (ARDS) in animal models [5–7] and patients [8,9]. To prevent hypoxaemia, these patients require sedation to be mechanically ventilated with high concentrations of oxygen. Although there are data on the influence of sedatives and analgesics on HPV in healthy lungs [10–12] only little is known about the influence of anaesthetics on sedation with ketamine in septic patients. Over recent years, ketamine has received increasing attention in intensive care because of its lack of cardiodepressive as well as bowel function properties . Moreover, being an N-methyl-D-aspartate (NMDA) receptor antagonist, it prevents opioid tolerance.
Ketamine is a potent analgesic with central sympathomimetic properties. The anaesthetic and analgesic effects of ketamine are mediated primarily by noncompetitive antagonism at the NMDA receptor . Other mechanisms include binding to opioid receptors with spinal and supraspinal components . One feature of ketamine is the overall lack of cardiovascular depression, with a balance between the direct negative inotropic effect and a central sympathetic stimulation, associated with increased plasma levels of epinephrine and norepinephrine . Ketamine has been suggested to be a direct vasoconstrictor  or vasodilator  in pulmonary arteries. Gassner et al. reported that changes in pulmonary haemodynamics are due to secondary effects on cardiac output rather than due to modulation of pulmonary vascular tone.
Halogenated inhalational anaesthetics like halothane and isoflurane have been reported to inhibit hypoxic pulmonary vasoconstriction and to depress cardiovascular and myocardial function even in nonseptic patients and animal models [10,11].
As patients with sepsis or ARDS may – on one hand – benefit from the cardiovascular stability during ketamine sedation , which – on the other hand – may lead to inhibition of HPV, increased pulmonary shunting and, thus, hypoxaemia, we studied the direct effects of ketamine on resting pulmonary vascular tone and on HPV responsiveness in septic and nonseptic mice in an isolated perfused lung model.
Materials and methods
All animal experiments were conducted under protocols reviewed and approved by the Subcommittee on Research Animal Care of the University of Heidelberg.
Four groups of mice (n = 7, respectively) received an intraperitoneal injection of endotoxin [lipopolysaccharide (LPS); Escherichia coli 0111: B4, 25 mg kg−1 body weight; Sigma, St Louis, Missouri, USA] 18 h before isolated lung perfusion experiments (LPS groups). During lung perfusion, ketamine was added to the perfusate to give a final perfusion rate of 0, 0.1, 1.0, and 10 mg kg−1 body weight min−1 ketamine in the respective groups. Additional groups of animals served as untreated controls (n = 7, respectively).
Isolated, perfused, and ventilated mouse lung model
Mice were killed by an intraperitoneal injection of pentobarbital sodium (200 mg kg−1 body weight) (Merial, Hallbergmoos, Germany) and the lungs were explanted and perfused as previously described . Lungs were included in this study if they had a homogeneous white appearance without signs of haemostasis or atelectasis and showed a stable perfusion pressure of less than 10 mmHg during the second 5 min of an initial 10 min baseline perfusion period. Using these two criteria, approximately 15% of lung preparations from each group were discarded before the study. Pulmonary artery pressure (PAP) and left atrial pressure were measured using saline-filled membrane pressure transducers connected to a side port of the inflow and outflow cannulae. Pressure transducers were connected to a biomedical amplifier and data were recorded at 150 Hz on a personal computer using an analogue-to-digital interface with a data acquisition system (DI-220; Dataq Instruments, Akron, Ohio, USA). The system was calibrated before each experiment. HPV responsiveness (ΔPAP) was quantified as the difference between basal PAP and PAP at the end of a 6 min ventilation with an FiO2 of 0.01 (Fig. 1).
Pulmonary vascular pressure–flow relationships were obtained during normoxia and hypoxia. After a 10 min equilibration period with normoxic ventilation (FiO2 = 0.21) and a perfusate flow of 50 ml kg−1 min−1, flow was set to 25, 50, 75, and 100 ml kg−1 min−1 in a randomized fashion for 30 s each and the corresponding PAP recorded. Then, ventilation was switched to hypoxic gas (FiO2 = 0.01, perfusate flow 50 ml kg−1 min−1) for measurement of ΔPAP and a second pressure–flow relationship was recorded in the same manner at the end of a 6 min ventilation with FiO2 of 0.01.
Pulmonary vascular pressure–flow relationships were analysed using the nonlinear distensible vessel model of Linehan et al. as described before . Briefly, this model describes the vascular pressure–flow characteristic using two parameters: R0, the pulmonary vascular resistance that would exist if the vessels were at their respective diameter at zero vascular pressure (intrinsic vascular resistance) and the vascular distensibility factor α.
Lung wet/dry weight ratio
At the end of the experiments, both lungs of the studied animals, excluding their hilar structures, were excised and immediately weighed. Thereafter, lungs were dried in an oven at 100°C overnight and then re-weighed. Lung wet/dry weight ratios were calculated by dividing the wet weight by the dry weight as described previously .
Data are reported as means ± SD. After approving the assumption of normality and equal variance across groups, differences were assessed using analysis of variance (ANOVA) followed by an appropriate post-hoc comparison test. Groups were compared by a two-way ANOVA statistical test. When significant differences were detected by ANOVA, a post-hoc least difference test for planned comparisons was used (SigmaStat, Jandel Corporation, San Rafael, California, USA). Statistical significance was assumed at a P value of less than 0.05.
Ketamine and resting pulmonary vascular tone
Lungs obtained from untreated control mice were perfused with 0, 0.1, 1.0, or 10 mg kg−1 body weight min−1 ketamine. As shown in Fig. 2, we did not find any difference in baseline PAP between these groups. However, analysis of pressure–flow relationships revealed that ketamine increased vascular distensibility at 10 mg kg−1 body weight min−1 as well as intrinsic vascular resistance R0 at 0.1 and 10 mg kg−1 body weight min−1 under normoxic conditions (Figs 3 and 4).
Ketamine and resting pulmonary vascular tone during endotoxaemia
LPS treatment 18 h before lung perfusion experiments did not change baseline PAP (Fig. 2). However, we could detect an endotoxin-induced increase in vessel distensibility α and basic vascular resistance R0, as shown in Figs 3 and 4. Of interest, the LPS-induced increase in α was abolished by perfusing lungs of LPS-pretreated mice with 1.0 or 10 mg kg−1 body weight min−1 ketamine (Fig. 3), and there was a tendency of ketamine to reduce the LPS-induced increase in R0 (Fig. 4).
Ketamine and hypoxic pulmonary vasoconstriction
To further study whether ketamine modulates the hypoxic pulmonary vasoconstrictor response, ΔPAP was measured in lungs of LPS-treated and control mice. As shown in Fig. 1, PAP began to increase within 2 min of hypoxic ventilation (FiO2 = 0.01) in untreated control animals. This increase in PAP was associated with an increase in intrinsic vascular resistance R0 (Figs 3 and 4). At a dose of 10 mg kg−1 body weight min−1 ketamine, perfusion decreased the HPV response (Fig. 2 and Table 1). Pressure–flow curve analysis showed that this reduction in HPV responsiveness was due to an abolished decrease in vessel distensibility α, whereas the hypoxia-induced increase in R0 remained unaffected.
In contrast to untreated animals, no significant change was detected in PAP during hypoxic ventilation in LPS-treated mice, regardless of whether ketamine was added to the perfusate or not (Fig. 2 and Table 1). There was no significant effect of ketamine perfusion on hypoxia-induced changes in the pulmonary vascular pressure–flow relationship (Table 1).
Lung wet/dry weight ratios revealed no significant difference between any of the studied controls.
This study was designed to assess the effect of ketamine on resting pulmonary vascular tone and the pulmonary vasoconstrictor response to hypoxia (HPV) during Gram-negative endotoxaemia. The main results of our study are that there was no effect of ketamine on baseline pulmonary perfusion pressure (PAP) in control or LPS-pretreated mice at any tested dose; nevertheless, under normoxic conditions, ketamine increased vessel distensibility α and intrinsic vascular resistance R0 in control animals, whereas it abolished the LPS-induced increase in vessel distensibility α in the lungs of endotoxin-challenged mice; and ketamine at high doses reduces HPV responsiveness in control but not further in LPS-pretreated mice.
There are only limited data on the influence of ketamine on pulmonary vascular tone. In-vivo studies suggested an increase in pulmonary vascular resistance, but it remains unclear whether this reflects a direct effect of ketamine on pulmonary vascular tone [10,22]. Takahashi et al.  demonstrated that ketamine possesses direct pulmonary vasoconstrictor properties, whereas Nakayama and Murray , who measured vascular pressure–flow curves in dogs in vivo, did not find any effect of ketamine on pulmonary vascular tone. In-vitro ketamine caused a dose-dependent relaxation of pulmonary artery rings of rats with chronic hypoxic pulmonary hypertension  and pulmonary artery rings of KCl-precontracted rabbits . In both studies, higher concentrations of ketamine than peak plasma levels used in clinical settings caused vasodilation. In our study, analysis of pulmonary vascular pressure–flow curves revealed that ketamine increased vessel distensibility, but also intrinsic vascular resistance R0 in untreated control mice (Figs 3 and 4); effects that in their net effect balanced each other as suggested by an unchanged baseline perfusion pressure (Fig. 2). This is supported by the data obtained in dogs in vivo that demonstrated that ketamine does not affect pulmonary vascular pressure–flow curves in dogs when anaesthetized with 1 mg kg−1 body weight min−1, a dose within the range used in our study.
As shown before , we found LPS treatment to result in an increase in vessel distensibility α as well as in intrinsic pulmonary vascular resistance R0 (Figs 3 and 4). Of interest, ketamine counteracted both LPS-induced effects, thus restoring pulmonary vascular properties. However, ketamine did not restore HPV responsiveness in LPS-treated mice, as would have been expected if ketamine possessed vasoconstrictor properties thereby augmenting HPV. In contrast, we found no effect of ketamine on HPV responsiveness, possibly because we already barely detected any HPV response in lungs obtained from mice pretreated with LPS (Fig. 2). Analysis of pulmonary vascular pressure–flow curves further showed that ketamine did not affect the hypoxia-induced changes in α and R0 in septic mice as well as in untreated control animals (Figs 3 and 4). These data suggest that the reduced pulmonary vascular response to hypoxia (ΔPAP) in untreated mice is rather due to changes in baseline properties of the pulmonary vasculature than to direct modulation of the HPV response itself. One of the possible mechanisms for reducing HPV with high doses of ketamine is via L-type calcium channels, as Kaye et al. found a dose-dependent vasodilation by ketamine in the lungs of precontracted isolated rats. Another potential mechanism is the nitric oxide dependent vasorelaxation of the pulmonary vascular bed. As ketamine can decrease nitric oxide release of endothelial cells  and because we inhibited nitric oxide synthesis with L-NAME, this effect is unlikely to be responsible for decreased HPV.
Sepsis and its complications continue to represent a major cause of mortality in ICUs. Sedation of ventilated patients is an integral part of critical care practice. Patients with sepsis and respiratory failure may benefit from sedatives with minimal side effects, maintaining cardiovascular stability and supporting pulmonary function by, for example, minimizing ventilation–perfusion mismatch. Even though ketamine has been included in recent sedation guidelines in general for patients in ICUs in Germany , there are no recommendations on how to specifically sedate patients with sepsis or septic shock. The most commonly used agents are a combination of opioids and benzodiazepines [29,30]. Opioid use is associated with bowel dysfunction . Sedation of septic patients with ketamine can avoid exacerbating these side effects and is an alternative drug in case of opioid tolerance. Our data suggest that patients with septic shock may benefit from the use of ketamine as a sedative, as it has minimally depressive effects on the cardiocirculatory system, it is not associated with bowel dysfunction, and it does not increase PAP or suppress HPV in murine sepsis. In terms of restoration of HPV during sepsis, however, ketamine does not seem advantageous over other sedatives. The observation that ketamine does not increase PAP in isolated perfused mouse lungs agrees with the results from a study of pulmonary hypertensive children, who did not show an increase in pulmonary vascular resistance with ketamine use .
In summary, our data suggest that ketamine modulates basal pulmonary vascular wall properties, which, moreover, results in an impaired pulmonary vascular response to hypoxia in untreated control mice. In LPS-treated mice, we demonstrated that ketamine restored baseline pulmonary vascular properties, but it did not augment HPV responsiveness. Further studies are needed to conclusively determine whether ketamine impairs HPV during sepsis or rather modulates other pathways involved in regulating pulmonary vascular tone during sepsis.
1 Voelkel NF. Mechanisms of hypoxic pulmonary vasoconstriction
. Am Rev Respir Dis 1986; 133:1186–1195.
2 Ward JP, Aaronson PI. Mechanisms of hypoxic pulmonary vasoconstriction
: can anyone be right? Respir Physiol 1999; 115:261–271.
3 Archer SL, Wu XC, Thebaud B, et al
. Preferential expression and function of voltage-gated, O2-sensitive K+ channels in resistance pulmonary arteries explains regional heterogeneity in hypoxic pulmonary vasoconstriction
: ionic diversity in smooth muscle cells. Circ Res 2004; 95:308–318.
4 Bonnet S, Archer SL. Potassium channel diversity in the pulmonary arteries and pulmonary veins: implications for regulation of the pulmonary vasculature in health and during pulmonary hypertension. Pharmacol Ther 2007; 115:56–69.
5 Chang SW, Feddersen CO, Henson PM, Voelkel NF. Platelet-activating factor mediates hemodynamic changes and lung injury in endotoxin-treated rats. J Clin Invest 1987; 79:1498–1509.
6 Theissen JL, Loick HM, Curry BB, et al
. Time course of hypoxic pulmonary vasoconstriction
after endotoxin infusion in unanesthetized sheep. J Appl Physiol 1991; 70:2120–2125.
7 Spöhr F, Cornelissen AJ, Busch C, et al
. Role of endogenous nitric oxide in endotoxin-induced alteration of hypoxic pulmonary vasoconstriction
in mice. Am J Physiol Heart Circ Physiol 2005; 289:H823–H831.
8 Dantzker DR, Brook CJ, Dehart P, et al
. Ventilation–perfusion distributions in the adult respiratory distress syndrome. Am Rev Respir Dis 1979; 120:1039–1052.
9 Marshall BE, Marshall C, Frasch F, Hanson CW. Role of hypoxic pulmonary vasoconstriction
in pulmonary gas exchange and blood flow distribution. 1. Physiologic concepts. Intensive Care Med 1994; 20:291–297.
10 Bjertnaes LJ. Hypoxia-induced vasoconstriction in isolated perfused lungs exposed to injectable or inhalation anesthetics. Acta Anaesthesiol Scand 1977; 21:133–147.
11 Lennon PF, Murray PA. Attenuated hypoxic pulmonary vasoconstriction
during isoflurane anesthesia is abolished by cyclooxygenase inhibition in chronically instrumented dogs. Anesthesiology 1996; 84:404–414.
12 Beck DH, Doepfmer UR, Sinemus C, et al
. Effects of sevoflurane and propofol on pulmonary shunt fraction during one-lung ventilation for thoracic surgery. Br J Anaesth 2001; 86:38–43.
13 Martin J, Franck M, Fischer M, Spies C. Sedation and analgesia in German intensive care units: how is it done in reality? Results of a patient-based survey of analgesia and sedation. Intensive Care Med 2006; 32:1137–1142.
14 Liu HT, Hollmann MW, Liu WH, et al
. Modulation of NMDA receptor function by ketamine
and magnesium. Part I. Anesth Analg 2001; 92:1173–1181.
15 Hirota K, Okawa H, Appadu BL, et al
. Stereoselective interaction of ketamine
with recombinant mu, kappa, and delta opioid receptors expressed in Chinese hamster ovary cells. Anesthesiology 1999; 90:174–182.
16 White PF, Way WL, Trevor AJ. Ketamine
: its pharmacology and therapeutic uses. Anesthesiology 1982; 56:119–136.
17 Ogawa K, Tanaka S, Murray PA. Inhibitory effects of etomidate and ketamine
on endothelium-dependent relaxation in canine pulmonary artery. Anesthesiology 2001; 94:668–677.
18 Lee TS, Hou X. Vasoactive effects of ketamine
on isolated rabbit pulmonary arteries. Chest 1995; 107:1152–1155.
19 Gassner S, Cohen M, Aygen M, et al
. The effect of ketamine
on pulmonary artery pressure. An experimental and clinical study. Anaesthesia 1974; 29:141–146.
20 Linehan JH, Haworth ST, Nelin LD, et al
. A simple distensible vessel model for interpreting pulmonary vascular pressure–flow curves. J Appl Physiol 1992; 73:987–994.
21 Weimann J, Bloch KD, Takata M, et al
. Congenital NOS2 deficiency protects mice from LPS-induced hyporesponsiveness to inhaled nitric oxide. Anesthesiology 1999; 91:1744–1753.
22 Tweed WA, Minuck M, Mymin D. Circulatory responses to ketamine
anesthesia. Anesthesiology 1972; 37:613–619.
23 Takahashi K, Shima T, Koga Y, Iwatsuki K. Effect of ketamine
hydrochloride (Ketalar) on the pulmonary hemodynamics. Masui 1971; 20:842–846.
24 Nakayama M, Murray PA. Ketamine
preserves and propofol potentiates hypoxic pulmonary vasoconstriction
compared with the conscious state in chronically instrumented dogs. Anesthesiology 1999; 91:760–771.
25 Maruyama K, Maruyama J, Yokochi A, et al
. Vasodilatory effects of ketamine
on pulmonary arteries in rats with chronic hypoxic pulmonary hypertension. Anesth Analg 1995; 80:786–792.
26 Kaye AD, Banister RE, Anwar M, et al
. Pulmonary vasodilation by ketamine
is mediated in part by L-type calcium channels. Anesth Analg 1998; 87:956–962.
27 Chen RM, Chen TL, Lin YL, et al
reduces nitric oxide biosynthesis in human umbilical vein endothelial cells by down-regulating endothelial nitric oxide synthase expression and intracellular calcium levels. Crit Care Med 2005; 33:1044–1049.
28 Martin J, Bäsell K, Bürkle H, et al
. Analgesia and sedation in intensive care medicine. Anästh Intensivmed 2005; 46:S1–S20.
29 Ostermann ME, Keenan SP, Seiferling RA, Sibbald WJ. Sedation in the intensive care unit: a systematic review. JAMA 2000; 283:1451–1459.
30 Payen JF, Chanques G, Mantz J, et al
. Current practices in sedation and analgesia for mechanically ventilated critically ill patients: a prospective multicenter patient-based study. Anesthesiology 2007; 106:687–695.
31 Kurz A, Sessler DI. Opioid-induced bowel dysfunction: pathophysiology and potential new therapies. Drugs 2003; 63:649–671.
32 Williams GD, Philip BM, Chu LF, et al
does not increase pulmonary vascular resistance in children with pulmonary hypertension undergoing sevoflurane anesthesia and spontaneous ventilation. Anesth Analg 2007; 105:1578–1584.