1

Introduction

Hyperbaric means relating to, producing, operating, or occurring at pressures higher than normal atmospheric pressure . In hyperbaric oxygen (HBO ₂ ) therapy, the patient breaths pure oxygen (100%) at a pressure greater than atmospheric pressure while in a steel or polymer chamber. HBO ₂ therapy is known mainly for its use as the treatment of choice in carbon monoxide poisoning, gas embolism, and decompression sickness. The experience of hyperbaric medicine specialist and, to a certain extent, the scientific literature also support the use of HBO ₂ as an adjuvant treatment for a number of other medical conditions, such as complex refractory wounds , intracranial abscess, radiation tissue injury, crush injuries, compartment syndrome, acute traumatic peripheral ischemia, burns, and other tissue damage resulting from ischemic reperfusion injury (IRI) . At present, there are no standard protocols for specific medical conditions. The therapeutic procedures vary according to the condition (acute or chronic) and the treatment centre. The treatment can be administered on a one-time basis and varies in duration by way of several daily or twice daily sessions of predetermined duration. The pressure at which HBO ₂ is administered depends on several factors such as the medical condition, the patient’s characteristics, the type of chamber, and the centre’s practices.

Hyperbaric oxygen results in an oxidative stress that is capable of increasing reactive oxygen species (ROS) generation . It has been previously suggested that part of the therapeutic effect of HBO ₂ may originate from the generation of ROS and that this ROS initiates a cascade of events that may lead to myocardial protection. This is paradoxical to the traditional premise that ROS plays an important role in IRI-mediated cellular damage . In a study involving IRI conducted by Sterling et al. , it was demonstrated that the animals exposed to HBO ₂ during ischemia only, reperfusion only, or ischemia and reperfusion had significantly smaller myocardial infarct sizes compared to the control animals, indicating they had been protected by HBO ₂ . This study suggested that HBO ₂ compared to normobaric hyperoxia was capable of inducing myocardial protection. Furthermore, it also provided one of the first experimental evidences that pretreatment with HBO ₂ (also known as HBO ₂ preconditioning) prior to a reperfusion injury was capable of inducing myocardial protection. The specific ability for HBO ₂ preconditioning prior to IRI to reduce myocardial infarct size in animals has also more recently been demonstrated by Kim et al. and Han et al. . The work by Kim et al. furthermore suggests that part of the protective effect of HBO ₂ preconditioning may involve ROS and its effects on antioxidants. Moreover, it has also been suggested that the cellular protective effects of HBO ₂ may stem from production of nitric oxide synthase (NOS) and heat shock proteins (Hsp). Both NOS and Hsp are known to be cardioprotective and have been shown to be induced by ROS , thus implicating their possible roles in HBO ₂ -induced myocardial protection via ROS.

In the Hyperbaric Oxygen Therapy in Percutaneous Coronary Intervention (HOT-PI) study , following stabilization with medical therapy, resolution of chest pain, and normalization of ST-segment changes, patients who presented with unstable angina or acute myocardial infarction (AMI) were randomized to a group treated with HBO ₂ or a group which was not treated with HBO ₂ . The results demonstrated that patients who received adjunctive HBO ₂ in the early peri-percutaneous coronary intervention period had a lower clinical restenosis rate. Significantly fewer patients ( P <.003) in the HBO ₂ group required revascularization of the target lesion, and the number of patients with recurrence of late angina symptoms was also less frequent ( P <.05) in this group. In addition, composite adverse cardiac events [death, myocardial infarction, coronary artery bypass graft surgery (CABG), or revascularization of target lesion] at 8 months were significantly higher in the control group compared to the HBO ₂ group ( P =.001). The results of this study also suggest that HBO ₂ preconditioning may have the capacity to reduce vascular ischemic events by perhaps limiting the pathological progression of atherosclerosis. This concept has been corroborated by animal findings demonstrating that HBO ₂ treatment halted the progress of atherosclerosis and appeared to facilitate it regression.

In the Hyperbaric Oxygen Therapy in Myocardial Infarction (HOT-MI) study , patients with an AMI who received recombinant tissue plasminogen activator (rTPA) were randomized to treatment consisting of HBO ₂ combined with rTPA or rTPA alone. In this study, the group that was also treated with HBO ₂ experienced an improvement in post MI ejection fraction, a 35% reduction in mean creatinine phosphokinase (CK) at 12 and 24 h post MI, a reduction in time to pain relief, and ST-segment resolution post MI. The authors of this study concluded by suggesting that adjunctive HBO ₂ was a feasible and safe treatment for AMI.

Based on the available clinical evidence , the hypothesis of this clinical study was that HBO ₂ preconditioning in patients with CAD, prior to first-time elective on-pump cardiopulmonary bypass (CPB) CABG, would be capable of improving myocardial function. The objective of this study was to determine whether preconditioning CAD patients with HBO ₂ prior to first-time elective on-pump CPB CABG leads to improved myocardial left ventricular stroke work (LVSW) post CABG.

The primary end point of this study was to demonstrate that preconditioning CAD patients with HBO ₂ prior to on-pump CPB CABG leads to a statistically significant ( P <.05) improvement in myocardial LVSW 24 h post CABG. The secondary end points of this study were to assess the effects of HBO ₂ preconditioning on

• a)
  

  the other measured parameters of cardiovascular hemodynamics which included:

  
  
  
  
  • i.
    

    Heart rate (HR)

    
    
  • ii.
    

    Mean arterial pressure (MAP)

    
    
  • iii.
    

    Stroke volume (SV)

    
    
  • iv.
    

    Cardiac output (CO)

    
    
  • v.
    

    Cardiac index (CI)

    
    
  • vi.
    

    Mean pulmonary artery pressure (MPAP)

    
    
  • vii.
    

    Pulmonary capillary wedge pressure (PCWP)

    
    
  • viii.
    

    Pulmonary vascular resistance (PVR)

    
    
  • ix.
    

    Pulmonary vascular resistance index (PVRI)

    
    
  • x.
    

    Systemic vascular resistance (SVR)

    
    
  • xi.
    

    Systemic vascular resistance index (SVRI)

    
    
  • xii.
    

    Left ventricular stroke work (LVSW)

    
    
  • xiii.
    

    Left ventricular stroke work index (LVSWI)

    
    
  • xiv.
    

    Right ventricular stroke Work (RVSW)

    
    
  • xv.
    

    Right ventricular stroke work index (RVSWI)

  
  
  
  
• b)
  

  serum troponin T

  
  
• c)
  

  myocardial NOS and Hsp

  
  
• d)
  

  serum soluble adhesion molecules (sPSGL-1, sP-selectin, sE-selectin, sICAM-1)

  
  
• e)
  

  post CABG length of intensive care unit (ICU) stay

  
  
• f)
  

  the incidence of the following post CABG complications:

  
  
  
  
  • i.
    

    low cardiac output syndrome (defined as difficulty in maintaining intra- and postoperative mean arterial pressure above 70 mmHg)

    
    
  • ii.
    

    inotrope usage

    
    
  • iii.
    

    atrial fibrillation

    
    
  • iv.
    

    intra- and postoperative blood loss

    
    
  • v.
    

    postoperative blood transfusion

    
    
  • vi.
    

    pulmonary, renal, gastrointestinal, and neurological complications

    
    
  • vii.
    

    infections

    
    
  • viii.
    

    perioperative MI

    
    
  • ix.
    

    reoperations for bleeding

    
    
  • x.
    

    mortality

  
  

A post hoc analysis was also done to determine the cost-effectiveness of HBO ₂ preconditioning in this study. This article will focus on the primary end point and all the secondary end points apart from the serum adhesion molecules and the myocardial NOS and Hsp, which will be reported on later.