Our meta-analysis revealed that the use of methylene blue in patients with septic shock was associated with significantly greater mean arterial pressure and PaO2/FiO2 ratio and reduced mortality, serum lactate and length of hospital stay.
A previous systematic review completed by Kwok et al included 2 randomized controlled trials and 11 case reports/series [28]. Another systematic review by Paciullo et al included 2 randomized controlled trials and 8 prospective observational studies [29]. However, the inclusion of non-RCTs would have downgraded the quality of evidence for the measured outcomes [30]. Previously, no quantitative meta-analysis of the data could be performed due to the limited number of RCTs focused on patients with septic shock. Given that more RCTs have been published, our team decided to perform an updated and comprehensive review and meta-analysis on the use of methylene blue in the adult population with septic shock.
Prompt treatment of hypotension is imperative in septic shock [31]. Observational studies of methylene blue in septic shock patients have generally shown short-term improvements in hemodynamic parameters [9, 12, 28, 32]. It is believed that methylene blue increases blood pressure via the inhibition of iNOS vasodilation instead of vasoconstriction in catecholamines [25]. Memis et al. reported that methylene blue significantly increased the mean arterial pressure in patients with severe sepsis [33]. Similarly, our pooled analysis revealed a significant increase in the mean arterial pressure in the methylene blue group compared to that in the control group. However, the increase in MAP after 24 hours was not statistically significant. This suggests that the hemodynamic effects of methylene blue may be short-lived and transient. While methylene blue has long been established as a treatment for methemoglobinemia at dosages of 1–2 mg/kg [34], the optimal dosing strategy for methylene blue in septic patients is still not well defined. A study on seven healthy individuals in which 100 mg of IV methylene blue was administered demonstrated that the estimated terminal half-life is 5.25 hours [35]. Three dosing strategies were identified in our study: single bolus, continuous infusion, and bolus dose followed by infusion. Doses based on weight ranged from 0.5 mg/kg/h to 2.0 mg/kg/h. The most common regimen, which was used in four of the five included studies, was continuous infusion [13, 14, 26, 27]. Ibarra-Estrada et al. was the only study in which methylene blue was administered for greater than 24 hours in repeated doses—once daily for a total of 3 days [25]. Juffermans et al demonstrated that the hemodynamic effects are dose dependent and that an infusion of 1–3 mg/kg is sufficient to increase MAP [36]. This study also suggested that the effects of a single infusion are transient, lasting an estimated 2 hours [36]. This finding is in keeping with our study, which demonstrated a statistically significant increase in MAP immediately after methylene blue infusion, while no statistical significance was achieved at 24 hours after infusion. Therefore, it may be useful to consider the use of repeated boluses or continuous infusions to overcome this limitation.
In the setting of sepsis, inflammatory cytokines have been theorized to suppress the cortisol response or compete with intracellular glucocorticoid receptors, resulting in peripheral tissue resistance and eventual vascular collapse [37, 38]. Low-dose corticosteroid therapy has been utilized in septic shock patients to enhance the vascular contractile response to pressor agents [39]. The utility of steroids was not explicitly described in any of the included studies, except for that of Ibarra-Estrada et al. [25]. This study was the only RCT that administered intravenous hydrocortisone at 200 mg/day by continuous infusion as part of the resuscitation protocol [25]. However, the combined effects of methylene blue and corticosteroids have yet to be studied. Therefore, the confounding effect of corticosteroids in this study cannot be adjusted at this review level.
Quantitative analysis of the three studies [14,25,26] reporting mortality demonstrated a statistically significant reduction in the overall mortality rate among patients receiving methylene blue. Similarly, the current evidence by Huang et al. and Pruna et al. showed that the use of methylene blue was also associated with a significant reduction in the mortality rate among patients with distributive shock and critically ill and perioperative patients, respectively [39, 40]. However, in this review, the total sample size (n = 259) was very small; hence, it was not possible to detect any clinically meaningful outcomes for the mortality rate. Serum lactate has been included in the recent definition of septic shock because it is considered an important biomarker of cellular stress in refractory hypotension [1]. Current guidelines recommend using lactate levels as biochemical indicators to guide resuscitation in the early phases of septic shock [31]. Our findings demonstrated that methylene blue was associated with a significantly lower level of serum lactate [14,25,26]. This indicates that methylene blue may have favorable effects on tissue perfusion and anaerobic metabolism [41, 42]. Therefore, the interpretation of lactate levels should be performed with caution, as should other hemodynamic parameters.
The time factor remains a critical element in the management of septic shock, and early intervention is crucial for improving outcomes [31]. In our included studies, the timing of methylene blue administration and vasopressor titration varied across the included studies. The majority of studies administered methylene blue within 24 hours after the initiation of vasopressors [13, 14, 26, 27]. It is difficult to determine whether there are possible delays between the onset of septic shock and the administration of methylene blue. It has been proposed that there is a window of opportunity and time sensitivity for methylene blue to effectively restore vascular resistance in septic shock [43]. In a mouse sepsis model, dynamic guanylate cyclase activity was detected in three 8-hour windows [43]. An increase in guanylate cyclase and iNOS is observed in the first 8-hour window, followed by an absence of guanylate cyclase expression and downregulation of iNOS in the second 8-hour window [44]. Then, there is an upregulation of guanylate cyclase and iNOS in the third 8-hour window [44]. This suggests that the use of methylene blue as a last rescue therapeutic option may not act (second window) or may act too late (third window) when the shock is irreversible with profound tissue hypoxia and intractable metabolic acidosis [45]. In the current literature, methylene blue has been administered as a late rescue treatment during septic shock, but the impact of earlier administration of methylene blue as an adjunct has yet to be established.
There are some concerns that the use of methylene blue causes ventilation-perfusion mismatch and vasoconstriction via the inhibition of cGMP [46]. A significant reduction in PaO2/FiO2 was demonstrated in previous studies [12, 33] in the methylene blue group. Weingartner and colleagues demonstrated a reduced PaO2/FiO2 ratio from 168 (131–215) to 132 (109–156) mmHg after the use of methylene blue (P < 0.05) [32]. However, it is worth noting that all the included patients in this study had acute lung injury, and seven of the ten included patients fulfilled the criteria for ARDS [32]. Hence, the observed respiratory dysfunction may be attributed to septic shock secondary to respiratory pathology rather than the adverse effect of methylene blue. Our pooled results support this hypothesis by showing that methylene blue increases PaO2/FiO2 compared to that in the control group [13,25]. Methylene blue is a safe drug when it is used at therapeutic doses < 2 mg/kg [35]. All our included studies used a dose of methylene blue < 2 mg/kg, and no significant adverse events were reported. There was an increase in the level of methemoglobin in one of the included studies [13], but this increase was not clinically significant. Other common side effects include blue skin and urine discoloration, which are transient and benign [35]. Clinicians should also be aware of the interference in pulse oximeter readings caused by methylene blue, as it absorbs most of the light emitted by the pulse oximeter [46]. This may be interpreted as a false reduction in circulating oxyhemoglobin and reduced oxygen saturation [47]. Furthermore, a study by Juffermans et al demonstrated that high doses (> 7 mg/kg) of methylene blue may compromise splanchnic perfusion despite global hemodynamic enhancement [37]. The use of methylene blue is contraindicated in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency because it increases the risk of hemolytic anemia [48]. Moreover, methylene blue also inhibits the monoamine oxidase enzyme, which may result in the manifestation of serotonin syndrome when it is used with monoamine oxidase inhibitors, such as antidepressant medications [49].
Markers to predict specific vasopressor responsiveness may be the ideal approach for managing hypotension in septic shock patients. Physiologically, diastolic arterial pressure and dynamic arterial elastance may be useful parameters to guide patients who will benefit from early vasopressors [50, 51]. Analysis of the kinetics of the norepinephrine dose increase and the response to vasopressin treatment may suggest the need for early initiation of other vasopressors with different mechanisms of action [52]. Catecholamine-resistant shock patients with high renin levels may benefit from early commencement of angiotensin II [53]. Measurement of the metabolites of the nitric oxide pathway in the form of nitrite and nitrate may suggest early initiation of methylene blue.
Our limitations include variations in the regimen of methylene blue administration and the protocol of vasopressor titration. All the included studies had small sample sizes and were not powered to detect the primary outcome of mean arterial pressure. The durations of follow-up periods were short and heterogeneous in the majority of RCTs, resulting in the risk of underreporting adverse outcomes. At the current review level, the causes of septic shock were not reported in any of the included studies, which can introduce bias to our findings. Transient urine discoloration, which is commonly observed in patients receiving methylene blue, could unintentionally unblind clinicians, potentially causing bias. None of the studies measured the levels of nitrite or nitrate to indicate that the effect on MAP could be attributed to methylene blue.
In this meta-analysis, our pooled findings demonstrated the promising effect of methylene blue in improving the mean arterial pressure, mortality rate, serum lactate level, length of hospital stay, and PaO2/FiO2 ratio. However, the sample size and low quality of evidence warrant future adequately powered RCTs to examine the effects of the optimal dose and timing of methylene blue treatment on the hemodynamic and clinical outcomes of patients with septic shock.