## 1 Introduction  
Technological development plays vital roles in areas of life such as healthcare, education, tourism, national security, and others. In the field of healthcare, healthcare agencies compete through applying advanced technologies to improve their throughput, management, control, and services with reduced costs. One important step toward this direction is to use electronic personal health records (EPHRs), which supports the use of other technologies [1]. The use of EPHRs involves cloud storage, which allows for more control, availability, and accessibility [2, 3]. However, having EPHRs in cloud storage is called a centralized parallel and distributed system, which has a single point of failure. Blockchain technology is one kind of distributed system that runs in a decentralized manner [4], in which multiple transactions are processed and grouped into one block. The blocks are listed in one ledger. Copies of the ledger are distributed among all nodes, such that every node has an updated copy of the ledger. The nodes are devices that belong to the blockchain network, and they are authorized to store and validate transactions and blocks. Actually, transactions are executed by users, but they cannot confirm (commit) those transactions. The blockchain nodes (miners) perform processing of transactions to confirm them. During this process, the miners compete in transaction processing to create a block of transactions using some consensus mechanisms, such as proof-of-work (PoW) or proof-of-stake (PoS) [5]. Then, the miner who succeeds in creating a new block proposes that block to other miners in order to verify it. If this block is verified, then it is added to the ledger; otherwise, it is ignored [6; 7]. The blockchain processes are executed as follows:  
* Proposal: A miner verifies transactions and proposes a new block to other miners (they will act as validators);
* Verification: The validators validate the proposed block and send their votes to the others to either confirm or decline (commit or abort) the proposed block;
* Consensus: After receiving the votes of all validators, every validator checks the votes of the majority. Accordingly, the block is committed and either added to the ledger or not. On the other hand, healthcare utilizes various data sources, such as healthcare professionals and the Internet of Things (IoT). Firstly, healthcare professionals have different levels of authority and trustworthiness, and this is also connected with the criticality of healthcare situations. For example, in emergency cases, doctors and nurses should access EPHRs to read or update them directly without any delay; for these purposes, blockchain procedures such as permission or voting cannot be applied. Secondly, IoT, including sensors, smartphones, and wearable mobile devices, provides real-time data as these devices sense and reflect data directly [8]. These devices are able to facilitate or perform some actions [9; 10; 11]. Actually, wearable mobile devices can be embedded in clothes or accessories such as watches, bracelets, glasses, jewelry, etc. [12]. There are also other, complicated kinds of wearable devices that can be embedded into the human body. This allows for the improvement of healthcare follow-up and services. It helps in tracking life signs and monitoring patients' situations [13]. However, such real-time technology is challenged by delays caused by blockchain procedures [14]. Moreover, these tools usually have limited storing, processing, and energy capabilities, which would also be challenged by blockchain procedures such as mining, validating, voting and storing processes [15]. Applying blockchain technology to e-healthcare has many advantages, such as decentralization, security, privacy, anonymity, transparency, reliability, and fault tolerance. However, the main challenge is to handle the impact of the latency that results from blockchain procedures during the communication and voting phases.