What is the role of lactate in arterial blood gases (ABG), venous blood gases (VBG), and mixed venous oxygen saturation in the Cardiovascular Intensive Care Unit (CVICU)?

Role of Lactate in ABG, VBG, and Mixed Venous Measurements in CVICU

Lactate is a critical marker of tissue hypoperfusion in the CVICU setting, serving as an objective surrogate for tissue perfusion that guides resuscitation and predicts mortality across various shock states.

Lactate as a Marker of Tissue Hypoperfusion

• Elevated lactate levels correlate with increased mortality in various types of shock and can be utilized as a marker of the adequacy of hemodynamic support 1
• Lactate-guided resuscitation has been consistently shown to be effective in improving outcomes, with a significant reduction in mortality compared to resuscitation without lactate monitoring 1
• Current guidelines recommend targeting resuscitation to normalize lactate in patients with elevated lactate levels as a marker of tissue hypoperfusion 1

Arterial Blood Gas (ABG) Lactate

• Arterial lactate is considered the gold standard measurement for assessing tissue hypoperfusion in critically ill patients 1
• In sepsis-induced tissue hypoperfusion, defined as hypotension persisting after initial fluid challenge or blood lactate concentration ≥4 mmol/L, arterial lactate is a key diagnostic criterion 1
• Arterial lactate levels should be normalized as rapidly as possible during resuscitation, with lactate clearance being an important goal of therapy 1

Venous Blood Gas (VBG) Lactate

• Venous lactate measurements show excellent agreement with arterial lactate values, with 96% of venous lactate measurements falling within clinically acceptable limits of agreement 2
• This agreement remains clinically acceptable even in critically ill patients with varying degrees of hypotension 3
• VBG lactate can be used as an acceptable substitute for arterial lactate, potentially eliminating the need for additional arterial sampling and its associated risks 2, 3

Mixed Venous Oxygen Saturation and Lactate

• Mixed venous oxygen saturation (SvO2) or central venous oxygen saturation (ScvO2) are used alongside lactate to assess tissue oxygenation 1
• During initial resuscitation of sepsis-induced hypoperfusion, targets include ScvO2 ≥70% or SvO2 ≥65%, alongside efforts to normalize lactate 1
• However, lactate and ScvO2 correlate poorly in most critically ill patients (r² = 0.0041), suggesting they are not interchangeable markers of tissue oxygenation 4
• Lactate can predict ScvO2 only when patients are at or below the critical oxygen delivery threshold, which represents only a small percentage of shock patients 4

Clinical Application in CVICU

• Lactate should be measured serially alongside other markers of systemic and organ perfusion, such as mixed or central venous oxygen saturations, urine output, skin perfusion, and other hemodynamic variables 1
• The gradient between central venous and mixed venous blood oxygen saturation (ΔSO2) and lactate concentration (Δ[Lac]) has prognostic value - positive gradients are associated with improved survival in critically ill patients 5
• Monitoring lactate clearance is particularly important, as all patients in whom lactate levels return to normal range (≤2 mmol/L) within 24 hours have shown improved survival 1

Pitfalls and Caveats

• Elevated lactate is not solely due to poor tissue perfusion - it can also result from accelerated aerobic glycolysis driven by excess beta-adrenergic stimulation or other causes (e.g., liver failure) 1
• In alcohol-associated trauma, base deficit may be a better predictor of prognosis than lactate, as alcohol itself can increase blood lactate levels 1
• When using lactate as a marker of shock, it should be interpreted alongside other clinical parameters rather than in isolation 1
• There is no consistent relationship between oxygen delivery (DO2) and lactate levels in sepsis patients, likely reflecting distributive flow abnormalities and differences in metabolic requirements 6