Blood Flow Restriction Training for Mechanically Ventilated Patients With ICU-Acquired Weakness

Intensive care unit-acquired weakness (ICUAW) is a prevalent neuromuscular complication in critically ill patients undergoing invasive mechanical ventilation, with reported incidence ranging from 25% to 60% in high-risk populations. Its pathophysiology is multifactorial, involving immobilization-induced muscle protein turnover imbalance, systemic inflammatory response-mediated myofiber injury, neuromuscular junction dysfunction, and metabolic disturbances. ICUAW not only prolongs mechanical ventilation and ICU length of stay but also increases nosocomial complication risk and leads to persistent long-term functional impairment, imposing substantial burden on patients and healthcare systems.

Current standard rehabilitation for ICUAW relies mainly on passive range-of-motion exercises, bed mobility training, and low-intensity assisted active movements. However, due to limited cardiopulmonary reserve, reduced baseline muscle strength, and hemodynamic instability in critically ill patients, conventional rehabilitation often fails to deliver sufficient mechanical stimulus to counteract rapid muscle atrophy, resulting in only modest clinical benefits. There remains an unmet need for safe, low-load rehabilitation modalities that can effectively mitigate muscle wasting in this vulnerable population.

Blood flow restriction training (BFRT) is an evidence-based rehabilitation modality that applies an inflatable cuff to the proximal limb to partially occlude venous return while preserving arterial inflow, paired with low-intensity resistance exercise. Prior studies in orthopedic, chronic disease, and healthy populations have consistently shown that BFRT produces muscle hypertrophy and strength gains comparable to high-load resistance training, despite using only 20-30% of maximal voluntary contraction intensity. Its mechanisms include local hypoxic stress, metabolic product accumulation, activation of the mTOR anabolic pathway, inhibition of ubiquitin-proteasome catabolic signaling, and increased secretion of myokines with systemic anti-inflammatory effects.

Limb occlusion pressure (LOP) - the minimum cuff pressure required to fully occlude distal arterial blood flow - is the gold standard for individualizing BFRT pressure prescription. Prescribing pressure as a percentage of individual LOP minimizes variability caused by differences in limb circumference, vascular tone, and body composition, which is particularly critical for the heterogeneous critically ill population. Available evidence suggests pressures below 30% LOP provide insufficient hypoxic stimulus to activate muscle anabolic pathways, while pressures above 70% LOP carry elevated risk of arterial occlusion and adverse events. The 40% LOP and 60% LOP levels selected for this trial represent low and moderate-low pressure ranges, hypothesized to match the poor physical tolerance of ICUAW patients while delivering graded therapeutic effects.

To date, most BFRT research has been conducted in outpatient or post-surgical populations. High-quality randomized controlled trials evaluating BFRT efficacy and safety in mechanically ventilated patients with ICUAW remain scarce. Furthermore, few studies have simultaneously examined effects on muscle morphology, systemic inflammation, and hard clinical endpoints, leaving the optimal pressure parameter and comprehensive clinical value of BFRT in critical care unclear.

This is a single-center, prospective, three-arm parallel-group randomized controlled trial conducted in a tertiary teaching hospital intensive care unit. The protocol has been approved by the hospital ethics committee. A total of 120 eligible mechanically ventilated adults with ICUAW will be enrolled and randomly assigned 1:1:1 to a usual care control group, a 40% LOP BFRT group, and a 60% LOP BFRT group. All participants will receive standard ICU medical management and routine rehabilitation for 2 weeks. The two intervention groups will receive additional daily BFRT sessions on bilateral lower extremities at their respective pressure levels.

Randomization will use a computer-generated random number sequence, with allocation concealment maintained via sequentially numbered, sealed opaque envelopes. Outcome assessors and ultrasound technicians will be blinded to group assignment to reduce measurement bias. Analyses will be performed on both intention-to-treat and per-protocol populations, with multiple imputation used to handle missing data.

The BFRT protocol follows standardized operational procedures. Prior to the first session, each patient's LOP will be measured in supine resting position using an automated pneumatic cuff system with built-in pressure sensors. Each session includes three lower-extremity exercises - straight leg raise, isometric quadriceps contraction, and glute bridge - performed for 3 sets of 10 repetitions, with 30-second rest between sets and 3-minute rest between limbs. Training intensity is monitored via the Borg Rating of Perceived Exertion scale, targeted at 9-11 to maintain low exertion. BFRT sessions are scheduled at least 6 hours apart from routine rehabilitation to avoid cumulative fatigue.

Comprehensive safety monitoring is implemented throughout the intervention. Continuous bedside cardiac monitoring tracks heart rate, invasive blood pressure, respiratory rate, and peripheral oxygen saturation. Predefined stopping criteria for hemodynamic instability, respiratory compromise, and limb intolerance are applied to ensure patient safety. All adverse events are documented and reviewed by an independent safety monitoring team.

Outcome assessments occur at baseline and after 2 weeks of intervention. The primary outcome is change in muscle strength measured by the Medical Research Council sum score. Secondary outcomes include changes in lower limb muscle architecture (rectus femoris and medial gastrocnemius cross-sectional area and thickness) via standardized musculoskeletal ultrasound, changes in serum pro-inflammatory cytokines (TNF-α, IL-6, IL-8) measured by enzyme-linked immunosorbent assay, and clinical prognosis indicators including mechanical ventilation duration, ICU length of stay, and ventilator weaning success rate.

This study aims to provide high-level clinical evidence on the efficacy, optimal pressure parameter, and safety profile of BFRT in mechanically ventilated ICUAW patients. It is hypothesized that BFRT will attenuate muscle atrophy, improve strength, reduce systemic inflammation, and accelerate clinical recovery in a pressure-dependent manner without increasing adverse events. Findings are expected to inform clinical practice and establish standardized BFRT protocols for early critical care rehabilitation.

Inclusion criteria

• Age ≥ 18 years;
• Received invasive mechanical ventilation for ≥ 48 hours;
• Meets the clinical diagnosis criteria for ICUAW: The patient has reached a level of consciousness that allows for reliable assessment of voluntary muscle strength, is able to execute standardized motor commands, has an MRC-SS < 48 points, and has no other clearly identifiable cause that explains the muscle weakness;
• RASS score of -1 to 0, and is able to cooperate with the prescribed muscle strength assessment and rehabilitation training;
• Current vital signs and respiratory and circulatory status are stable, meeting the safety conditions for rehabilitation training, as assessed by clinical physicians and rehabilitation therapists;
• The patient or their legal representative has signed the informed consent form, voluntarily participating in the study.

Exclusion criteria

• The presence of pre-existing central or peripheral nervous system diseases, neuromuscular diseases, or severe motor function disorders that significantly affect limb strength or motor function prior to ICU admission;
• The presence of active deep vein thrombosis, pulmonary embolism, significant peripheral arterial disease, limb ischemia, or other severe lower limb vascular diseases;
• Severe coagulation function abnormalities, active bleeding, or a clinically judged unacceptable risk of thrombosis/hemorrhage;
• The presence of fractures, open wounds, local infections, significant skin damage, or other conditions in the target lower limb that are unsuitable for cuff compression and exercise training;
• Recent acute coronary syndrome, decompensated heart failure, uncontrolled severe arrhythmias, or other active cardiovascular diseases that are unsuitable for BFRT;
• Instability in respiratory or circulatory status at the time of screening, or the need for new or significantly increased use of vasoactive drugs, failing to meet the safety criteria for rehabilitation training; such individuals may be rescreened after their condition stabilizes;
• Anticipation of imminent death, or a decision to limit or withdraw life support treatment;
• Allergy to the cuff material, or the circumference of the target limb is not compatible with the cuff being used;
• Inability to reliably complete specified assessments such as MRC-SS after adequate arousal.