1.1 Intra-Abdominal Hypertension and Abdominal Compartment Syndrome The abdominal compartment is a closed anatomical space bounded by the abdominal wall, diaphragm, and pelvic floor. Under normal physiological conditions, intra-abdominal pressure (IAP) ranges between 0 and 5 mmHg, rising transiently during coughing, straining, or Valsalva manoeuvre. In critically ill surgical patients — those admitted following emergency abdominal surgery, severe acute pancreatitis, major trauma, bowel obstruction, peritonitis, or massive fluid resuscitation — IAP rises pathologically, threatening perfusion to every organ within and beyond the abdominal compartment. The World Society of Abdominal Compartment Syndrome (WSACS) defines Intra-Abdominal Hypertension (IAH) as a sustained IAP ≥ 12 mmHg, and Abdominal Compartment Syndrome (ACS) as a sustained IAP > 20 mmHg with new onset organ dysfunction. IAH is graded into four levels: Grade IAP (mmHg) Clinical Significance Grade I 12 – 15 mmHg Mild elevation — initiates renal and hepatic microvascular compromise Grade II 16 – 20 mmHg Moderate — significant cardiac output reduction; renal impairment worsens Grade III 21 – 25 mmHg Severe — oliguria, respiratory compromise; early ACS territory Grade IV > 25 mmHg Critical — full ACS; anuria, respiratory failure; high mortality IAH is far more prevalent than ACS: in a prospective 503-patient high-risk ICU cohort, 33% of patients developed IAH while only 3.6% developed true ACS, with pancreatitis patients carrying the highest ACS burden at 57%. The critical take-away is that IAH begins damaging organs well before the IAP threshold for ACS is reached — making early detection and serial monitoring of paramount importance in the surgical ICU. 1.2 The Organ Cascade of Raised IAP — Including the Eye The pathophysiological cascade of raised IAP is multisystemic. As IAP rises, it compresses the inferior vena cava and portal vein, reducing cardiac preload and output. It splints the diaphragm upward, impairing ventilation. It reduces renal cortical perfusion, causing acute kidney injury. Critically for this study, raised IAP transmits pressure cranially via the inferior vena cava → right heart → superior vena cava → jugular venous system. This raises central venous pressure (CVP), which in turn: • Increases episcleral venous pressure → impedes aqueous humour outflow → raises Intraocular Pressure (IOP) • Raises intracranial pressure (ICP) via reduced cerebral venous drainage → CSF is transmitted to the perioptic subarachnoid space → expands Optic Nerve Sheath Diameter (ONSD) • CO₂ retention from impaired ventilation (hypercapnia) → cerebral vasodilation → further ICP elevation This physiology has been elegantly demonstrated in the laparoscopic surgery literature: acute IAP elevation to 12–15 mmHg during CO₂ pneumoperitoneum significantly increases both IOP and ONSD in a pressure-proportional manner. A 2015 study in Surgical Endoscopy confirmed that acute elevation of IAP during laparoscopy significantly increased ONSD, reflecting a temporary and reversible rise in ICP proportional to the magnitude of IAP elevation. However, laparoscopic pneumoperitoneum lasts only 45–90 minutes. In the surgical ICU, pathological IAP elevation may persist for hours to days — creating a far more dangerous and potentially irreversible ocular pressure burden. 1.3 Current Monitoring Standard and Its Limitation The WSACS gold standard for IAP monitoring is the intravesicular (bladder) pressure technique: 25 mL of sterile saline is instilled into the bladder via the indwelling Foley catheter (already present in all ventilated ICU patients), the catheter clamped, and pressure transduced at the level of the mid-axillary line at the iliac crest, read 30–60 seconds after instillation. This is accurate, reproducible, and over 90% of worldwide IAP measurements use this method. However, it requires a dedicated pressure transducer setup and trained nursing staff. In this study, IAP will be measured at only 3 designated time points (M1, M2, M3) paired simultaneously with IOP and ONSD, rather than the routine clinical frequency. This minimises nursing workload and study complexity while maintaining full scientific validity. This creates a powerful clinical question: can IOP and ONSD — measurable in under 5 minutes at the bedside without any special consumables — serve as rapid non-invasive screening tools to flag patients with raised IAP before a formal bladder pressure measurement is performed? If a strong correlation exists between IAP and IOP/ONSD across the IAH grade spectrum, these ophthalmic parameters could become part of routine ICU nursing observations — a significant advance in patient safety, particularly in resource-limited settings. 1.4 The Research Gap Studies to date have validated ONSD as a non-invasive ICP surrogate using neurosurgical gold standards (external ventricular drains), and have demonstrated IAP–ONSD correlation in the controlled laparoscopic setting. However, no published Indian study has prospectively validated the correlation between bladder-measured IAP and simultaneously measured IOP and ONSD in surgical ICU patients across all four IAH grades. Furthermore, no study has assessed whether these ophthalmic parameters can detect the transition point between normal IAP and IAH — a finding with direct bedside triage utility. This study is designed to fill that gap with a simple two-group design. 2. AIMS AND OBJECTIVES 2.1 Primary Aim To validate the correlation between simultaneously measured intra-abdominal pressure (IAP) by bladder technique and intraocular pressure (IOP) by Tono-Pen / ONSD by ultrasonography in surgical ICU patients across IAH Grades I–IV and controls, establishing Pearson/Spearman correlation coefficients for both parameters. 2.2 Secondary Aims • To determine the sensitivity, specificity, and diagnostic accuracy (AUC-ROC) of IOP and ONSD cut-off values in detecting IAH (IAP ≥ 12 mmHg) and ACS (IAP > 20 mmHg). • To describe the temporal kinetics of IOP and ONSD changes as IAP rises and falls during ICU stay, and to assess whether IOP/ONSD recovery precedes, coincides with, or follows IAP normalization. • To correlate the magnitude of IOP and ONSD elevation with clinical severity markers: SOFA score, serum lactate, urine output, and mean arterial pressure (MAP). • To assess the feasibility of bedside Tono-Pen and ONSD measurement in a surgical ICU setting by recording time taken, technical failure rate, and inter-observer agreement between two trained observers. • To establish baseline reference data for IOP and ONSD in ventilated surgical ICU patients with normal IAP — a subgroup for which published Indian data are absent. 2.3 Hypotheses Null Hypothesis (H₀): There is no statistically significant correlation between IAP (bladder pressure) and IOP or ONSD in surgical ICU patients. Alternate Hypothesis (H₁): IAP correlates positively and significantly with both IOP and ONSD in a grade-proportional manner, such that IOP and ONSD can serve as non-invasive bedside surrogates for IAP monitoring in surgical ICU patients. 3. STUDY DESIGN AND SETTING Parameter Details Study Type Prospective observational validation study Study Setting Surgical ICU and General Surgery Wards, [Institution Name] Institution Type Any medical college with a functioning Surgical ICU and indwelling urinary catheter monitoring Study Population Adult patients admitted to surgical ICU with indwelling urinary catheter (IAP measurable) Study Duration 12 months: Month 1–2 (ethics/preparation), Month 3–11 (data collection), Month 12 (analysis/writing) Ethics IEC approval mandatory before first enrolment; CTRI registration required Blinding Ophthalmologist measuring IOP and ONSD blinded to IAP value at time of measurement; ICU team blinded to ophthalmic measurements during study period Consent Written informed consent from patient or legally authorized representative (LAR) if patient incapacitated 4.2 Inclusion Criteria • Age ≥ 18 years • Admitted to surgical ICU with indwelling urethral Foley catheter in situ (prerequisite for bladder pressure measurement) • IAP measured by standard WSACS intravesicular technique as part of clinical care • Mechanically ventilated or spontaneously breathing — both eligible • Written informed consent from patient or LAR obtained within 6 hours of ICU admission 4.3 Exclusion Criteria • Known pre-existing glaucoma or structural ocular disease causing baseline IOP or ONSD abnormality • Severe periorbital oedema, facial fractures, or conjunctival/corneal pathology preventing IOP measurement • Previous intraocular surgery in either eye • Contraindication to ocular ultrasonography (penetrating eye injury, open globe) • Patients with primary neurological injury (traumatic brain injury, intracranial haemorrhage, meningitis) as confounding direct ICP elevation independent of IAP • Pregnancy • Moribund patients (expected to die within 24 hours) — consent not obtainable and serial measurements not feasible Urological pathology preventing reliable bladder pressure measurement (neurogenic bladder, cystectomy, pelvic haematoma compressing bladder) 4. STUDY GROUPS AND ELIGIBILITY CRITERIA 4.1 Study Groups Group IAP Criterion Definition n IAH Group IAP ≥ 12 mmHg Two consecutive bladder pressure readings ≥ 12 mmHg, 1–6 hours apart 20 Control Group IAP < 12 mmHg Post-operative surgical ICU patients with normal IAP on two consecutive readings 20 TOTAL 40 Note: All patients with IAP ≥ 12 mmHg are enrolled in a single IAH group regardless of grade. This two-group design simplifies enrolment, analysis, and IEC approval while maintaining full statistical validity. 5. SAMPLE SIZE CALCULATION AND FEASIBILITY 5.1 Statistical Basis The primary analysis is a Pearson/Spearman correlation between IAP and IOP, and between IAP and ONSD. Using the following parameters for the minimum adequate sample: Parameter Value Expected correlation coefficient (r) between IAP and ONSD 0.50 (conservative estimate) Alpha (α) 0.05 (two-tailed) Power (1−β) 80% Minimum n calculated (Fisher’s Z transformation) 29 patients Adjusted for 20% dropout / measurement failure ~35 patients Rounded to equal groups (20 IAH + 20 Control) 40 patients FINAL SAMPLE SIZE 40 patients 6. DETAILED METHODOLOGY 6.1 Patient Identification and Enrolment Every patient admitted to the surgical ICU will be screened daily by the research team (PG resident or co-investigator) for study eligibility. The eligibility checklist will be completed within 6 hours of ICU admission. Eligible patients (or their LAR) will be approached for informed consent. Following consent, patients will be enrolled and assigned to the appropriate IAP group based on their first two bladder pressure readings, taken 1–6 hours apart as per WSACS protocol. Group assignment may be updated if IAP changes grade during the ICU stay — all measurements will be retained and the patient's final group classified by their peak IAP grade achieved during the study period. 6.2 IAP Measurement — Standard WSACS Intravesicular (Bladder) Technique All IAP measurements will use the WSACS-recommended intravesicular method via the indwelling Foley catheter, performed by the ICU nursing team as part of routine clinical monitoring: • Patient positioned in ◦ complete supine position • External pressure transducer zeroed at the ◦ mid-axillary line at the level of the iliac crest • Foley drainage clamped; ◦ 25 mL of sterile normal saline instilled into the bladder • Pressure read ◦ 30–60 seconds after instillation (to allow detrusor muscle relaxation) • IAP recorded in mmHg; two readings taken 1–6 hours apart; mean used for group assignment IAP is measured exactly 3 times per patient — once at each study time point (M1, M2, M3) — always simultaneously with IOP and ONSD. This simplifies protocol adherence and reduces nursing burden. The ICU nurse or resident performs IAP measurement; the ophthalmology team performs IOP and ONSD simultaneously, blinded to the IAP reading. 6.3 Measurement Time Points Time Point Label Clinical Moment IAP IOP ONSD M1 Baseline Within 6h of ICU admission — first two IAP readings determine group Yes Yes Yes M2 24 hours 24h post-admission or post-operative Yes Yes Yes M3 ICU Discharge / Resolution At IAP normalisation or ICU discharge, whichever first Yes Yes Yes Additional measurements will be taken at any point of clinical IAP measurement if the team suspects a significant change (e.g., post-decompressive laparotomy, after abdominal closure, after aggressive diuresis). These will be recorded as supplementary data points. 6.4 IOP Measurement Protocol — Tono-Pen AVIA (All Patients, All Time Points) Tono-Pen AVIA applanation tonometer will be used exclusively for IOP measurement at all time points (M1–M3) and in all patients, regardless of conscious level or ventilatory status. This ensures complete instrument standardization across the study and eliminates inter-device variability. The Tono-Pen is ideally suited for the ICU setting: it functions on supine patients, works through a lightly retracted or closed eyelid in sedated patients, requires no patient fixation or cooperation, and produces readings within seconds. The probe tip is cleaned with an isopropyl alcohol swab between patients. For awake patients, a single drop of topical proparacaine 0.5% is instilled before measurement. For sedated or ventilated patients, no topical anaesthesia is required. Three readings per eye per time point are averaged; the mean of both eyes constitutes the study value. IOP > 21 mmHg is flagged as elevated; IOP ≥ 25 mmHg is a critical value requiring immediate ophthalmology attending notification and documentation in the patient's clinical record. 6.5 ONSD Measurement Protocol A 7.5–10 MHz linear ultrasound probe with sterile probe cover and aqueous coupling gel is applied to the closed eyelid. The globe is imaged in B-mode in transverse orientation. The hyperechoic posterior globe wall is identified as the landmark. The ONSD is measured at 3 mm posterior to the posterior globe wall using caliper measurement from inner sheath to inner sheath. Three measurements per eye averaged; mean of both eyes used. Measurements are performed by the same trained ophthalmology resident at all time points (intra-observer consistency). A second observer will independently perform ONSD on 20 patients for inter-observer reliability (ICC target > 0.85). ONSD ≥ 5.5 mm is the threshold for clinically raised ICP. The entire IOP + ONSD bedside assessment takes less than 5 minutes, requires no patient movement or repositioning, and creates no interruption to ICU care or mechanical ventilation. 6.6 Concurrent Clinical Parameters Recorded at Each Time Point Parameter Source Relevance IAP (mmHg) Bladder pressure — WSACS technique Primary comparator Mean Arterial Pressure (MAP, mmHg) Arterial line / NIBP Abdominal Perfusion Pressure = MAP − IAP Abdominal Perfusion Pressure (APP) Calculated: MAP − IAP Organ perfusion surrogate; target APP > 60 mmHg SOFA Score Clinical scoring at each time point Overall illness severity Urine Output (mL/h) Foley catheter measurement Renal perfusion marker Serum Lactate (mmol/L) ICU blood gas Tissue hypoperfusion marker Central Venous Pressure (CVP, mmHg) Central line (if in situ) Mediator of IAP → IOP/ONSD pathway GCS / Sedation Score (RASS) Clinical assessment Confounds ONSD; documented as covariate 6.7 Case Record Form A structured paper Case Record Form (CRF) with four measurement sections (M1–M3) will be used. Fields include: demographics, admission diagnosis and WSACS risk factors for IAH, surgical procedure details, all parameters above at each time point, complications, and outcomes (ICU LOS, in-hospital mortality, renal replacement therapy, mechanical ventilation duration). CRFs will be stored in a locked cabinet and data entered into a password-protected Excel/SPSS database. 7. STATISTICAL ANALYSIS PLAN 7.1 Primary Analysis — Correlation The primary analysis will use Pearson correlation coefficient (r) if data are normally distributed (Shapiro-Wilk test, p > 0.05), or Spearman's rank correlation (ρ) if non-normal. Two separate correlations will be computed: (1) IAP vs. IOP across all 60 patients at all time points; (2) IAP vs. ONSD across all 60 patients at all time points. A scatter plot with regression line and 95% confidence interval will be generated for each. Correlation will also be computed within each IAH grade subgroup. 7.2 Diagnostic Accuracy — ROC Analysis Receiver Operating Characteristic (ROC) curves will be constructed to determine the optimal IOP and ONSD cut-off values for detecting: (a) IAH (IAP ≥ 12 mmHg) vs. controls; and (b) ACS (IAP > 20 mmHg) vs. lower grades. Area Under the Curve (AUC), sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) will be reported for each cut-off. The Youden Index (sensitivity + specificity − 1) will identify the optimal threshold. 7.3 Group Comparisons • IOP and ONSD values between IAH group and Control group: Independent samples t-test (or Mann-Whitney U if non-normal). • Longitudinal IOP and ONSD changes over M1–M3 within each group: Paired t-test or Wilcoxon signed-rank test. • Abdominal Perfusion Pressure (APP = MAP − IAP) vs. IOP and ONSD: Spearman correlation. • SOFA score vs. IOP and ONSD elevation: Spearman correlation. 7.4 Feasibility Metrics • Mean time per IOP + ONSD bedside assessment (target < 5 minutes) • Technical failure rate (probe placement failure, periorbital oedema preventing measurement) • Inter-observer ICC for ONSD (target ≥ 0.85) 7.5 Software Statistical analysis will be performed using JASP (free, open-source) or IBM SPSS (institutional licence) for correlation and group comparisons, and MedCalc free online ROC calculator for AUC and diagnostic cut-off analysis. All tests two-tailed; p < 0.05 considered significant. Bonferroni correction for multiple comparisons. 9. ETHICAL CONSIDERATIONS 9.1 Study Nature and Risk Assessment This is a non-interventional, add-on observational study. No modifications to ICU management or surgical care, or any clinical decision will be made based on study findings during the study period. IAP measurement by bladder technique is already part of standard ICU care for these patients. IOP by Tono-Pen AVIA is non-invasive and safe. ONSD by B-mode ultrasound carries no risk when performed by trained personnel with sterile probe covers. The entire add-on assessment takes less than 5 minutes and creates no disruption to clinical care. 9.2 Consent in Critically Ill Patients Obtaining consent in critically ill patients requires special consideration. The following hierarchy will be followed: • If the patient has ◦ decision-making capacity (GCS ≥ 13, oriented): written informed consent obtained directly from patient • If the patient lacks capacity (sedated/intubated/GCS < 13): ◦ written consent from Legally Authorized Representative (LAR) — next of kin (spouse, adult child, parent, or sibling, in that order) • If LAR is unavailable within 6h and patient meets eligibility: ◦ the Principal Investigator may enrol under a waiver of consent provision, to be ratified by IEC in the protocol; retrospective consent from LAR will be obtained at first available opportunity ◦ All consent conversations will be conducted in the patient's or LAR's preferred language. A minimum of 30 minutes will be given for reflection before consent is signed. 8. STUDY TIMELINE Month Phase Key Activities Month 1–2 Preparation & Ethics IEC submission and approval; CTRI registration; CRF design; team training in ONSD technique (inter-observer reliability session with 10 healthy volunteers); IAP bladder pressure training for ICU nursing staff; standard operating procedures for simultaneous measurements finalized Month 3 Pilot Enrolment Enrol first 10 patients; validate blinding protocol; confirm CRF completeness; assess technical failure rate; adjust protocol if needed Month 4–9 Main Enrolment Recruit remaining 50 patients; target 6–8 patients/month; daily screening of ICU admissions; monthly data quality review by PI; all IAH grades to be tracked to ensure adequate subgroup representation Month 10–11 Complete Recruitment + Data Lock Achieve n=60; database finalized; data cleaning; query resolution; anonymization; statistical dataset prepared Month 12 Analysis & Writing Full statistical analysis; ROC curves; scatter plots; table and figure preparation; manuscript drafting; internal departmental review; submission to target journal 9.3 Safety Monitoring If IOP ≥ 25 mmHg is detected at any measurement time point, the ophthalmology co-investigator will be notified immediately. A formal ophthalmology review will be arranged within 24 hours. Study data will be shared with the clinical team at ICU discharge to enable appropriate ophthalmic follow-up. A Data Safety Monitoring Board (DSMB) — comprising the Head of Surgery and Head of Ophthalmology — will review cumulative data at Month 6 for any emerging safety signal. 9.4 Regulatory Framework • Declaration of Helsinki (World Medical Association, revised 2013) • ICMR National Ethical Guidelines for Biomedical and Health Research Involving Human Participants, 2017 • Schedule Y, CDSCO Good Clinical Practice Guidelines • CTRI registration prior to first enrolment (mandatory) • Data stored for 5 years post-publication per ICMR guidelines 10. EXPECTED OUTCOMES AND SIGNIFICANCE 10.1 Expected Primary Findings Based on published laparoscopy data demonstrating a significant positive correlation between IAP (12–15 mmHg) and ONSD, we expect to demonstrate a moderate-to-strong positive correlation (r ≥ 0.55–0.65) between bladder-measured IAP and both IOP and ONSD across IAH grades. We expect IOP and ONSD values to increase in a grade-proportional pattern — lowest in controls, progressively higher through Grade I–IV — establishing a dose-response relationship between abdominal compartment pressure and ocular/intracranial pressure transmission. 10.2 Diagnostic Cut-Off Values We expect to establish IOP and ONSD threshold values that, with acceptable sensitivity and specificity, identify patients with IAH (≥ 12 mmHg) and ACS (> 20 mmHg). If IOP > 18–20 mmHg or ONSD > 5.2–5.5 mm correlates reliably with IAH, these could be incorporated into a rapid 'ocular IAP screening protocol' for use by ICU nurses — a completely novel contribution to critical care practice. 11. TEAM, EQUIPMENT AND BUDGET Role Department Responsibilities Principal Investigator General Surgery Overall coordination; patient screening and recruitment; IAP interpretation; data oversight Co-Investigator 1 Ophthalmology IOP (Tono-Pen AVIA) and ONSD measurements at all time points; blinded to IAP values; DSMB member Co-Investigator 2 Head of Surgery / Ophthalmology DSMB chair; monthly data review; ethical oversight PG Resident / Research Fellow General Surgery Daily ICU screening; CRF data entry; consent process; IAP measurement coordination with ICU nursing; ONSD inter-observer measurements ICU Nursing Staff Surgical ICU Nursing IAP bladder pressure measurements (trained by PI at study start) Biostatistician Community Medicine / External Sample size, ROC analysis, correlation statistics, manuscript tables 10.3 Clinical Impact This study addresses a genuine unmet clinical need in Indian surgical ICUs: the absence of a rapid, non-invasive, zero-consumable bedside tool for IAP screening. Bladder pressure measurement, while the gold standard, is not performed in many ICUs due to lack of training, equipment, or awareness. If IOP and ONSD can reliably flag IAH, they provide a first-line screen that triggers formal bladder pressure measurement — potentially preventing progression from undetected IAH to full ACS with irreversible organ failure. The clinical impact would be greatest in resource-limited and new-institution settings — precisely the context of this study. 10.4 Academic Significance This study is the first prospective validation study of IOP and ONSD as IAP surrogates in a surgical ICU from India. It provides: (a) Indian normative data for IOP and ONSD in surgical ICU patients; (b) a two-group correlation dataset comparing IAH and normal IAP; (c) ROC-based diagnostic cut-offs validated in an Indian population. Target journals: Indian Journal of Critical Care Medicine, Surgical Endoscopy, Journal of Intensive Care Medicine, Indian Journal of Ophthalmology. Conference presentations: IAGES (Indian Association of Gastrointestinal Endo-Surgeons), DOS (Delhi Ophthalmological Society), ISS (Indian Surgical Society). Suitable as an MS (General Surgery) or MS (Ophthalmology) thesis with joint supervision. Equipment Availability Role in Study Foley catheter (all ICU patients) Already in situ — standard ICU care IAP measurement conduit Foley catheter manometer kit or 3-way stopcock + IV pressure line ICU — standard equipment already in situ Bladder pressure measurement; no additional procurement if already available in ICU Tono-Pen AVIA applanation tonometer Ophthalmology OPD/OT — available IOP at all time points M1–M3; all patients regardless of conscious level Topical proparacaine 0.5% drops Hospital pharmacy — available For awake patients only; not needed for sedated patients 7.5–10 MHz linear ultrasound probe Radiology / Emergency / ICU — available ONSD at all time points; sterile probe cover required Sterile ultrasound probe covers Procure — consumable ~Rs. 150–200 per case × 60 = Rs. 9,000–12,000 Sterile saline 25 mL (for IAP) Hospital pharmacy — standard Already used in IAP measurement; no additional cost SPSS / MedCalc statistical software Institutional or external Data analysis and ROC curves Budget Item Estimated Cost (INR) Sterile ultrasound probe covers (60 cases × 3 measurements × Rs.40 each) Rs. 7,200 Topical proparacaine 0.5% eye drops (for awake patients) Rs. 1,000 Printed CRFs, stationery, files (simple format, 2 pages/patient) Rs. 1,500 Statistical software (JASP — free; or institutional SPSS licence) Rs. 0 ROC analysis (MedCalc online — free) Rs. 0 Ethics committee fee Rs. 5,000–10,000 CTRI registration Free Open-access publication fee (Cureus: free; Indian J Ophthalmol: free) Rs. 0 Miscellaneous (printing, binding, travel within institution) Rs. 2,000 TOTAL ESTIMATED BUDGET Rs. 16,700 – 21,700

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