Critical care physiology: concepts and unit conventions

Educational work by a pre-medical student — not medical advice. Not a clinical protocol, prescription, or patient handout, and not for patient care. See DISCLAIMER.

Flow and oxygen transport

Cardiac output equals heart rate multiplied by stroke volume. Stroke volume, ejection fraction, LVOT velocity-time integral, and perfusion markers are influenced by loading conditions; none is an isolated measure of intrinsic contractility. A normal ejection fraction does not exclude shock.

With hemoglobin in g/dL, arterial saturation as a fraction from 0 to 1, and PaO2 in mmHg:

CaO2 (mL O2/dL) = 1.34 × Hb × SaO2 + 0.003 × PaO2.

With CO in L/min, DO2 (mL O2/min) = CO × CaO2 × 10. The factor of 10 converts litres to decilitres. The systemic oxygen extraction ratio is VO2/DO2; extraction can rise as delivery falls before consumption becomes delivery-dependent. There is no universal ScvO2 threshold that defines this transition. ScvO2 and mixed venous SvO2 are not interchangeable and can be normal or high despite tissue hypoperfusion.

Pressure and preload

If SVR is in mmHg·min/L, MAP − RAP ≈ CO × SVR. If SVR is in dyn·s/cm5, SVR = 80 × (MAP − RAP)/CO. Omitting right atrial pressure is an approximation. Blood pressure alone neither establishes nor excludes adequate perfusion.

CVP, wedge pressure, and IVC appearance are context-dependent observations, not stand-alone tests of volume status or fluid responsiveness. Fluid responsiveness also does not establish that fluid is indicated or tolerable.

Heart–lung interaction

PEEP may recruit collapsed lung or overdistend aerated lung. Recruitment can reduce hypoxic pulmonary vasoconstriction; overdistension can raise pulmonary vascular resistance, and positive pressure can reduce venous return. Its net effect on RV function is context-dependent. “Always lower PEEP in RV failure” is not a valid general rule.

Plateau and driving pressure interpretation requires attention to patient effort, chest-wall mechanics, total PEEP, and the measurement conditions. A driving-pressure association (Amato et al., N Engl J Med 2015;372:747-755; higher risk commonly cited above roughly 15 cm H2O) does not establish a universal independent treatment target.

Renal, acid–base, and neurologic concepts

AKI in critical illness can reflect several overlapping mechanisms, including altered perfusion, venous congestion, inflammation, nephrotoxins, and obstruction; it should not be presumed prerenal. Urine output is one contextual measure, not a stand-alone explanation.

Winter’s formula estimates expected respiratory compensation in metabolic acidosis: PaCO2 ≈ 1.5 × bicarbonate + 8 ± 2 (PaCO2 in mmHg; bicarbonate in mmol/L). A discrepancy prompts consideration of a mixed disorder; the formula does not prescribe ventilation.

CPP ≈ MAP − ICP is a simplified relationship. A target derived from traumatic brain injury should not be generalized to every neurologic condition.

Sources and applicability

SCCM/ESICM 2026 adult sepsis guideline (Prescott et al.; co-published in Intensive Care Med, DOI, and Crit Care Med, DOI): the current guideline supersedes the 2021 edition. For ongoing fluid decisions, dynamic responsiveness assessment is preferable to isolated filling pressures.
ATS 2024 ARDS guideline: conditionally favors higher PEEP without recruitment maneuvers in moderate-to-severe ARDS and recommends against prolonged recruitment maneuvers.
ATS/ESICM/SCCM 2017 ventilation guideline: lower tidal volumes use predicted body weight, not actual body weight. Its PEEP, recruitment-maneuver and ECMO questions were re-addressed and are superseded by the ATS 2024 update; the low-tidal-volume/predicted-body-weight and prone-positioning recommendations remain in use.
ESICM 2023 ARDS guideline (Grasselli et al., Intensive Care Med 2023;49:727-759): a European comparator whose recommendations on PEEP and other adjuncts differ in places from ATS 2024.
ESICM 2025 circulatory shock and hemodynamic monitoring guideline (Monnet et al., Intensive Care Med 2025;51:1971-2012): current society guidance on shock assessment, including dynamic over static preload markers, echocardiography as first-line imaging for shock type, and cardiac-output monitoring in patients not responding to initial therapy. It updates the 2014 ESICM consensus.

These summaries are educational and do not reproduce the full guideline, its exceptions, or its implementation requirements.