Inclusion Criteria:

• - Voluntary written informed consent.

• - Men or women, age ≥ 18 years of age, with upper limit of 75 years old.

• - Subjects with acute myeloid leukemia (AML) or Myelodysplastic Syndrome (MDS) for cohort 1.

• - Subjects with chronic myelomonocytic leukemia (CMML), atypical chronic myeloid leukemia (aCML), CML, chronic neutrophilic leukemia (CNL), myelofibrosis, and myelodysplastic/myeloproliferative (MDS/MPN) overlap syndrome for cohort 2.

• - Karnofsky performance status (KPS) of ≥ 70% - Patients should have New York Heart Association (NYHA) Functional Classification, Class I (ordinary physical activity does not cause undue fatigue, palpitation, dyspnea, or anginal pain) or Class II (ordinary physical activity results in fatigue, palpitation, dyspnea, or anginal pain).

• - Adequate hepatic, renal, cardiac and pulmonary function to be eligible for transplant.

Minimum criteria include: Hepatic: ALT, AST < 4x IULN and serum total bilirubin ≤ 2.0 mg/dL; Renal: serum creatinine: ≤ 2.0 mg/dL; Left ventricular ejection fraction ≥ 45% measured by 2D-ECHO or MUGA scan; EKG with no clinically significant arrhythmia; FEV1, FVC and DLCO ≥ 50% of predicted value (corrected to serum hemoglobin)

• - Women of child-bearing potential and men with partners of child-bearing potential must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation, and for 30 days following completion of therapy.

Should a woman or partner become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician and the investigator immediately.

• - A woman of child-bearing potential is any female (regardless of sexual orientation, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria: Has not undergone a hysterectomy or bilateral oophorectomy; or Has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months) - Women of child-bearing potential should have a negative urine or serum pregnancy test within 4 weeks of starting preparative regimen.

Exclusion Criteria:

• - Pregnant or breastfeeding.

• - Severe chronic obstructive pulmonary disease requiring oxygen supplementation.

• - History of spontaneous pneumothorax, prior chest surgery requiring thoracotomy or direct chest irradiation to the lungs.

• - Evidence of pneumothorax or significant pulmonary fibrosis on chest imaging within 60 days of transplant.

• - Active malignancy excluding AML, MDS, CMML, aCML CML, CNL, MF and MDS/MPN overlap syndrome.

• - Active ear/sinus infection.

Patients with chronic sinusitis or sinus headaches are excluded unless cleared by ear, nose, and throat specialist.

• - Recent sinus surgery (within the last 5 years).

• - Ear surgery excluding myringotomy or ear tubes.

• - Subjects must agree to refrain from active tobacco or e-cigarette use 72 hours prior to transplant until complete transplant recovery.

Nicotine replacement therapy is allowed.

• - Claustrophobia.

• - History of recurrent seizures within 5 years of study enrollment.

• - Uncontrolled asthma.

• - Uncontrolled viral or bacterial infection at the time of study enrollment.

• - Active or recent (prior 6 months) invasive fungal infection without interdisciplinary (ID) consult and approval.

- Patients who had intrathecal chemotherapy within 2 weeks of starting preparative regimen or cranial irradiation within 4 weeks of starting preparative regimen

Arms

Experimental: Cohort 1- AML or MDS

Patients with will receive HBO therapy one time on day 0 of the transplant. The treatment consists of exposure to hyperbaric oxygen at 2.5 atmospheric absolutes (ATA) for a total of 90 minutes after compression to 2.5 atmosphere absolutes (ATA) in a monoplace hyperbaric chamber (Model 3200/3200R, Sechrist Industries, Inc., USA), breathing 100% oxygen. The subjects will be in the chamber for a total of 120 minutes as approximately 10-15 minutes were spent during the compression and decompression phases and subjects had 10 minute room air breaks every 30 minutes of hyperbaric oxygen treatment.

Experimental: Cohort 2- CMML, aCML, CML, CNL, MDS/MPN

Patients with will receive HBO therapy one time on day 0 of the transplant. The treatment consists of exposure to hyperbaric oxygen at 2.5 atmospheric absolutes (ATA) for a total of 90 minutes after compression to 2.5 atmosphere absolutes (ATA) in a monoplace hyperbaric chamber (Model 3200/3200R, Sechrist Industries, Inc., USA), breathing 100% oxygen. The subjects will be in the chamber for a total of 120 minutes as approximately 10-15 minutes were spent during the compression and decompression phases and subjects had 10 minute room air breaks every 30 minutes of hyperbaric oxygen treatment.

Interventions

Drug: - Hyperbaric oxygen

Reduced intensity conditioning Fludarabine and Melphalan with Hyperbaric Oxygen and Allogeneic Hematopoietic Stem Cell Transplant