Haemodynamic monitoring

CO2 gap, lactate and perfusion targets

Reviewed Nov 2026·6 references
New· views rounded down, updated weekly

Use lactate, capillary refill time and the venous-arterial CO2 gap together to judge tissue perfusion and guide resuscitation.

Definition
Lactate > 2 mmol/L = hyperlactataemia (septic shock definition includes lactate > 2 despite adequate fluids). CO2 gap (Pcv-aCO2) = central venous PCO2 minus arterial PCO2; > 6 mmHg suggests inadequate cardiac output for metabolic demand. CRT > 3 s suggests poor peripheral perfusion.

At the bedside

  1. 1

    Measure lactate early and repeat

    Measure within 1 h of suspected sepsis/shock; repeat every 2-4 h if raised. SSC 2026 suggests serial lactate to guide resuscitation, but individualise fluid after the first bolus rather than chase a normal value.

  2. 2

    Check capillary refill time

    Press the index finger pulp with a glass slide for 10 s, release, time to return of colour: normal ≤ 3 s. Repeat every 30 min during resuscitation.

  3. 3

    Calculate CO2 gap

    Paired central venous (SVC) and arterial gas drawn together: Pcv-aCO2 > 6 mmHg = low flow relative to CO2 production → look for low CO (fluids if responsive, inotrope).

  4. 4

    Combine with ScvO2

    Low ScvO2 + high CO2 gap: low DO2/flow. Normal/high ScvO2 + high CO2 gap: flow still inadequate (microcirculatory). Normal CO2 gap + high lactate: consider non-hypoxic causes.

  5. 5

    Consider non-perfusion causes of high lactate

    Adrenaline (beta-2), salbutamol, metformin, liver failure, seizures, thiamine deficiency, malignancy, mesenteric ischaemia, cyanide, methanol, propofol infusion syndrome, linezolid.

  6. 6

    Avoid over-resuscitation

    Lactate clears slowly (hours); do not give repeated fluid boluses solely to 'normalise lactate' if the patient is not fluid responsive. CRT normalises faster.

Causes of hyperlactataemia

Type A (hypoperfusion/hypoxia)Type B (other)
Shock of any typeAdrenaline/salbutamol (beta-2 agonists)
Severe hypoxaemiaLiver failure (reduced clearance)
Severe anaemiaMetformin, linezolid, propofol infusion syndrome, NRTIs
Regional ischaemia (mesenteric, limb)Thiamine deficiency (beri-beri, alcohol)
CO / cyanide poisoningSeizures, malignancy, toxic alcohols, DKA (D-lactate rarely)
Red flag · call your senior now
Lactate > 4 mmol/L, or rising despite resuscitation, carries high mortality: senior review, look for uncontrolled source or ischaemic bowel.
Red flag
Disproportionate lactate with mild abdominal tenderness: suspect mesenteric ischaemia.
Pitfall
Treating a lactate of 4 from an adrenaline infusion or salbutamol nebs as worsening shock.
Pitfall
Venous lactate is usually close to arterial; a normal venous lactate excludes hyperlactataemia, but tourniquet time can raise it.
Pearl
ANDROMEDA-SHOCK (2019): CRT-targeted resuscitation was at least as good as lactate-targeted, with less organ dysfunction at 72 h; ANDROMEDA-SHOCK-2 (2025) tested a personalised CRT-targeted protocol.

References

  1. 1

    Mallat J, Lemyze M, Tronchon L, Vallet B, Thevenin D. Use of venous-to-arterial carbon dioxide tension difference to guide resuscitation therapy in septic shock. World J Crit Care Med 2016;5:47-56.

  2. 2

    Hernandez G, Ospina-Tascon GA, Damiani LP, et al. Effect of a resuscitation strategy targeting peripheral perfusion status vs serum lactate levels on 28-day mortality among patients with septic shock: the ANDROMEDA-SHOCK randomized clinical trial. JAMA 2019;321:654-64.

  3. 3

    ANDROMEDA-SHOCK-2 Investigators. Personalized hemodynamic resuscitation targeting capillary refill time in early septic shock: the ANDROMEDA-SHOCK-2 randomized clinical trial. JAMA 2025.

  4. 4

    Prescott HC, Antonelli M, Alhazzani W, et al. Surviving Sepsis Campaign: international guidelines for management of sepsis and septic shock 2026. Intensive Care Med. 2026;52(5):863-936. Co-published in Crit Care Med. 2026 (doi:10.1097/CCM.0000000000007075)

  5. 5

    Singer M, Deutschman CS, Seymour CW, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA 2016;315:801-10.

  6. 6

    Farkas J. The Internet Book of Critical Care (IBCC). EMCrit Project; continuously updated.

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